KR102402133B1 - Continuous-wave organic thin-film distributed feedback laser and electrically driven organic semiconductor laser diode - Google Patents

Abstract

A laser is disclosed having an organic layer on a dispersive feedback grating structure, the organic layer comprising an organic grain medium. Lasers include continuous wave lasers, pseudo continuous wave lasers, and electrically driven semiconductor laser diodes.

Description

CONTINUOUS-WAVE ORGANIC THIN-FILM DISTRIBUTED FEEDBACK LASER AND ELECTRICALLY DRIVEN ORGANIC SEMICONDUCTOR LASER DIODE

) 유형의 에너지 전달을 가능하게 한다. BSBCz:CBP 블렌드막에서 CBP 분자는 주로 337 nm 빛에 의해 여기된다. 그런 다음, 효율적인 에너지 전달 때문에 BSBCz에서 방출이 발생하지만 CBP에서는 방출이 관찰되지 않는다.
도 2. 20 Hz에서의 0.8 ns 넓이의 펄스 여기를 사용했을 때 분산형 피드백 구조에 내장된 (a, c) BSBCz:CBP 블렌드막 및 (b, d) BSBCz 니트막의 여기 강도에 따른 방출 세기 및 반치전폭(FWHM)의 방출 스펙트럼 및 도표. (a, b)에서의 삽도는 레이저 동작 임계 근처에서 측정된 방출 스펙트럼을 도시한다.
도 3. BSBCz:CBP 블렌드막에서 (a) 레이저 진동 및 (b, c) 레이저 강도의 시간적 변화의 스트리크 카메라 영상. 반복률이 0.01 MHz에서 8 MHz로 변경되었다. 여기 광 강도는 0.25 μJcm^{- 2} 의 E_th 보다 1.8 배 높은 약 0.44 μJcm^-2로 고정되었다. 시간 척도는 (a, b)에서 500 μs이고 (c)에서 2 μs였다.
도 4. BSBCz:CBP 블렌드막 및 BSBCz 니트막을 기반으로 한 유기 분산형 피드백 레이저에서 (a) 반복률 및 (b) 레이저 진동의 작동 안정성에 따른 레이저 동작 임계의 도표. 안정성을 측정하기 위해서 레이저 장치는 8 MHz에서 연속적으로 준 연속파(quasi cw) 작동되었다. 여기 광 강도는 레이저 동작 임계보다 1.5 배 더 높았다.
도 5. (a) 액체 카르바졸(ECHz) 호스트와 헵타플루오렌 유도체의 화학 구조. (b) 1차 1D 그레이팅 패턴을 사용하여 준비된 액체 분산형 피드백 레이저의 개략도. (c) 주기 140 nm 및 높이 100 nm의 PUA 패턴의 SEM 영상. 축척 막대는 200 nm를 나타낸다. (d) 1 MHz의 반복률에서 임계 이상으로 측정된 질소화 및 산소화 액체 분산형 피드백 레이저의 방출 스펙트럼. (e) 준 연속파 영역에서 작동하는 산소화 청색 액체 분산형 피드백 레이저의 사진.
도 6. 질소화 및 산소화 EHCz:헵타플루오렌 분산형 피드백 레이저에서 (a, b) 레이저 동작 방출 및 (c, d) 레이저 강도의 시간 변화의 스트리크 카메라 영상. 반복률이 0.01 MHz에서 4 MHz로 증가되었다. 여기 광 강도는 2.5 μJcm^-2로 일정하게 유지되었다.
도 7. 질소화 EHCz:헵타플루오렌 분산형 피드백 레이저에서 서로 다른 반복률에서의 여기 강도에 따른 출력 방출 강도 및 반치전폭.
도 8. 산소화 EHCz:헵타플루오렌 분산형 피드백 레이저에서 서로 다른 반복률에서의 여기 강도에 따른 출력 방출 강도 및 반치전폭.
도 9. (a) 반복률에 따른 질소화 및 산소화 액체 분산형 피드백 유기 반도체 레이저의 레이저 동작 임계. (b) 1 MHz의 반복률에서 준 연속파 영역에서 작동하는 산소화 및 질소화 액체 유기 반도체 분산형 피드백 레이저의 광 안정성. 여기 광 강도는 2.5 μJcm^-2이었다.
도 10. (a) 산소화 및 질소화 클로로포름 용액에서 헵타플루오렌의 삼중항-삼중항 흡수 스펙트럼. 삼중항-삼중항 흡수는 분자 산소에 의한 삼중항의 ??칭 (quenching)으로 인해 산소화 용액에서 사라지게 된다. 산소화 분산형 피드백 장치에서 측정된 대표적인 레이저 스펙트럼 또한 도시되어 있다. (b) 1 ms 이상의 시간 상수를 갖는 삼중항-삼중항 흡수 기여의 일시적 감쇄.
도 11. 50 내지 800 μs의 네 가지 서로 다른 펄스 지속시간 동안의 질소화 및 산소화 EHCz:헵타플루오렌 블렌드에서 0.5 kWcm^-2에서 측정된 포토루미네선스 강도의 역학. 이 실험을 위해, 게인 매체가 두 동일한 평면 용융 실리카 기판들 사이에 위치되었다. 그 후 샘플들은 325 nm에서 방출되는 연속파 HeCd 레이저로부터 기인하는 레이저 펄스에 의해 광학적으로 펌핑되었다. 스폿 면적은 2.0 x 10^-5 cm²이었다. 샘플을 기판 평면에 대해 수직 방향으로 조사하고 PL 강도를 광전자증배관을 사용하여 45도 각도로 측정하였다. 이러한 결과는 산소화가 분자 산소에 의한 삼중항의 ??칭으로 인한 STA 손실의 억제로 이어진다는 것을 명확히 증명하는 것이다.
도 12. 유기 반도체 분산형 피드백 레이저의 제조 방법. 유기 분산형 피드백 레이저를 제조하는데 사용되는 방법의 개략도. 서로 다른 연속 단계들이 전자빔 리소그래피에 의한 분산형 피드백 공진기 구조의 제조, 유기 반도체 박막의 열증발, 및 CYTOP 중합체 막의 스핀 코팅과 이어지는 높은 열전도성 (TC) 사파이어 리드를 이용한 장치의 밀봉을 포함한다.
도 13. 혼합차수 분산형 피드백 공진기의 구조, (A) 이 연구에서 사용된 혼합차수 분산형 피드백 그레이팅 구조의 개략도. (B) 2500X 및 (C) 100000X 배율의 SEM 영상, 및 200 nm 두께의 BSBCz:CBP 블렌드막의 증착 후 장치의 (D) SEM 영상 및 (E) 단면 SEM 영상.
도 14. 준 연속파 영역에서의 유기 분산형 피드백 레이저의 레이저 동작 특성. 대표적인 BSBCz:CBP 캡슐화 혼합차수 분산형 피드백 장치의 (A) 500 μs 또는 (B) 200 ns (오직 80 MHz)의 주기 동안 0.01 내지 80 MHz의 반복률에서의 레이저 진동을 도시하고 있는 스트리크 카메라 영상. 여기 강도는 레이저 동작 임계 (E_th)보다 높은 ~0.5 μJcm^-2로 고정되었다. (C) 캡슐화 BSBCz:CBP 혼합차수 분산형 피드백 레이저에서 다양한 반복률(f)에서의 레이저 출력 강도의 시간적 변화. (D) 반복률에 따른 다양한 분산형 피드백 장치에서의 레이저 동작 임계. 실선은 눈에 대한 기준선이다.
도 15. 롱 펄스 영역에서의 유기 분산형 피드백 레이저의 레이저 동작 특성. (A) 게인 매체로서 BSBCz:CBP (20:80 wt%) 막을 사용하고 30 ms 및 2.0 kWcm^-2 (상단) 또는 800 μs 및 200 Wcm^-2 (하단)의 펄스로 광학적으로 펌핑되는 캡슐화 혼합차수 분산형 피드백 장치로부터 100 펄스 이상으로 레이저 방출을 통합하는 것을 도시하고 있는 스트리크 카메라 영상. (B) 롱 펄스 영역 (30 ms 여기)에서 작동하는 분산형 피드백 장치의 사진. (C) 여기 지속시간에 따른 다양한 분산형 피드백 장치에서의 레이저 동작 임계 (E_th). 점선은 눈에 대한 기준선이다. (D) 입사 펄스 수 (100 μs, 200 Wcm^{- 2})에 따른 유기 분산형 피드백 레이저로부터의 레이저 출력 강도의 변화.
도 16. 광학 시뮬레이션에 사용되는 기하학의 도식.
도 17. 서로 다른 디멘션의 2차 그레이팅에 대한 BSBCz:CBP (6:94 wt%) 200 nm 두께의 막을 기반으로 하는 혼합차수 분산형 피드백 레이저의 레이저 동작 임계. 장치는 20 Hz의 반복률 및 337 nm의 파장에서 800 ps 펄스를 전달하는 질소 레이저에 의해 광학적으로 펌핑되었다.
도 18. (A) 흡수 스펙트럼, (B) 정상 상태 포토루미네선스 스펙트럼 및 (C) 캡슐화 및 비캡슐화 BSBCz:CBP (6:94 wt%) 막의 CYTOP 막에 의한 일시적인 포토루미네선스 부식.
도 19. (A) BSBCz:CBP (6:94 wt%) 및 (B) BSBCz 니트막을 기반으로 하는 대표적인 유기 혼합차수 분산형 피드백 레이저의 레이저 동작 임계 이하 및 이상에서 다양한 펌핑 강도에서의 방출 스펙트럼. 삽도는 레이저 동작 임계 근처의 방출 스펙트럼을 도시한다. (C) 블렌드막 및 (D) 니트막 분산형 피드백 레이저에서의 펌핑 강도에 따른 방출 출력 강도 및 FWHM. 이 실험에서, 유기막은 스핀 코팅된 CYTOP 막 및 사파이어 리드로 덮였다. 광 펌핑 소스는 20 Hz의 반복률 및 337 nm의 파장에서 0.8 ns 폭의 펄스 여기를 방출하는 질소 레이저였다. 레이저 장치로부터의 방출은 기판 평면에 대해 수직 방향으로 수집되었다.
도 20. (A) 0.01 MHz 및 (B) 80 MHz의 반복률에서의 대표적인 캡슐화 블렌드 혼합차수 분산형 피드백 레이저의 펌핑 강도에 따른 방출 출력 강도 및 FWHM.
도 21. 반복률에 따른 순방향 (반복률 증가) 및 역방향 (반복률 감소)으로 측정된 레이저 동작 임계. 레이저 동작 임계의 비가역적 변화는 강한 광 여기 하에서의 게인 매체의 열화에 기인하는 것이다.
도 22. 80 MHz의 반복률 및 0.5 μJcm^-2의 펌핑 강도에서의 비캡슐화 2차, 비캡슐화 혼합차수, 및 캡슐화 혼합차수 분산형 피드백 장치로부터의 방출을 도시하고 있는 (A) 스트리크 카메라 영상 및 (B) 대응 방출 스펙트럼.
도 23. (A) BSBCz:CBP (6:94 wt%) 블렌드막을 기반으로 하는 서로 다른 유기 분산형 피드백 레이저로부터의 레이저 출력 진동의 작동 안정성. 안정성을 측정하기 위해, 레이저 장치는 8 MHz 또는 80 MHz에서 20 분 동안 연속적으로 준 연속파 작동되었다. 각각의 장치에 대해, 여기 광 강도는 레이저 동작 임계의 1.5 배였다. (B) 80 MHz에서의 준 연속파 영역에서 작동하는 캡슐화 블렌드 분산형 피드백 레이저의 사진.
도 24. (A) 200 nm 두께의 BSBCz:CBP (20:80 wt%) 막의 펌프 에너지 및 스트라이프 길이에 따른 472 nm를 중심으로 하는 자연증폭방출(ASE)의 출력 강도. 점선은 참고 문헌 (7, 8)에 보고된 식을 사용하는 데이터에 적합한 것이다. 도파관의 순 게인은 적합도로부터 결정되었다. (B) 도파관 막의 가장자리로부터 방출된 ASE 강도 대 펌프 스트라이프와 블렌드막의 가장자리 사이의 거리. 막의 ASE 특성은 서로 다른 (0.1, 10 및 50 μs) 펄스 폭으로 405 nm에서 방출되는 무기 레이저 다이오드를 사용하여 조사되었다.
도 25. 800 μs 및 30 ms의 롱 펄스 지속 시간 동안 각각 200 Wcm^-2 및 2.0 kWcm^-2의 펌핑 강도로 측정된 캡슐화 20 wt% 블렌드 혼합차수 분산형 피드백 레이저의 방출 스펙트럼.
도 26. 2.0 kWcm^-2의 펌프동력으로 30 ms의 긴 광 여기 동안 캡슐화 혼합차수 분산형 피드백 장치로부터 100 펄스에 대해 통합된 레이저 방출을 보여주는 스트리크 카메라 영상. 게인 매체는 BSBCz:CBP (20:80 wt%)이었다. 여기 파장은 405 nm이었다. 본 스트리크 카메라 시스템으로 30 ms 동안 레이저 동작을 관찰하기 위해서 가동 주기 백분율을 2%로 변경하여 1 ms 프레임 (0.02 X 30 ms = 0.6 ms)에서 30 ms 펄스를 시각화하였다.
도 27. (A) 레이저 동작 출력 강도의 시간적 변화, (B) 폭 30 ms 및 펌핑 강도 2.0 kWcm^-2의 평균 펄스 10 회 및 500 회 이후 측정된 캡슐화 혼합차수 분산형 피드백 레이저의 방출 스펙트럼. (C) BSBCz의 유도 방출 및 삼중항 흡수 단면 스펙트럼. 분산형 피드백 레이저의 방출 스펙트럼은 E_th 위의 BSBCz 니트막으로부터 측정되었다. (D) 삼중항 흡수 스펙트럼은 아르곤 아래에서 BSBCz를 함유하는 용액에서 측정되었다. (E) 용액 (디클로로메탄에서의 BSBCz 및 벤젠에서의 벤조페논)에서의 일시적 흡수 스펙트럼의 여기력 의존성.
도 28. (A) (B) 임계 근처 및 (C) 임계 이상에서 분산형 피드백 레이저의 방출된 빔의 발산을 보여주는 도면. 활성 게인 매체는 BSBCz:CBP (20:80 wt%) 막이었다. 장치를 롱 펄스 영역 (10 ms 여기)에서 펌핑하였다.
도 29. (A) 편광 각에 따른 방출 스펙트럼 및 (B) 편광 각에 따른 방출 강도. 장치는 BSBCz:CBP (20:80 wt%) 블렌드를 기반으로 하였으며 혼합차수 분산형 피드백 그레이팅을 사용하였다. 펌핑 강도는 200 Wcm^-2이고 펌프 펄스 지속 시간은 800 μs 이었다. 0o는 분산형 피드백 그레이팅의 홈에 평행한 방향에 대응한다는 것을 유의한다.
도 30. (A, B) 1 μs 및 (C, D) 800 μs의 롱 펄스 여기 폭에 대한 펌핑 강도에 따른 캡슐화 혼합차수 블렌드 장치의 방출 스펙트럼, 레이저 출력 강도 및 FWHM.
도 31. (A) 사파이어 또는 (B) 유리 리드로 캡슐화된 혼합차수 블렌드 BSBCz:CBP (20:80 wt%) 분산형 피드백 장치로부터의 레이저 방출을 보여주는 스트리크 카메라 영상. (C) 펌핑 강도가 2.0 kWcm^-2인 다양한 펄스 폭에 대한 유리 리드로 캡슐화된 장치의 방출 스펙트럼. 방출 스펙트럼은 펄스 폭이 길어질수록 넓어지는데, 이 펄스 폭을 가지는 레이저 동작 임계의 상당한 증가로 설명될 수 있다. 예를 들어, 2 및 3 ms의 펄스 폭의 경우, 장치는 레이저 동작 임계 이하에서 작동한다. (d) 사파이어 또는 유리 리드로 캡슐화된 분산형 피드백 장치에서 여기 펄스 폭에 따라 측정된 레이저 동작 임계 (E_th). 점선은 눈에 대한 기준선이다.
도 32. 폭 1 ms의 100 여기 펄스에 의한 (A) 조사 전 및 (B) 조사 후의 비캡슐화 블렌드 혼합차수 분산형 피드백 레이저의 레이저 현미경 영상. (B)의 삽도의 두께 프로파일은 고밀도 연속파 조사 동안 유기 박막이 제거되는 것을 보여준다. 펌핑 강도는 200 Wcm^-2이었다. (C)와 (D)의 스트리크 카메라 영상은 동일한 조사 조건 (1 ms 펄스 폭, 200 Wcm^-2의 펌핑 강도, 및 100 펄스 이상의 스트리크 카메라 영상의 통합)하에서 캡슐화 및 비캡슐화 블렌드로부터의 방출을 보여준다.
도 33. 분산형 피드백 유기 레이저의 구조.
도 34. (a) 3 층 슬래브 도파관의 구조. (b) 레이저 동작 파장 477 nm에서 막 두께 d에 따른 TE 및 TM 모드에 대한 유효 굴절률 (n_eff).
도 35. 서로 다른 값의 막 두께에 대한 수직 입사 TE 편광 하에서 수치 계산된 (실선) 및 파노 (Fano)-프로파일 피팅 (파선) 반사 스펙트럼.
도 36. (a) 서로 다른 d_f를 가진 제작된 레이저 장치들의 실험적 레이저 방출 스펙트럼. (b) 실험적 레이저 동작 파장과 계산된 공진 파장.
도 37. (a) E_th (사각형), 수치 계산으로부터 추출된 Q-인자, QFEM-계산 (삼각형), 파노 (Fano) 피팅, Q파노 (QFano) 피팅 (별), d_f에 따른 Γ (원)의 도표, (b) 레이저 방출의 실험적 FWHM 및 파노 공명으로부터 계산된 것.
도 38. 열 시뮬레이션에 사용되는 기하학의 도식.
도 39. 각 펄스의 단부에서의 최대 온도.
도 40. 게인 영역에서 (A) 1 ms, (B) 10 ms, (C) 30 ms 및 (D) 40 ms의 서로 다른 펄스 폭으로의 시간의 함수로서의 온도 상승.
도 41. 캡슐화 및 비캡슐화 장치에서 10 ms의 펄스 폭에 대한 시간에 따른 온도 상승.
도 42. 게인 영역에서 시간 또는 30 ms의 펄스 수 τ_p 에 따른 온도 상승.
도 43. 전기적으로 구동되는 유기 반도체 분산형 피드백 레이저 다이오드의 분산형 피드백 그레이팅 및 유기 레이저 다이오드를 제조하는 데 사용되는 방법의 개략도. 서로 다른 연속단계들은 패턴화된 ITO 전극 상부에 100 nm 두께의 SiO₂ 층을 스퍼터링하는 것, 전자빔 리소그래피 및 건식 에칭에 의해 SiO₂에 분산형 피드백 공진기 구조를 제조하는 것, 및 유기 반도체 박막 및 상부 전극을 열 증발시키는 것을 포함한다.
도 44. ITO 위에서 분산형 피드백 제조의 여러 단계 이후의 기판의 개략도. (B) ITO 위에 SiO₂를 스퍼터링 한 후, (C) ITO 위에 분산형 피드백를 제조한 후, 및 (D) 분산형 피드백 구조체를 제조한 후의 (A) 패턴화된 ITO.
도 45. 나노임프린트 분산형 피드백 그레이팅 제작 방법의 개략도. 서로 다른 연속 단계들은 패턴화된 ITO 전극의 상부에 70 nm 두께의 중합체 층을 마련하고, 이어서 간단한 나노임프린트 리소그래피 공정에 의해 저비용으로 중합체에 분산형 피드백 공진기 구조를 제작하는 것을 포함한다.
도 46. 유기 레이저 다이오드에 사용되는 혼합차수 분산형 피드백 공진기의 구조적 특성.
ITO 패턴화된 유리 기판 상부에 마련된 혼합차수 분산형 피드백 SiO₂ 그레이팅 구조의 (A) 레이저 현미경 및 (B) SEM 영상 (5000X 및 200000X 배율 (삽도에서)). (C, D) ITO 상부에 마련된 혼합차수 분산형 피드백 그레이팅의 EDX 및 SEM 분석. 이들 영상은 장치와의 접촉에 대한 ITO의 노출을 확인시켜준다.
도 47. 전기적으로 구동되는 유기 반도체 분산형 피드백 레이저의 구조와 특성.
(A) 유기 반도체 레이저 다이오드의 개략도 및 에너지 준위도. (B) 유기 분산형 피드백 레이저 다이오드의 현미경 사진 및 (C) 4V에서 직류 작동하는 및 직류 작동하지 않는 기준 장치 (그레이팅이 없는 OLED). 장치 면적은 140 x 200 μm이다. 직류 및 펄스 작동 하에서의 기준 OLED 장치 및 유기 분산형 피드백 레이저 다이오드에서 측정된 (D) 전류 밀도-전압 (J-V) 곡선 및 (E) 외부 양자 효율-전류 밀도 (EQE-J) 곡선.
도 48. (a) 전자 전용 장치, (b) 정공 전용 장치 및 (c) 양극성 장치의 에너지 준위도.
도 49. (a) BSBCz에 대한 정공 (파란색)과 전자 (빨간색)의 풀-프란켈 (Pool-Frankel) 필드 종속 모델 (실선)에 대한 보고된 (기호) 및 적합 (실선) 이동성. (b) 정공 전용 장치 (파란색), 전자 전용 장치 (빨간색) 및 양극성 장치 (검은색)에 대한 실험적 (기호) 및 시뮬레이션된 (실선) J(V) 곡선.
도 50. 모든 층들의 증착 후 레이저 다이오드 구조의 SEM (A, B) 표면 형태 영상 및 (C, D) 단면 영상.
도 51. 유기 반도체 레이저 다이오드의 일부 가능한 구성의 개략도. 분산형 피드백 공진기 구조 (2차 및 혼합차수 그레이팅)는 (A) 전자빔 리소그래피 및 건식 에칭에 의해 SiO₂에서, (B) 전자빔 리소그래피 및 건식 에칭에 의해 ITO에서, (C) 나노임프린트 리소그래피 공정에 의해 패턴화된 ITO 전극 상부의 중합체에서, 또는 (D) 나노임프린트 리소그래피에 의해 활성층 상부에서 제조될 수 있었다.
도 52. 2D-분산형 피드백 레이저용 2 차원 분산형 피드백 공진기 구조 (2차 및 혼합차수 그레이팅)를 가지는 유기 반도체 레이저 다이오드의 개략도.
도 53. (A-D) 4V에서 직류 작동하는 및 직류 작동하지 않는 유기 분산형 피드백 레이저 다이오드의 현미경 사진 (A, B의 장치는 324 1차 주기로 둘러싸인 36 2차 주기를 가지는 분산형 피드백 구조로 마련되었으며 D, C의 장치는 12 1차 주기로 둘러싸인 4 2차 주기의 반복 구조로 마련되었다). 장치 면적은 30 x 101 μm이다. (E) 서로 다른 주입된 전류 밀도에 대한 기판 평면에 수직으로 수집된 전기적으로 구동된 유기 반도체 분산형 피드백 레이저 (1차 및 2차에 대한 이 장치의 그레이팅 주기는 각각 140 nm 및 280 nm 였다)의 방출 스펙트럼 및 (F) 전류 밀도의 함수로서의 출력 강도.
도 54. BSBCz의 전계발광 및 PL 스펙트럼 (흑색-PL 스펙트럼, 기준 장치의 적색 EL 스펙트럼, 및 레이저 동작 임계 이하의 그레이팅을 가지는 청색 EL 스펙트럼).
도 55. 전기적으로 구동되는 유기 반도체 분산형 피드백 레이저 다이오드의 레이저 동작 특성.
서로 다른 주입된 전류 밀도에 대한 기판 평면에 수직으로 수집된 전기적으로 구동된 유기 반도체 분산형 피드백 레이저 (1차 및 2차에 대한 이 장치의 그레이팅 주기는 각각 140 nm 및 280 nm 였다)의 방출 스펙트럼 및 (B) 전류 밀도의 함수로서의 그것의 출력 강도. 각각 150 nm 및 300 nm의 1차 및 2차의 그레이팅 주기를 사용하는 유기 분산형 피드백 레이저 다이오드에서 얻어지는 (C) 방출 스펙트럼 및 (D) 출력 강도 대 전류 밀도.
도 56. 전기적으로 구동되는 유기 반도체 분산형 피드백 레이저 다이오드의 레이저 동작 특성.
(A) 서로 다른 주입된 전류 밀도에 대한 기판 평면에 수직으로 수집된 전기적으로 구동되는 유기 반도체 분산형 피드백 레이저 (1차 및 2차의 장치의 그레이팅 주기는 각각 140 nm 및 280 nm였다)의 방출 스펙트럼, 및 (B) 전류 밀도에 따른 그것의 출력 밀도.
도 57. (A) 분산형 피드백이 있는 및 분산형 피드백이 없는 전류 밀도-전압 (J-V) 곡선. 분산형 피드백을 가지는 장치는 각각 150 nm 및 300 nm의 1차 및 2차의 차수에 대한 그레이팅 주기를 가진다. (B) 500 ns 펄스 하에서 분산형 피드백 없이 전기적으로 구동되는 유기 분산형 피드백 고체 레이저에서의 외부 양자 효율 대 전류 밀도.
도 58. 1차 및 2차 영역에서 서로 다른 주기 수를 가지는 혼합차수 그레이팅의 SEM 영상 (A, B, C, D 및 E). (F) 설계 분산형 피드백에 대한 1차 및 2차 영역에서의 서로 다른 주기 수를 가지는 혼합차수 그레이팅의 표.
도 59. (A) 서로 다른 주기 수를 가지는 혼합차수 그레이팅의 레이저 스펙트럼. 각 그래프 상단의 주기 수는 2차 주기 수에 해당하며, 1차 주기 수는 도 58에서 확인될 수 있다. 하단 그래프는 장치의 출력 특성을 도시한다. (B) 2차 영역에서의 서로 다른 주기 수에 대한 혼합차수 그레이팅 레이저의 임계 에너지. 광학 펌핑의 경우, 1차 및 2차 영역에서는 각각 4 및 12 주기의 사용이 최저 임계값을 나타내게 된다. 광학 펌핑 소스는 20 Hz의 반복률 및 337 nm의 파장에서 0.8 ns 폭의 펄스 여기를 방출하는 질소 레이저였다. 레이저 장치로부터의 방출은 기판 평면에 대해 수직 방향으로 수집되었다.
도 60. (A) 1차 및 2차 영역에서 각각 12 주기 및 4 주기를 가지는 혼합차수 그레이팅의 SEM 영상. (B, C) 5 V에서 직류 작동하는 및 직류 작동하지 않는 유기 분산형 피드백 레이저 다이오드. 장치 면적은 2.9 x 10 μm이다. (D) 기준 OLED에 대한 방출 스펙트럼.
도 61. (A) 레이저의 편광을 검사하기 위한 실험 기구의 개략도. 분산형 피드백 레이저 다이오드의 (B) 레이저 동작 임계 이하 (415 Acm^-2에서), (C) 레이저 동작 임계 이상 (823 Acm^-2에서), 및 기준 (800 Acm^-2 에서, 분산형 피드백 없음)에 대해, 및 (E) 편광 각에 따른 EL 강도에 대한 EL 스펙트럼의 편광 의존성. EL은 장치의 평면에서 편광된다 (TE 모드).
도 62. (A) 분산형 피드백을 가지는 OLED 구조의 개략도. (B) 디바이스 면적이 560 x 800 ㎛인 ITO 기판상의 분산형 피드백의 현미경 영상. (C) 여기 밀도에 따른 분산형 피드백 레이저의 광학적 펌프 출력 강도; 405 nm에서 연속파 레이저로 여기.
도 63. (A, B) 4 V에서 직류 작동하는 및 직류 작동하지 않는 유기 분산형 피드백 레이저 다이오드 및 (C, D) 기준 장치 (그레이팅이 없는 OLED)의 현미경 사진.
도 64. ITO 패턴 기판 위에 SiO₂를 사용하여 마련된 환형 혼합차수 분산형 피드백 그레이팅 구조의 레이저 현미경 영상.
도 65. 구동하는 및 구동하지 않는 유기 환형 분산형 피드백 레이저의 현미경 영상. 환형 분산형 피드백을 가지는 및 환형 분산형 피드백을 갖지 않는 장치에 대한 전류 밀도-전압 (J-V) 곡선. 분산형 피드백을 가지는 및 분산형 피드백을 갖지 않는 OLED에서의 외부 양자 효율 대 전류 밀도.
도 66. (a) 분산형 피드백 그레이팅 OLED의 개략도 및 (b) 분산형 피드백 그레이팅 및 기준 OLED에 대한 실험적 (기호) 및 시뮬레이션된 (실선) J(V) 곡선.
도 67. (a) 10 V 및 (b) 70 V에서의 전하 캐리어 밀도, n, p 및 전계 F의 공간 분포.
도 68. 0.11 μm의 y에서 (a) 70 V에서의 정공 밀도, p, (b) 전자 밀도, n, (c) 2D 단면의 n, p 절개선의 공간 분포 도면.
도 69. 70 V에서의 (a) 전계 F, 및 (b) 전류 밀도 J의 프로파일의 도면.
도 70. 분산형 피드백 장치의 0.10 μm의 y에서 (a) 분산형 피드백 장치에 대한 프로파일 재조합률 R, (b) 기준 장치의 R, (c) 2D 단면의 절개선의 R, 및 (d) 70 V에서 분산형 피드백 장치의 0.164 μm의 y에서 2D 단면의 절개선 n, p의 도면.
도 71. (a) (a) 기준 장치의 서로 다른 E_b 및 (b) 분산형 피드백 및 0.6 eV의 E_b를 가지는 기준 장치에 대해 EFQ가 없는 및 EFQ가 있는 경우의 S(J) 특성.
도 72. 70 V에서 (a) EFQ를 가지는 및 EFQ를 갖지 않는 기준장치 내부, (b) EFQ를 갖지 않는 분산형 피드백 장치의 내부, (b) EFQ를 가지는 분산형 피드백 장치의 내부에서의 엑시톤 분포.
도 73. ??칭이 없는 (왼쪽 상단), EFQ를 가지는 (왼쪽 하단), PQ를 가지는 (오른쪽 상단), PQ 및 EFQ를 가지는 (오른쪽 하단) 분산형 피드백 장치 내부의 엑시톤 밀도 분포.
도 74. 분산형 피드백 장치 내부의 공기/BSBCz/SiO₂ (왼쪽 상단), 공기/BSBCz/SiO₂/ITO (오른쪽 상단), 실제 장치 Al/Ag/MoO₃/BSBCz/SiO₂/ITO (하단)의 광학 밀도 분포.
도 75. (a) 옥타플루오렌 유도체의 화학 구조. (b) 스핀 코팅된 옥타플루오렌 니트막에서 실온에서 측정된 흡수 및 정상상태 PL 스펙트럼. PL 스펙트럼의 측정을 위한 여기 파장은 376 nm였다. 자외선 조명 하의 옥타플루오렌 니트막의 사진이 삽도에 도시되어 있다. (c) 가변 입사각 분광 타원법에 의해 측정된 옥타플루오렌 니트막의 전형적 및 비전형적 광학 상수 (k 및 n). 막 두께는 약 75 nm이었다.
도 76. CBP 호스트에서 옥타플루오렌 10 및 20 wt%를 함유하는 블렌드막의 (a) 흡수 및 (b) 정상상태 PL 스펙트럼. 방출 스펙트럼에 사용된 여기 파장은 두 막 모두에서 424 nm이었다.
도 77. 옥타플루오렌 니트막에서, 및 CBP 호스트에 옥타플루오렌 10 및 20 wt%를 함유하는 블렌드막에서 측정된 PL 감쇄. 여기 파장은 365 nm이었다.
도 78. 스핀 코팅된 옥타플루오렌 니트막에서 다양한 입사각으로 측정된 실험 및 시뮬레이션 타원편광 반사 데이터 ψ 및 Δ.
도 79. (a) 유기 박막의 ASE 특성을 특징화하는데 사용되는 실험 구성의 개략도. (b) ASE 임계 이하 및 이상의 서로 다른 여기 강도에 대해 유기층의 모서리로부터 수집된 260 nm 두께의 옥타플루오렌 니트막의 방출 스펙트럼. 정상상태 PL 스펙트럼은 파선으로 표시되어 있다. 강한 광 조사 하에서의 옥타플루오렌 니트막의 사진이 삽도에 도시되어 있다. (c) 여기 밀도에 따른 260 nm 두께의 막 (모든 파장에 대해 통합)의 가장자리로부터의 출력 강도. (d) 옥타플루오렌 니트막 두께에 따른 ASE 임계. 여기 파장은 337 nm이었다.
도 80. 53 내지 540 nm 범위의 다양한 두께를 가지는 여러 옥타플루오렌 니트막의 여기 밀도에 따른 유기층 (모든 파장에 대해 통합)의 가장자리로부터의 출력 강도. 이들 데이터는 도 2d에 도시된 ASE 임계의 두께 의존성을 검사하는 데 사용되었다.
도 81. 펌프 스트라이프와 260 nm 두께의 옥타플루오렌 니트막의 가장자리 사이의 거리에 대해 플롯된 ASE 강도. 실선은 손실계수를 결정하기 위해 단일 지수 감쇄 함수에서 얻은 적합도에 해당한다.
도 82. 공기 또는 질소 기층 안에 놓인 260 nm 두께의 옥타플루오렌 니트막에서 ASE 임계를 초과하는 방출 강도의 시간적 감쇄. 펌핑 강도는 873 μJ/cm² 및 10 Hz이었다.
도 83. (a) 옥타플루오렌 분산형 피드백 레이저의 특성을 특징화하는데 사용되는 실험 구성의 개략도. (b) 이 작동에 사용된 혼합차수 분산형 피드백 그레이팅의 SEM 영상. (c) 레이저 동작 임계 이하 및 이상의 서로 다른 여기 강도에 대해 기판 평면에 대해 수직으로 수집된 옥타플루오렌 니트막을 기반으로 하는 분산형 피드백 레이저의 방출 스펙트럼. (d) 여기 밀도에 따른 분산형 피드백 레이저의 출력 세기.
도 84. (a) 이 연구에서 사용되는 OLED 구조의 개략도. 이들 장치에서 사용되는 유기 재료의 최고준위 점유 분자궤도(HOMO) 및 최저준위 비점유 분자궤도(LUMO)도 제공되고 있다. (b) 옥타플루오렌 니트막 및 CBP 블렌드막을 기반으로 하는 OLED에서의 외부 양자 효율 대 전류 밀도.
도 85. 공기 중 광전자 분광법을 사용하여 니트막에서 옥타플루오렌의 이온화 포텐셜 결정. 니트막의 흡수 스펙트럼으로부터 결정된 광학 밴드갭 값을 고려하여, 니트막에서의 옥타플루오렌의 전자 친화도를 근사화하였다. 그러나 수직 이온화 포텐셜이 HOMO 에너지의 계산된 절대값과 동일하다는 코프만스(Koopmans)의 정리가 종종 이온화 과정 및 전자 상관 동안의 완화 과정 때문에 충족되지 못한다는 것을 유념하는 것이 중요하다. 광학 밴드갭은 일반적으로 실제 전자 갭과 다르지만 이온 친화도와 LUMO는 이온화 포텐셜과 광학 밴드갭 값 사이의 차이로부터 근사화될 수 있다.
도 86. (a) 10 mA/cm²의 전류 밀도에서 측정된 전계발광(EL) 스펙트럼 및 (b) 옥타플루오렌 니트막 및 CBP 블렌드막을 기반으로 하는 OLED에서의 J-V-L 곡선.
도 87. 옥타플루오렌 니트막에서의 연속파 ASE.
도 88. 옥타플루오렌 니트막에서의 연속파 ASE.Fig. 1. (a) chemical structures of BSBCz and CBP, (b) schematic diagram of BSBCz:CBP film embedded in a second-order distributed feedback (DFB) structure, (c) dispersive feedback resonator structures by electron beam lithography and dry etching. Schematic of the method used to prepare, (d) scanning electron microscope (SEM) images of the distributed feedback structure at 4500X and (e) 120000X magnification, and (f) absorption and photoluminescence of BSBCz and CBP neat membranes. ) spectrum. The distributed feedback structure is etched directly on the silicon dioxide surface. When the second-order mode (m = 2) is used in the Bragg equation (mλ = 2n _eff Λ), the diffracted beam is emitted in a direction perpendicular to the film plane as shown in (b). CBP release and BSBCz uptake strongly overlapped, resulting in an efficient forster of CBP to BSBCz (Fig.

) to enable tangible energy transfer. In the BSBCz:CBP blend film, CBP molecules are mainly excited by 337 nm light. Then, emission occurs in BSBCz but not in CBP because of efficient energy transfer.
Figure 2. Emission intensity according to excitation intensity of (a, c) BSBCz:CBP blend film and (b, d) BSBCz neat film embedded in a distributed feedback structure using 0.8 ns wide pulse excitation at 20 Hz Emission spectrum and plot of full width at half maximum (FWHM). Insets in (a, b) show the measured emission spectra near the laser operating threshold.
Figure 3. Streak camera images of (a) laser oscillation and (b, c) temporal change of laser intensity in BSBCz:CBP blend films. The repetition rate was changed from 0.01 MHz to 8 MHz. The excitation light intensity was fixed at about 0.44 μJcm ⁻² , which is 1.8 times higher than the E _th of 0.25 ^{μJcm −2} . The time scale was 500 μs in (a, b) and 2 μs in (c).
Figure 4. Plot of laser operation threshold as a function of (a) repetition rate and (b) operational stability of laser oscillations in organic dispersive feedback lasers based on BSBCz:CBP blend film and BSBCz neat film. To measure the stability, the laser device was continuously operated quasi cw at 8 MHz. The excitation light intensity was 1.5 times higher than the laser motion threshold.
Figure 5. (a) Chemical structures of liquid carbazole (ECHz) hosts and heptafluorene derivatives. (b) Schematic diagram of a liquid-dispersive feedback laser prepared using a first-order 1D grating pattern. (c) SEM image of the PUA pattern with a period of 140 nm and a height of 100 nm. Scale bars represent 200 nm. (d) Emission spectra of nitrated and oxygenated liquid dispersive feedback lasers measured above critical at a repetition rate of 1 MHz. (e) Photograph of an oxygenated blue liquid dispersive feedback laser operating in the quasi-continuous wave region.
Figure 6. Streak camera images of (a, b) laser action emission and (c, d) time change of laser intensity in a nitrogenated and oxygenated EHCz:heptafluorene dispersive feedback laser. The repetition rate was increased from 0.01 MHz to 4 MHz. The excitation light intensity was kept constant at 2.5 μJcm ⁻² .
Figure 7. Output emission intensity and full width at half maximum as a function of excitation intensity at different repetition rates in a nitrogenated EHCz:heptafluorene dispersive feedback laser.
Figure 8. Output emission intensity and full width at half maximum as a function of excitation intensity at different repetition rates in an oxygenated EHCz:heptafluorene dispersive feedback laser.
Fig. 9. (a) Laser operation thresholds of nitrogenated and oxygenated liquid dispersive feedback organic semiconductor lasers as a function of repetition rate. (b) Photostability of oxygenated and nitrogenated liquid organic semiconductor dispersive feedback lasers operating in the quasi-continuous wave region at a repetition rate of 1 MHz. The excitation light intensity was 2.5 μJcm ^-2 .
Figure 10. (a) Triplet-triplet absorption spectra of heptafluorene in oxygenated and nitrogenated chloroform solutions. The triplet-triplet absorption disappears in the oxygenated solution due to quenching of the triplet by molecular oxygen. Representative laser spectra measured in an oxygenated dispersive feedback device are also shown. (b) Temporal attenuation of triplet-triplet absorption contributions with time constants greater than or equal to 1 ms.
Figure 11. Kinetics of photoluminescence intensity measured at 0.5 kWcm ^-2 in nitrogenated and oxygenated EHCz:heptafluorene blends for four different pulse durations from 50 to 800 μs. For this experiment, a gain medium was placed between two identical planar fused silica substrates. The samples were then optically pumped by a laser pulse resulting from a continuous wave HeCd laser emitting at 325 nm. The spot area was 2.0 x 10 ^-5 cm ² . The sample was irradiated in a direction perpendicular to the substrate plane and the PL intensity was measured at a 45 degree angle using a photomultiplier tube. These results clearly demonstrate that oxygenation leads to inhibition of STA loss due to quenching of triplets by molecular oxygen.
12. A method of manufacturing an organic semiconductor distributed feedback laser. A schematic diagram of the method used to fabricate an organic dispersive feedback laser. The different successive steps include fabrication of a distributed feedback resonator structure by electron beam lithography, thermal evaporation of organic semiconductor thin films, and spin coating of CYTOP polymer films followed by sealing of the device with high thermal conductivity (TC) sapphire leads.
Figure 13. Structure of a mixed-order distributed feedback resonator, (A) Schematic diagram of the mixed-order distributed feedback grating structure used in this study. (B) SEM images at 2500X and (C) 100000X magnifications, and (D) SEM images and (E) cross-sectional SEM images of the device after deposition of a 200 nm thick BSBCz:CBP blend film.
Fig. 14. Laser operation characteristics of an organic dispersive feedback laser in the quasi-continuous wave region. Streak camera images showing laser oscillations at repetition rates from 0.01 to 80 MHz for periods of (A) 500 μs or (B) 200 ns (80 MHz only) of a representative BSBCz:CBP encapsulated mixed-order distributed feedback device. The excitation intensity was fixed at ~0.5 μJcm ⁻² , which is higher than the laser action threshold (E _th ). (C) Temporal change of laser power intensity at various repetition rates (f) in an encapsulated BSBCz:CBP mixed-order dispersive feedback laser. (D) Threshold of laser operation in various distributed feedback devices according to repetition rate. The solid line is the reference line for the eye.
Fig. 15. Laser operation characteristics of organic dispersive feedback laser in the long pulse region. (A) Encapsulation mixed orders using BSBCz:CBP (20:80 wt%) membrane as gain medium and optically pumped with pulses of 30 ms and 2.0 kWcm ^-2 (top) or 800 μs and 200 Wcm ^-2 (bottom) Streak camera image showing integrating laser emission into more than 100 pulses from a distributed feedback device. (B) Photograph of the distributed feedback device operating in the long pulse region (30 ms excitation). (C) Laser operation threshold (E _th ) in various distributed feedback devices as a function of excitation duration. The dotted line is the reference line for the eye. (D) Changes in laser output intensity from organic dispersive feedback lasers with the number of incident pulses (100 μs, 200 Wcm ^{− 2} ).
Fig. 16. Schematic of the geometry used for optical simulation.
Figure 17. Laser operation threshold of a mixed-order dispersive feedback laser based on a BSBCz:CBP (6:94 wt%) 200 nm thick film for secondary gratings of different dimensions. The device was optically pumped by a nitrogen laser delivering 800 ps pulses at a repetition rate of 20 Hz and a wavelength of 337 nm.
18. (A) Absorption spectra, (B) steady-state photoluminescence spectra, and (C) transient photoluminescence corrosion by CYTOP membranes of encapsulated and unencapsulated BSBCz:CBP (6:94 wt%) membranes.
Figure 19. Emission spectra at various pumping intensities below and above the laser operating threshold of representative organic mixed-order dispersive feedback lasers based on (A) BSBCz:CBP (6:94 wt%) and (B) BSBCz neat films. The inset shows the emission spectrum near the laser operating threshold. Emission output intensity and FWHM as a function of pumping intensity in (C) blend film and (D) neat film distributed feedback laser. In this experiment, the organic film was covered with spin-coated CYTOP film and sapphire lead. The light pumping source was a nitrogen laser emitting 0.8 ns wide pulsed excitation at a repetition rate of 20 Hz and a wavelength of 337 nm. Emissions from the laser device were collected in a direction perpendicular to the substrate plane.
Figure 20. Emission output intensity and FWHM as a function of pumping intensity of representative encapsulated blend mixed-order dispersive feedback lasers at repetition rates of (A) 0.01 MHz and (B) 80 MHz.
21. Thresholds of laser motion measured in the forward (increasing repetition rate) and in the reverse direction (decreasing repetition rate) as a function of repetition rate. The irreversible change in the laser operating threshold is due to degradation of the gain medium under strong optical excitation.
22. (A) streak camera image showing emission from an unencapsulated secondary, unencapsulated mixed-order, and encapsulated mixed-order distributed feedback device at a repetition rate of 80 MHz and a pumping intensity of 0.5 μJcm ^-2 and (B) Corresponding emission spectra.
Figure 23. (A) Operational stability of laser output oscillations from different organic dispersive feedback lasers based on a BSBCz:CBP (6:94 wt%) blend film. To measure the stability, the laser device was operated continuously quasi-continuous wave at 8 MHz or 80 MHz for 20 min. For each device, the excitation light intensity was 1.5 times the laser operating threshold. (B) Photograph of an encapsulated blended dispersive feedback laser operating in the quasi-continuous wave region at 80 MHz.
Figure 24. (A) Output intensity of spontaneously amplified emission (ASE) centered at 472 nm as a function of the pump energy and stripe length of a 200 nm thick BSBCz:CBP (20:80 wt%) film. The dotted line is the fit for the data using the formula reported in refs (7, 8). The net gain of the waveguide was determined from the fit. (B) ASE intensity emitted from the edge of the waveguide membrane versus the distance between the pump stripe and edge of the blend membrane. The ASE properties of the membranes were investigated using inorganic laser diodes emitting at 405 nm with different (0.1, 10 and 50 μs) pulse widths.
Figure 25. Emission spectra of an encapsulated 20 wt% blend mixed-order dispersive feedback laser, measured with pumping intensities of 200 Wcm ^-2 and 2.0 kWcm ^-2 , respectively, for long pulse durations of 800 μs and 30 ms.
Figure 26. Streak camera image showing integrated laser emission for 100 pulses from an encapsulated mixed-order distributed feedback device during long optical excitation of 30 ms with a pump power of 2.0 kWcm ^-2 . The gain medium was BSBCz:CBP (20:80 wt%). The excitation wavelength was 405 nm. To observe laser motion for 30 ms with this streak camera system, 30 ms pulses were visualized in 1 ms frames (0.02 X 30 ms = 0.6 ms) by changing the duty cycle percentage to 2%.
Figure 27. (A) Temporal change in laser operating output intensity, (B) Emission spectra of an encapsulated mixed-order dispersive feedback laser measured after 10 and 500 average pulses of 30 ms width and 2.0 kWcm ^-2 pumping intensity. (C) Stimulated emission and triplet absorption cross-sectional spectra of BSBCz. The emission spectra of the distributed feedback laser were measured from the BSBCz neat film on E _th . (D) Triplet absorption spectra were measured in solutions containing BSBCz under argon. (E) Excitation force dependence of transient absorption spectra in solution (BSBCz in dichloromethane and benzophenone in benzene).
Figure 28. (A) A diagram showing the divergence of the emitted beam of a distributed feedback laser at (B) near the threshold and (C) above the threshold. The active gain medium was a BSBCz:CBP (20:80 wt%) membrane. The device was pumped in a long pulse region (10 ms excitation).
Figure 29. (A) Emission spectrum as a function of polarization angle and (B) Emission intensity as a function of polarization angle. The apparatus was based on a BSBCz:CBP (20:80 wt%) blend and used mixed-order distributed feedback grating. The pumping intensity was 200 Wcm ^-2 and the pump pulse duration was 800 μs. Note that 0o corresponds to the direction parallel to the groove of the distributed feedback grating.
Figure 30. Emission spectra, laser power intensity and FWHM of encapsulated mixed-order blend devices as a function of pumping intensity for long pulse excitation widths of (A, B) 1 μs and (C, D) 800 μs.
Figure 31. Streak camera image showing laser emission from (A) sapphire or (B) mixed-order blend BSBCz:CBP (20:80 wt%) dispersive feedback device encapsulated with (B) glass lid. (C) Emission spectra of devices encapsulated in glass leads for various pulse widths with a pumping intensity of 2.0 kWcm ⁻² . The emission spectrum broadens as the pulse width increases, which can be explained by a significant increase in the threshold of laser operation with this pulse width. For example, for pulse widths of 2 and 3 ms, the device operates below the laser operating threshold. (d) Laser motion threshold (E _th ) measured as a function of excitation pulse width in a distributed feedback device encapsulated in sapphire or glass leads. The dotted line is the reference line for the eye.
Figure 32. Laser microscopy images of an unencapsulated blend mixed-order dispersive feedback laser before (A) irradiation and (B) after irradiation with 100 excitation pulses of 1 ms in width. The thickness profile of the inset in (B) shows that the organic thin film is removed during high-density continuous wave irradiation. The pumping intensity was 200 Wcm ^-2 . Streak camera images in (C) and (D) show release from encapsulated and non-encapsulated blends under the same irradiation conditions (1 ms pulse width, 200 Wcm ⁻² pumping intensity, and integration of 100 or more streak camera images). shows
Fig. 33. Structure of a distributed feedback organic laser.
Fig. 34. (a) Structure of a three-layer slab waveguide. (b) Effective refractive indices (n _eff ) for TE and TM modes as a function of film thickness d at laser operating wavelength 477 nm.
Figure 35. Numerically calculated (solid line) and Fano-profile fit (dashed line) reflection spectra under normal incidence TE polarization for different values of film thickness.
Fig. 36. (a) Experimental laser emission spectra of fabricated laser devices with different d _f . (b) Experimental laser operating wavelength and calculated resonant wavelength.
Figure 37. (a) E _th (square), Q-factor extracted from numerical computation, QFEM-calculation (triangle), Fano fit, _QFano fit (star), Γ ( Plot in circle), (b) calculated from experimental FWHM of laser emission and Fano resonance.
Fig. 38. Schematic of the geometry used for thermal simulation.
Fig. 39. Maximum temperature at the end of each pulse.
Figure 40. Temperature rise as a function of time with different pulse widths of (A) 1 ms, (B) 10 ms, (C) 30 ms and (D) 40 ms in the gain domain.
Figure 41. Temperature rise with time for a pulse width of 10 ms in encapsulated and non-encapsulated devices.
Fig. 42. Temperature rise as a function of time or the number of pulses τ _p of 30 ms in the gain domain.
Figure 43. Schematic diagram of the distributed feedback grating of an electrically driven organic semiconductor distributed feedback laser diode and the method used to fabricate the organic laser diode. The different successive steps include sputtering a 100 nm thick SiO ₂ layer on top of the patterned ITO electrode, fabricating a distributed feedback resonator structure in SiO ₂ by electron beam lithography and dry etching, and organic semiconductor thin film and top thermal evaporation of the electrode.
44. Schematic of the substrate after various stages of distributed feedback fabrication on ITO. (A) Patterned ITO after (B) sputtering of SiO ₂ on ITO, (C) after dispersive feedback on ITO, and (D) after dispersive feedback structure is prepared.
45. Schematic diagram of a method for fabricating a nanoimprint distributed feedback grating. The different successive steps include preparing a 70 nm thick polymer layer on top of a patterned ITO electrode, followed by fabricating a dispersive feedback resonator structure in polymer at low cost by a simple nanoimprint lithography process.
Fig. 46. Structural characteristics of a mixed-order distributed feedback resonator used in an organic laser diode.
(A) Laser microscopy and (B) SEM images (5000X and 200000X magnification (in inset)) of a mixed-order distributed feedback SiO ₂ grating structure prepared on top of an ITO patterned glass substrate. (C, D) EDX and SEM analysis of mixed-order distributed feedback gratings prepared on top of ITO. These images confirm the exposure of the ITO to contact with the device.
Fig. 47. Structure and characteristics of an electrically driven organic semiconductor distributed feedback laser.
(A) Schematic and energy level diagram of an organic semiconductor laser diode. (B) Micrographs of organic dispersive feedback laser diodes and (C) DC-operated and non-DC-operated reference devices (OLEDs without gratings) at 4 V. The device area is 140 x 200 μm. (D) current density-voltage (JV) curves and (E) external quantum efficiency-current density (EQE-J) curves measured in a reference OLED device and organic dispersive feedback laser diode under direct current and pulsed operation.
Figure 48. Energy level diagrams of (a) electron-only devices, (b) hole-only devices, and (c) bipolar devices.
Figure 49. (a) Reported (symbol) and fitted (solid line) mobility for a Pool-Frankel field dependent model (solid line) of holes (blue) and electrons (red) for BSBCz. (b) Experimental (symbol) and simulated (solid line) J(V) curves for hole-only devices (blue), electron-only devices (red) and bipolar devices (black).
Figure 50. SEM (A, B) surface morphology images and (C, D) cross-sectional images of the laser diode structure after deposition of all layers.
51. Schematic diagram of some possible configurations of an organic semiconductor laser diode. Dispersive feedback resonator structures (second and mixed order gratings) were fabricated on (A) SiO ₂ by electron beam lithography and dry etching, (B) in ITO by electron beam lithography and dry etching, and (C) nanoimprint lithography process. It could be fabricated from the polymer on top of the patterned ITO electrode, or (D) on top of the active layer by nanoimprint lithography.
Figure 52. Schematic diagram of an organic semiconductor laser diode with a two-dimensional distributed feedback resonator structure (second and mixed-order gratings) for a 2D-dispersive feedback laser.
Fig. 53. (AD) Micrographs of organic distributed feedback laser diodes with and without DC operating at 4 V (devices A and B were prepared with a distributed feedback structure with 36 second cycles surrounded by 324 first cycles and The devices in D and C were prepared with a repeating structure of 4 secondary cycles surrounded by 12 primary cycles). The device area is 30 x 101 μm. (E) Electrically driven organic semiconductor distributed feedback lasers collected perpendicular to the substrate plane for different injected current densities (the grating periods of this device for primary and secondary were 140 nm and 280 nm, respectively) The emission spectrum of and (F) the output intensity as a function of current density.
Figure 54. Electroluminescence and PL spectra of BSBCz (black-PL spectra, red EL spectra of the reference device, and blue EL spectra with grating below the laser operating threshold).
Fig. 55. Laser operating characteristics of an electrically driven organic semiconductor distributed feedback laser diode.
Emission spectra of an electrically driven organic semiconductor distributed feedback laser (the grating periods of this device for the 1st and 2nd order were 140 nm and 280 nm, respectively) collected perpendicular to the substrate plane for different injected current densities. and (B) its output intensity as a function of current density. (C) Emission spectra and (D) output intensity versus current density obtained from organic dispersive feedback laser diodes using primary and secondary grating periods of 150 nm and 300 nm, respectively.
Fig. 56. Laser operating characteristics of an electrically driven organic semiconductor distributed feedback laser diode.
(A) Emission of electrically driven organic semiconductor distributed feedback lasers (the grating periods of the primary and secondary devices were 140 nm and 280 nm, respectively) collected perpendicular to the substrate plane for different injected current densities. spectrum, and (B) its power density as a function of current density.
Figure 57. (A) Current density-voltage (JV) curves with and without distributed feedback. Devices with distributed feedback have grating periods for the first and second orders of 150 nm and 300 nm, respectively. (B) External quantum efficiency versus current density in an electrically driven organic dispersive feedback solid-state laser without dispersive feedback under a 500 ns pulse.
Fig. 58. SEM images (A, B, C, D and E) of mixed-order gratings with different number of cycles in the first and second domains. (F) Table of mixed-order gratings with different number of cycles in the first and second domains for design distributed feedback.
Fig. 59. (A) Laser spectra of mixed-order gratings with different number of cycles. The number of cycles at the top of each graph corresponds to the number of secondary cycles, and the number of primary cycles can be confirmed in FIG. 58 . The bottom graph shows the output characteristics of the device. (B) Critical energy of mixed-order grating laser for different number of cycles in the secondary region. In the case of optical pumping, the use of 4 and 12 cycles in the 1st and 2nd regions, respectively, represents the lowest threshold. The optical pumping source was a nitrogen laser emitting 0.8 ns wide pulsed excitation at a repetition rate of 20 Hz and a wavelength of 337 nm. Emissions from the laser device were collected in a direction perpendicular to the substrate plane.
Fig. 60. (A) SEM images of mixed-order gratings with 12 and 4 cycles, respectively, in the first and second regions. (B, C) Organic distributed feedback laser diodes with and without DC operating at 5 V. The device area is 2.9 x 10 μm. (D) Emission spectrum for a reference OLED.
Figure 61. (A) Schematic diagram of the experimental apparatus for examining the polarization of the laser. (B) below the laser operating threshold (at 415 Acm ^-2 ), (C) above the laser operating threshold (at 823 Acm ^-2 ), and reference (at 800 Acm ^-2 , no distributed feedback) of a distributed feedback laser diode , and (E) polarization dependence of the EL spectrum on the EL intensity as a function of polarization angle. The EL is polarized in the plane of the device (TE mode).
Figure 62. (A) Schematic diagram of OLED structure with distributed feedback. (B) Microscopy image of dispersive feedback on an ITO substrate with a device area of 560 x 800 μm. (C) optical pump output intensity of a dispersive feedback laser as a function of excitation density; Excitation with a continuous wave laser at 405 nm.
Figure 63. (A, B) Micrographs of organic dispersive feedback laser diodes with and without direct current operating at 4 V and (C, D) reference device (OLED without grating).
64. A laser microscope image of an annular mixed-order distributed feedback grating structure prepared using SiO ₂ on an ITO patterned substrate.
Figure 65. Microscopy images of an organic annular dispersive feedback laser with and without driving. Current density-voltage (JV) curves for devices with and without annular distributed feedback. External quantum efficiency versus current density in OLEDs with and without distributed feedback.
Figure 66. (a) Schematic of a distributed feedback grating OLED and (b) experimental (symbol) and simulated (solid line) J(V) curves for a distributed feedback grating and reference OLED.
Figure 67. Spatial distribution of charge carrier density, n, p and electric field F at (a) 10 V and (b) 70 V.
Figure 68. Diagram of spatial distribution of hole density at (a) 70 V, p, (b) electron density, n, (c) n, p incision in 2D cross section at y of 0.11 μm.
Figure 69. Plot of the profile of (a) electric field F, and (b) current density J at 70 V.
Figure 70. Profile recombination rate R for the distributed feedback device at 0.10 μm y of the distributed feedback device, (b) R of the reference device, (c) R of the cut line of the 2D section, and (d) Figures of incisions n, p of the 2D section at 0.164 μm y of the distributed feedback device at 70 V.
Figure 71. (a) S(J) characteristics with and without EFQ and EFQ for a reference device with different E _b and (b) distributed feedback and E _b of 0.6 eV of (a) reference device.
Figure 72. Excitons at 70 V (a) inside a reference device with and without EFQ, (b) inside a distributed feedback device without EFQ, (b) inside a distributed feedback device with EFQ Distribution.
Figure 73. Exciton density distribution inside a distributed feedback device without quenching (top left), with EFQ (bottom left), with PQ (top right), with PQ and EFQ (bottom right).
Figure 74. Air/BSBCz/SiO ₂ inside a distributed feedback device (top left), optical density distribution of air/BSBCz/SiO ₂ /ITO (top right) and real device Al/Ag/MoO ₃ /BSBCz/SiO ₂ /ITO (bottom).
Figure 75. (a) Chemical structure of an octafluorene derivative. (b) Absorption and steady-state PL spectra measured at room temperature on spin-coated octafluorene neat films. The excitation wavelength for the measurement of the PL spectrum was 376 nm. A photograph of the octafluorene knit membrane under ultraviolet illumination is shown in the inset. (c) Typical and atypical optical constants (k and n) of octafluorene neat films measured by variable incidence angle spectroscopic ellipticity. The film thickness was about 75 nm.
Figure 76. (a) absorption and (b) steady-state PL spectra of blend films containing 10 and 20 wt% of octafluorene in CBP host. The excitation wavelength used for the emission spectra was 424 nm for both films.
77. PL attenuation measured on octafluorene neat membranes and on blend membranes containing 10 and 20 wt% octafluorene in the CBP host. The excitation wavelength was 365 nm.
Fig. 78. Experimental and simulated elliptically polarized reflection data ψ and Δ measured at various angles of incidence on spin-coated octafluorene knit films.
Figure 79. (a) Schematic of the experimental setup used to characterize the ASE properties of organic thin films. (b) Emission spectra of a 260 nm thick octafluorene neat film collected from the edges of the organic layer for different excitation intensities below and above the ASE threshold. The steady-state PL spectrum is indicated by the dashed line. A photograph of the octafluorene knit film under strong light irradiation is shown in the inset. (c) Output intensity from the edge of a 260 nm thick film (integrated for all wavelengths) as a function of excitation density. (d) ASE threshold according to octafluorene knit film thickness. The excitation wavelength was 337 nm.
Figure 80. Output intensity from the edge of the organic layer (integrated for all wavelengths) as a function of excitation density of different octafluorene neat films with varying thicknesses in the range of 53 to 540 nm. These data were used to examine the thickness dependence of the ASE threshold shown in Fig. 2d.
81. ASE intensity plotted against the distance between the pump stripe and the edge of a 260 nm thick octafluorene knit membrane. The solid line corresponds to the fit obtained from a single exponential decay function to determine the loss factor.
82. Temporal decay of emission intensity above the ASE threshold in a 260 nm thick octafluorene neat film placed in an air or nitrogen base layer. The pumping intensity was 873 μJ/cm ² and 10 Hz.
Figure 83. (a) Schematic of the experimental setup used to characterize the properties of an octafluorene dispersive feedback laser. (b) SEM image of the mixed-order distributed feedback grating used in this operation. (c) Emission spectra of a distributed feedback laser based on an octafluorene neat film collected perpendicular to the substrate plane for different excitation intensities below and above the laser operating threshold. (d) The output intensity of the distributed feedback laser as a function of excitation density.
Figure 84. (a) Schematic diagram of the OLED structure used in this study. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for organic materials used in these devices are also provided. (b) External quantum efficiency versus current density in OLEDs based on octafluorene neat films and CBP blend films.
Figure 85. Determination of the ionization potential of octafluorene in neat membranes using photoelectron spectroscopy in air. Considering the optical bandgap value determined from the absorption spectrum of the neat film, the electron affinity of octafluorene in the knit film was approximated. It is important to note, however, that Koopmans' theorem that the perpendicular ionization potential is equal to the calculated absolute value of the HOMO energy is often not satisfied because of the relaxation process during the ionization process and electron correlation. The optical bandgap is usually different from the actual electron gap, but the ion affinity and LUMO can be approximated from the difference between the ionization potential and the optical bandgap value.
Figure 86. (a) Electroluminescence (EL) spectrum measured at a current density of 10 mA/cm ² and (b) JVL curve in OLED based on octafluorene neat film and CBP blend film.
Fig. 87. Continuous wave ASE in an octafluorene neat film.
Fig. 88. Continuous wave ASE in an octafluorene neat film.

[1] Continuous Wave Organic Thin Film Dispersive Feedback Laser

Since the discovery of organic solid-state lasers ^[1-6] , much effort has been devoted to developing continuous-wave laser operation in organic materials, including small molecules, oligomers and polymers. ^{[ 7-10]} However, operating an organic solid-state laser under optical continuous-wave excitation or pulsed excitation at very high repetition rates (quasi-continuous-wave excitation) is a great challenge. When organic films are optically pumped under these conditions, accumulation of long-period triplet excitons and charge carriers typically occurs, ^{[ 11-14]} increasing absorption losses by triplet exciton formation and single excitons by triplet excitons. (ie, single-triplet annihilation). ^[11-16] These absorption losses and emission quenching are important problems to be solved in achieving continuous-wave and quasi-continuous-wave operation because they dramatically increase the laser operation threshold and, in the worst case, completely stop the laser operation. ^{[ 17-19]} Mixing of triplet catalysts such as oxygen in organic films, ^{[ 15,16]} cyclooctatetraenes, ^{[ 20]} or anthracene derivatives ^[19] to suppress absorption loss and emission quenching This has been suggested. However, as suggested by Schols et al. ^[20] , what is required for triplet dislocation is low triplet energy, short triplet lifetime, and large differences between singlet and triplet state energies, and the laser It makes it difficult to find a suitable triplet anchor that satisfies these conditions without interfering with operation. Rabe et al. disclose a poly containing 12% (BN-PFO) of 6,6′-(2,2′-octyloxy-1,1′-binaphthalene)binaphthyl spacer groups without triplet A quasi-continuous wave operation with a repetition rate of 5 MHz was demonstrated in the (9,9-dioctylfluorene) derivative. ^[9] This high repetition rate can be obtained because of the small spectral overlap between emission and triplet absorption in BN-PFO. ^{[ 10]} Therefore, it is important to develop organic laser dyes with little spectral overlap between excited state absorption and emission to realize continuous wave and quasi-continuous wave laser operation with low threshold.

In this group, the optical and spontaneous amplified emission (ASE) properties of many organic materials have been continuously investigated to realize electrically pumped organic laser diodes. ^{[ 21-27]} Among them, 4,4'-bis[(N-carbazole)styryl]biphenyl (BSBCz) is one of the most promising candidates, 6 wt% BSBCz and Vacuum deposited films of the blended host material 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) exhibited high photoluminescence quantum yield (ΦPL) of nearly 100% and approximately 1.0 ns This is because it has excellent optical and ASE properties such as a short ^PL lifetime ( ^{τPL )} of ^{[ 23,26]} In this paper, we report the operation of a quasi-continuous wave front-emitting laser in a distributed feedback device based on such a BSBCz:CBP blend film. In this laser device, the highest repetition rate (up to 8 MHz) and the lowest threshold (on the order of 0.25 μJ cm ^-2 ) reported so far for quasi-continuous wave lasers based on organic thin film systems were obtained. Incorporation of triplet junctions is not essential in this blend film, as ΦPL is high and there is no significant spectral overlap between emission and triplet absorption of BSBCz. ^{[ 24]}

In a DFB structure, laser oscillation occurs when the following Bragg condition is met: mλ _Bragg = 2n _eff Λ, where m is the diffraction order, λ _Bragg is the Bragg wavelength, n _eff is the effective refractive index of the gain medium, and Λ is the grating is the cycle of ^{[ 28,29]} Considering the second-order mode (m = 2), the grating period is calculated to be Λ of 280 nm using n _eff and λ _Bragg reported for BSBC _Z. ^{[ 21,22]} A grating with a Λ of 280 nm provides a surface emitting laser motion in the direction perpendicular to the substrate plane as shown in Fig. 1b. Although secondary gratings generally lead to higher laser operation thresholds compared to primary gratings, surface emitting laser operation using secondary gratings can be used to produce electrically pumped organic laser diodes with organic light emitting diode structures exhibiting the same surface emission. suitable for ^{[ 30,31]} Using electron beam lithography and reactive ion etching, gratings were directly etched into the silicon dioxide surface for an area of 5 X 5 mm ² (Fig. 1c). 1D and 1E show SEM images of representative gratings prepared in this study. From the SEM image, a Λ of 280 2 nm and a grating depth d of 70±5 nm were obtained, which perfectly match this specification. To fabricate a laser device, a 6 wt% BSBCz:CBP blend film or BSBCz neat film with a thickness of 200 nm was prepared on the grating by vacuum deposition.

First, the operating characteristics of the surface emitting laser of the DFB system from a nitrogen gas laser under pulse excitation of 0.8 ns width at 20 Hz were investigated. This excitation light having a wavelength of 337 nm is mainly absorbed by CBP in the blend film. However, the large spectral overlap between CBP emission and BSBCz absorption ensures efficient Forster-type energy transfer between the two molecules (Fig. 1f). ^{[ 26]} Therefore, no emission from CBP was observed even under high excitation. Figures 2a and 2b show emission spectra measured from laser devices, (a) BSBCz:CBP and (b) neat BSBCz films at different excitation intensities. For a particular excitation light intensity, both devices showed laser-driven emission with very narrow peaks. It was confirmed that there was no surface emission laser operation from the area without grating on the same substrate. It was found that τPL and full width at full width at half maximum (FWHM) are greatly reduced due to stimulated emission ^[32–35] under high excitation energy for E _th in the present laser device (Figs. It indicates that light is outcoupled from the film. In emission spectra measured at low excitation intensities (insets in FIGS. 2A and 2B ), Bragg dips were observed at about 478 nm for the blend film and 474 nm for the neat film. The Bragg dip is due to suppression of propagation of waveguide light by the grating and can be expected as a photonics stopband for waveguide mode. ^{[ 36]} The laser action occurs at the short-wavelength edge of the Bragg dip (477 nm for the blend film and 473 nm for the neat film). The difference in the Bragg dip positions is probably because the refractive indices of the blend film and the knit film are different from each other. The emission intensity increases linearly with increasing excitation intensity and then begins to be amplified for laser operation as the FWHM decreases to less than 0.30 nm for the blend film and less than 0.40 nm for the neat film (Fig. 2c and see 2d). The laser motion threshold energies (E _th ) measured from the intersection of two straight lines fitted to the emission intensity were 0.22 _μJcm ^-2 for the blend film and 0.66 _µJcm ^-2 for the neat film, and 275 and 825 Wcm ^-2 for the neat film. It corresponds to the power density of These values are lower than the ASE critical power densities of 375 and 1625 Wcm ^-2 without grating because of the good quality of the grating. ^{[ 23,26] The} obtained E _th values are the lowest values reported for all quasi-continuous wave organic thin-film lasers. The lower E _th in the blend film than in the neat film is because ?PL is higher for the blend film (98%) than the neat film (76%) because concentration quenching is suppressed. ^{[ 36]} In general, E _th and laser gain are inversely proportional to ΦPL. ^[37,38]

The device was operated in quasi-continuous wave mode using light pulses with a wavelength of 365 nm and a width of 10 ps from a Ti sapphire laser. Fig. 3 shows a streak camera image of the laser oscillation and the time change of the corresponding laser intensity in a BSBCz:CBP blend film at the laser operating wavelength. The excitation light intensity was fixed at about 0.44 μJcm ⁻² , which was about two times higher than that of E _th . At a repetition rate of 0.01 MHz, laser motion oscillations were observed at intervals of 100 μs. At higher repetition rates, the time interval between laser oscillations was shortened. Adjacent laser oscillations appear continuously at 8 MHz for a wide time range of 500 μs (Figs. 3a and 3b); However, individual laser oscillations at intervals of 125 ns could still be discerned for a short time span of 2 μs even at 8 MHz (Fig. 3c). It was confirmed that a similar quasi-continuous wave operation is possible with the BSBCz neat membrane.

The emission intensity of the two laser devices having a blend film and a neat film was almost constant at a maximum of 8 MHz as shown in FIG. 3 . This maximum repetition rate is the highest reported so far and is believed to be due to the small absorption loss and emission quenching due to triplet exciton formation. The ΦPL of BSBCz is very high, which minimizes the occurrence of triplet excitons through intersystem crossing, especially for blend films. In addition, the spectral overlap between emission and triplet absorption is negligible, reducing the likelihood of collisions between singlet and triplet excitons. When the laser device was operated at 80 MHz (the highest frequency possible in the device), the emission intensity decreased sharply, and it was impossible to estimate an unambiguous laser operating threshold due to the rapid material degradation. Also, the FWHM of the emission peak observed at 80 MHz is about twice that of the emission peak at lower frequencies. At this stage, it is not clear whether this is a laser operation.

Figure 4a shows a plot of the laser operation threshold as a function of repetition rate for a blend film and a neat film. Interestingly, the laser motion threshold was almost independent of the repetition rate of the blend film due to negligible absorption loss and emission quenching. However, in the case of the knit membrane, it was observed that the criticality gradually increased as the repetition rate increased. The exact reason for the gradual increase in threshold is not known, and therefore further studies are needed to clarify this observation.

The operational stability of the laser operating oscillations was investigated when the device was continuously operated at 8 MHz (Fig. 4b). The emission intensity gradually decreased with time. The change was irreversible and indicates optical degradation of the material. The lifetime until the emission intensity was reduced to 90% of the initial time was 900 seconds, longer than 480 seconds for the neat film for the blend film. Stronger excitation light was required to obtain laser action in the neat film than in the blend film due to the higher threshold. Therefore, the optical degradation can be expected to be faster in the neat film. Lowering the threshold is important to suppress optical degradation.

In summary, a DFB laser device combining a BSBCz:CBP blend film as a gain medium and a secondary grating was fabricated and evaluated. Excellent surface-emitting laser behavior was obtained from the device under quasi-continuous wave operation, and the emission intensity and laser operation threshold were independent of the repetition rate. For this laser device, the maximum repetition rate is 8 MHz, which is the highest reported so far, and the laser operation threshold is about 0.25 μJcm ^-2 , which is the lowest reported so far. Because there is little accumulation of triplet excitons and the spectral overlap between emission and triplet absorption is small, the triplet antenna commonly used in the fabrication of organic thin-film lasers was not required in this device. Therefore, BSBCz is considered as the most promising candidate for the first realization of an electrically pumped organic laser diode in terms of optical properties. However, electrical properties such as charge carrier mobility and charge carrier trapping cross-section are also very important, and further investigation and improvement are needed for the realization of electrically pumped organic lasers.

Experiment section

A silicon substrate covered with a 1 μm-thick thermally grown silicon dioxide layer was ultrasonically cleaned using neutral detergent, pure water, acetone and isopropanol, followed by UV-ozone treatment. The silicon dioxide surface was treated by spin coating with hexamethyldisilazane (HMDS) at 4000 rpm for 15 seconds. A resist layer with a thickness of about 70 nm was spin-coated from a ZEP520A-7 solution (ZEON Co.) at 4000 rpm for 30 sec on the substrate and baked at 180 °C for 240 sec. Electron beam lithography was performed to derive a grating pattern on the resist layer using a JBX-5500SC system (JEOL) with an optimized dose of 0.1 nCcm ^-2 . After electron beam irradiation, the pattern was developed in a developer (ZED-N50, Zeon Co.) at room temperature. The patterned resist layer was used as an etch mask and the substrate was plasma etched with CHF ₃ using an EIS-200ERT etch system (ELIONIX). To completely remove the resist layer from the substrate, the substrate was plasma etched with O ₂ using a FA-1EA etching system (SAMCO). The grating formed on the silicon dioxide surface was observed with a scanning electron microscope (SU8000, Hitachi). To complete the laser device, a 200 nm thick 6 wt% BSBCz:CBP blend film and BSBCz neat film were prepared on the grating by thermal evaporation with a total evaporation rate of 0.1 to 0.2 nms ^-1 under a pressure of 4.0 x 10 ^-4 Pa. did.

For laser actuation, pulsed excitation light from a nitrogen gas laser (USHO, KEN-2020) was focused through a lens and a slit into a device with an area of 6 X 10 ^-3 cm ² . The excitation wavelength was 337 nm, the pulse width was 0.8 ns, and the repetition rate was 20 Hz. The excitation light was incident on the device at about 20 degrees to the normal to the device plane. Light emitted perpendicular to the device surface was collected with an optical fiber connected to a multichannel spectrometer (PMA-50, Hamamatsu Photonics) located 3 cm away from the device. The excitation intensity was controlled using a series of neutral density filters. For quasi-continuous wave operation, excitation light with an excitation wavelength of 365 nm, a pulse width of 10 ps, and a repetition rate of 0.01 to 8 MHz was generated using a mode-fixed frequency dual Ti sapphire laser (Millennia Prime, Spectra physics). The excitation light was focused through a lens and a slit to a device with an area of 1.9 × 10 ⁻⁴ cm ² , and the emitted light was focused on a streak scope (C9300, Hamamatsu Photonics) connected to a digital camera (C9300, Hamamatsu Photonics) with a temporal resolution of 15 ps. C10627, Hamamatsu Photonics). The same irradiation and detection angles were used in this measurement as previously described. The size of the excitation area was carefully checked using a beam profiler (WimCamD-LCM, DataRay). All measurements were performed in a nitrogen atmosphere to prevent degradation due to moisture and oxygen.

A solution containing BSBCz in 0.15 mM CH ₂ Cl ₂ was prepared and bubbled with argon before use. Third harmonic laser light of wavelength 355 nm and FWHM 5 ns from Nd:YAG laser (Quanta-Ray GCR-130, Spectra-Physics) was used as pump light, and pulsed white light from Xe lamp was used as a streak camera (C7700, Hamamatsu Photonics) was used as a probe for triplet absorption measurements in solution phase.

, A. J. Heeger, Chem. Phys. Lett. 1996, 256, 424.[1] F. Hide, BJ Schwartz, MA

, AJ Heeger, Chem. Phys. Lett. 1996, 256, 424.

, B. J. Schwartz, M. Andersson, Q. Pei, A. J. Heeger, Science 1996, 273, 1833.[2] F. Hide, MA

, BJ Schwartz, M. Andersson, Q. Pei, AJ Heeger, Science 1996, 273, 1833.

[3] N. Tessler, G. J. Denton, R. H. Friend, Nature 1996, 382, 695.

[4] M. D. McGehee, A. J. Heeger, Adv. Mater. 2000, 12, 1655.

[5] N. Tessler, Adv. Mater. 1999, 11, 363.

[6] I. D. W. Samuel, G. A. Turnbull, Chem. Rev. 2007, 107, 1272.

[7] R. Bornemann, U. Lemmer, E. Thiel, Opt. Lett. 2006, 31, 1669.

, Opt. Express. 2011, 19, 26383.[8] R. Bornemann, E. Thiel, P.H.

, Opt. Express. 2011, 19, 26383.

[9] T. Rabe, K. Gerlach, T. Riedl, H.-H. Johannes, W. Kowalsky, J. Niederhofer, W. Gries, J. Wang, T. Weimann, P. Hinze, F. Galbrecht, U. Scherf, Appl. Phys. Lett. 2006, 89, 081115.

[10] M. Lehnhardt, T. Riedl, U. Scherf, T. Rabe, W. Kowalsky, Org. Electron. 2011, 12, 1346.

[11] N. C. Giebink, S. R. Forrest, Phys. Rev. B 2009, 79, 073302.

[12] M. A. Baldo, R. J. Holmes, S. R. Forrest, Phys. Rev. B 2002, 66, 035321.

[13] M. Lehnhardt, T. Riedl, T. Weimann, W. Kowalsky, Phys. Rev. B 2010, 81, 165206.

[14] M. A. Stevens, C. Silva, D. M. Russell, R. H. Friend, Phys. Rev. B 2001, 63, 165213.

[15] M. Inoue, T. Matsushima, H. Nakanotani, C. Adachi, Chem. Phys. Lett. 2015, 624, 43.

[16] L. Zhao, M. Inoue, K. Yoshida, A. S. D. Sandanayaka, J.-H. Kim, J.-C. Ribierre, C. Adachi, IEEE Quantum Electronics 2016, 22, 1, 1300209.

[17] P. P. Sorokin, J. R. Lankard, E. C. Hammond, V. L. Moruzzi, IBM J. Res. Dev. 1967, 11, 130.

[18] O. G. Peterson, S. A. Tuccio, B. B. Snavely, Appl. Phys. Lett. 1970, 17, 245.

[19] Y. Zhang, S. R. Forrest, Phys. Rev. B 2011, 84, 241301.

[20] S. Schols, A. Kadashchuk, P. Heremans, A. Helfer, U. Scherf, Chem. Phys. Chem. 2009, 10, 1071.

[21] D. Yokoyama, M. Moriwake, C. Adachi, J. Appl. Phys. 2008, 103, 123104.

[22] H. Yamamoto, T. Oyamada, H. Sasabe, C. Adachi, Appl. Phys. Lett. 2004, 84, 1401.

[23] M. Inoue, K. Goushi, K. Endo, H. Nomura, C. Adachi, J. Lumin. 2013, 143, 754.

[24] H. Nakanotani, C. Adachi, S. Watanabe, R. Katoh, Appl. Phys. Lett. 2007, 90, 231109.

[25] D. Yokoyama, H. Nakanotani, Y. Setoguchi, M. Moriwake, D. Ohnishi, M. Yahiro, C. Adachi, Jpn. J. Appl. Phys. 2007, 46, 826.

[26] T. Aimono, Y. Kawamura, K. Goushi, H. Yamamoto, H. Sasabe, C. Adachi, Appl. Phys. Lett. 2005, 86, 071110.

[27] H. Nakanotani, S. Akiyama, D. Ohnishi, M. Moriwake, M. Yahiro, T. Yoshihara, S. Tobita, C. Adachi, Adv. Funct. Mater. 2007, 17, 2328.

[28] I. D. W. Samuel, G. A. Turnbull, Chem. Rev. 2007, 107, 1272.

, S. Forget, Polym. Int. 2012, 61, 390.[29] S.

, S. Forget, Polym. Int. 2012, 61, 390.

[30] A. E. Vasdekis, G. A. Turnbull, I. D. W. Samuel, P. Andrew, W. L. Barnes, Appl. Phys. Lett. 2005, 86, 161102.

[31] S. Riechel, C. Kallinger, U. Lemmer, J. Feldmann, A. Gombert, V. Wittwer, Appl. Phys. Lett. 2000, 77, 2310.

[32] Y. Takahashi, H. Hagino, Y. Tanaka, B.-S. Song, T. Asano, S. Noda, Opt. Express. 2007, 15, 17206.

[33] M. Koschorreck, R. Gehlhaar, V. G. Lyssenko, M. Swoboda, M. Hoffmann, K. Leo Appl. Phys. Lett. 2005, 87, 181108.

[34] P. A. Morton, V. Mizrahi, S. G. Kosinski, L. F. Mollenauer, T. Tanbuu-Ek, R. A. Logan, D. L. Cobletz, A. M. Sergent, K. W. Wecht, Electron. Lett. 1992, 28, 561.

[35] P. A. Morton, V. Mizrahi, T. Tanbun-Ek, R. A. Logan, P. J. Lemaire, H. M. Presby, T. Erdogan, S. L. Woodward, J. E. Sipe, M. R. Phillips, A. M. Sergent, K. W. Wecht, Appl. Phys. Lett. 1994, 64, 2634.

[36] G. A. Turnbull, P. Andrew, M. J. Jory, W. L. Barnes, I. D. W. Samuel, Phys. Rev. B 2001, 64, 125122.

[37] B. Zhen, S.-L. Chua, J. Lee, A. W. Rodriguez, X. Liang, S. G. Johnson, J. D. Joannopoulos, M. Soljacic, O. Shapira, Proc. Natl. Acad. Sci. USA 2013, 110, 13711.

[38] J. C. Ribierre, G. Tsiminis, S. Richardson, G. A. Turnbull, H. S. Barcena, P. L. Burn, I. D. W. Samuel, Appl. Phys. Lett. 2007, 91, 081108.

[2] triplet as wife Improvement of Quasi-Continuous Wave Laser Operating Characteristics in Oxygen-Using Organic Semiconductor Lasers

We demonstrate quasi-continuous-wave laser operation in a solvent-free liquid organic semiconductor dispersive feedback laser based on a blend containing a liquid 9-(2-ethylhexyl)carbazole host doped with a blue emitting heptafluorene derivative. By bubbling a liquid gain medium with oxygen or nitrogen, we investigate the role of triplet emitters, such as molecular oxygen, on the quasi-continuous-wave laser operating characteristics of organic semiconductor lasers. The oxygenating laser device exhibits a low threshold of 2 μJcm ⁻² , lower than that measured in the nitridation device and is independent of the repetition rate in the range of 0.01 to 4 MHz.

Since the demonstration of the ^first optically pumped organic solid-state semiconductor lasers in 1996 ^{, 1,2} organic lasers have been the subject of intensive research mainly because of several attractive properties of organic semiconductor materials, such as broad absorption and emission spectra and high optical gain coefficients. ^{3 ,4} The performance of organic solid-state lasers has improved significantly over the past two decades, and emerging applications are currently emerging, including the development of integrated light sources for spectroscopy and vapor chemistry sensors. ⁵ -pulse inorganic light-emitting diodes can currently be used to optically pump organic solid-state lasers ⁶ , but further development is needed to demonstrate optically pumped organic semiconductor lasers operating in the continuous wave region and ultimately to realize electrically pumped organic laser diodes. A breakthrough is still needed.

It is well known that generation of long-lived triplet excitons through intercalation causes high photons and single losses, preventing laser operation in the continuous wave optical pumping region. ^{7 -12} To solve this important problem, it has been proposed to incorporate a triplet actuator into the organic semiconductor gain medium. Zhang et al. in 4-(dicynomethylene)-2-methyl-6-juloridyl-9-enyl-4H-pyran (DCM2) doped tris(8-hydroxyquinoline) aluminum (Alq ₃ ) Anthracene derivatives were used as triplet actuators of , and their dispersive feedback (DFB) organic devices were able to extend the laser duration to nearly 100 μs. ⁸ At the same time, several other studies have demonstrated that triplet losses in optically pumped organic semiconductor lasers can be reduced by using oxygen or cyclooctatetraene (COT) as the triplet actuator. Although the use of ^9-11 triplet antennas is very promising for the development of true continuous wave organic solid-state laser technology, it should be noted that other approaches have also been proposed to achieve this goal. Recently, 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) doped with 4,4'-bis[(N-carbazole)styryl]biphenyl (BSBCz) We demonstrated the operation of a quasi-continuous wave laser with a repetition rate of 8 MHz in a host-based organic DFB laser. ¹³ This achievement is explained not only by the photoluminescence quantum yield reaching 100% of the material, but also by the negligible overlap between the laser-operated emission and triplet absorption spectra of BSBCz, thus reducing the generation of triplets under optical pumping. very weak. Another method used to realize high-power continuous-wave organic solid-state dye lasers is based on very fast rotation of the device during operation, but the long-term output stability of these devices appears to be too limited for practical use. ¹⁴

In this study, we report the fabrication of an organic semiconductor DFB laser operating in the quasi-continuous wave region using a solvent-free liquid organic semiconductor material as the laser gain medium. ^16-23 This laser-operated material consists of 9-(2-ethylhexyl)carbazole (EHCz) host ¹⁷ doped with a heptafluorene derivative. ²⁴ The chemical structure of this molecule is shown in Figure 5a. The choice of this blend was followed by a photoluminescence quantum yield (PLQY) of 85% and a low spontaneous amplification emission (ASE) threshold of 0.4 μJcm ⁻² under pulsed optical pumping. ²² In this context, the effect of oxygenation on the quasi-continuous wave DFB laser operating properties of EHCz:heptafluorene blends is investigated. These results provide clear evidence that the use of triplet emitters such as molecular oxygen is very promising for the further realization of optically pumped continuous wave organic semiconductor lasers.

A liquid carbazole, EHCz, was purchased (Sigma-Aldrich), and a heptafluorene derivative was synthesized according to a previously known method in document ²⁵ while using without further purification. EHCz ¹⁷ , which is liquid at room temperature and whose glass transition temperature is much lower than 0° C., was mixed with heptafluorene in chloroform solution. The EHCz:heptafluorene (90:10 wt%) blend solution was then bubbled with oxygen or nitrogen for about 20 minutes. Gas was introduced into the solution at a pressure of about 0.02 MPa using a needle with an inner diameter of 0.7 mm. The blend was used as the gain medium in the laser device after the solvent was completely evaporated. The device structure of the liquid DFB laser is schematically shown in Fig. 5b. To fabricate these devices, ultraviolet (UV) curable polyurethane acrylate (PUA) mixtures were synthesized according to previously reported methods. A ²⁶ wavy polymer DFB pattern was readily fabricated on a polyethylene terephthalate (PET) substrate by replicating a grating master mold of silicone with a PUA mixture. ²⁷ The grating period Λ for the desired laser operating wavelength λ must satisfy the Bragg condition Λ = mλ/(2n _eff ), where m is the order and n _eff is the effective refractive index of the induced mode. To achieve low-threshold laser actuation, a first-order feedback corresponding to m = 1 was chosen, allowing the laser to be emitted from the device edge. It is worth noting that the refractive indices of the PUA film and the EHCz blend are about 1.54 and 1.7, respectively, meaning that the specific refractive index difference is 0.16. ²² As shown in Fig. 5c, the patterned wavy structure on the PUA layer consists of a 1D grating with a period of 140 nm and a height of 100 nm. This grating period was selected for first order DFB laser operation with an emission wavelength of about 450 nm based on the Bragg formula and the emission spectrum of the blue emitting heptafluorene derivative. Then, the wavy PUA layer was covered with a fused silica substrate, and the interstitial distance between the PUA replica and the cover was fixed using silica microspheres with a diameter of 1 μm. The voids were filled with liquid gain medium through capillary action. In order to investigate its quasi-continuous wave laser properties, a Ti sapphire laser system (Millennia Prime, Spectra Physics), which transmits optical pulses at 365 nm with a pulse width of 10 ps, was used with nitrogenated and oxygenated EHCz:heptafluorene DFB lasers. was optically pumped. The repetition rate of the photoexcitation was varied from 0.01 MHz to 4 MHz. The spot area of the laser pump beam focused on the device was 1.9 x 10 ^-4 cm ² . Emissions from the edge of the device were detected using a Hamamatsu Streak Scope (C10627) coupled with a Hamamatsu digital camera (C9300).

Heptafluorene derivatives have previously been used in solution-processed fluorescent organic light-emitting diodes (OLEDs) with high external quantum efficiencies of 5.3%. ²⁸ This excellent electroluminescent performance could be achieved due to the horizontal orientation of the heptafluorene emitter in the 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host. In another study, heptafluorene molecules were blended into an EHCz host to demonstrate a solvent-free liquid organic secondary DFB laser operating in the blue region of the visible spectrum. ²² For this, the device was optically pumped using a pulsed nitrogen laser (λ = 337 nm, pulse duration 800 ps and repetition rate 8 Hz) and the laser power emission was detected perpendicular to the surface. The same liquid composite material was used to fabricate an edge-emitting primary DFB laser. 5D and 5E, the blue laser emission detected from the edge of the nitrogenated and oxygenated liquid DFB lasers have peak wavelengths of 450 and 449 nm, respectively. The very small difference between the laser operating wavelengths of the two devices is probably due to the slight change in the thickness of the organic liquid layer. ²⁹

6A and 6B show streak camera images of laser emission for nitrogenated and oxygenated solvent-free liquid organic DFB lasers at various repetition rates. For these measurements, the excitation intensity was kept constant at a value of 2.5 μJcm ⁻² . The laser pulses emitted from the DFB laser can be clearly observed in the 100 μs timescale window, but the time interval between pulses gradually decreases as the repetition rate increases. For the highest repetition rates of 1 MHz and 4 MHz, the DFB laser output emission in Figs. 6c and 6d is considered to be continuous within this time range, providing evidence that both the nitridation and oxygenation devices operate adequately in the quasi-continuous wave region. . It is worth noting, however, that we have found that the output intensity of the oxygenator in the quasi-continuous wave region, especially at 4 MHz, is always much higher than that of the nitrifier. ³⁰

The laser power intensity and full width at half maximum (FWHM) of the emission spectra were plotted for excitation intensities at different repetition rates in nitrogenated and oxygenated solvent-free liquid organic DFB lasers, respectively ( FIGS. 7 and 8 ). ³⁰ The FWHM of the emission peaks of both samples was shown to be lowered to 1.8 nm at high excitation density due to amplification by stimulated emission. This linewidth is higher than the 0.7 nm resolution of the spectrometer. Looking at the curve showing the output intensity versus the excitation intensity, the sharp change in the slope efficiency is directly related to the laser operation threshold. ^29,31-34 These data were used to determine ^the laser operation threshold according to the repetition rate in both devices. The results in FIG. 9a show that the laser operation threshold is lower and almost independent of the repetition rate in the oxygenated sample with a value of 2 μJcm ⁻² . Interestingly, it was found that the laser action threshold of the nitrogenated sample gradually increased from 2.8 μJcm ⁻² to 4.4 μJcm ⁻² as the repetition rate of optical picosecond pulsed excitation increased from 0.01 MHz to 4 MHz.

A non-negligible overlap is observed between the triplet-triplet absorption spectrum of the heptafluorene molecule in chloroform solution and the representative laser spectrum of the gain material ( FIG. 10 ). ³⁰ In fact, a previous study reported that the stimulated emission cross-section of heptafluorene was 7 times larger than the triplet absorption cross-section at the ASE/laser operating wavelength. ¹⁰ Remarkably, the triplet-triplet absorption completely disappears in the oxygenated solution due to the presence of oxygen molecules acting as triplet sites. To provide additional evidence that molecular oxygen can efficiently quench triplets in heptafluorene-based laser gain media, singlet-triplet exciton annihilation (STA) in liquid blend materials bubbled with oxygen or nitrogen was analyzed. The quenching of singlet excitons by For this purpose, a nitrogenated and oxygenated gain material was sandwiched between two planar fused silica substrates. The sample was irradiated with a 325 nm light pulse (pulse duration varied from 50 to 800 μs) at an excitation density of 0.5 kWcm ^-2 , and the temporal change of photoluminescence intensity was monitored. Temporal curves of ^{8-10 nitrated} samples show that after the optical pump is started, the emission intensity decreases significantly by almost 60% after 300 μs before reaching a steady state (Fig. 11). ³⁰ These data demonstrate that singlet excitons are quenched by STAs in nitrogenated liquid materials. ^{8 -10} In contrast, the oxygenated liquid gain medium shows no such quenching and no sign of degradation under high-intensity continuous wave irradiation for 800 μs. This is consistent with the results reported in a previous study ¹⁰ and provides clear evidence that, in fact, molecular oxygen can be used to quench triplets without affecting singlets in heptafluorene-based materials. The suppression of singlet quenching by STAs in oxygenated samples is also consistent with the fact that the intensity of the DFB laser emission appears to be stronger in the oxygenator than in the nitrifier.

From these considerations, it can be concluded that the highest threshold and this critical repetition rate dependence in the nitrogenated DFB laser device can be attributed to the generation and accumulation of long-lived triplet excitons in the gain medium, leading to triplet absorption and singlet-triplet excitons. It causes additional losses related to extinction. It should be noted that the liquid blend exhibits a high PLQY of 85% and a low ASE/laser operation threshold. In addition, interstitial crossover yields are typically small (about 3%) for oligofluorene and polyfluorene derivatives. ³⁵ In this context, to observe laser operation in the quasi-continuous wave region for a repetition rate of 4 MHz, it is clear that the concentration of triplets generated through intersystem crossing under optical pumping is kept sufficiently low in the nitrogenated heptafluorene-based gain material. That's very reasonable. Importantly, the fact that the laser operating threshold is lowered and is independent of the repetition rate in the oxygenated DFB apparatus can be directly explained by the presence of molecular oxygen acting as a triplet actuator.

In addition, the photostability of quasi-continuous-wave laser operation emission was evaluated by monitoring the time change of the output intensity from the liquid layer edge above the laser operation threshold in both nitrogenated and oxygenated DFB lasers at a repetition rate of 1 MHz. The characteristic light stability time constant was evaluated by measuring the duration associated with a 10% decrease from the initial value of the output intensity. As shown in Figure 9b, the time constants for the nitrification and oxygenation units were found to be 4 and 5 minutes, respectively. The decrease in output laser operating intensity with time is probably due to bleaching of heptafluorene molecules. This problem of optical degradation can be reliably solved by the use of microfluidic circuits to achieve true quasi-continuous wave solvent-free liquid organic semiconductor laser technology. ²² Interestingly, the presence of oxygen does not induce rapid optical degradation of the liquid device, despite the formation of chemically reactive oxygen singlets after quenching of the triplet excitons. ³⁶ This is well supported by the results that the photoluminescence intensity from the oxygenated sample remains almost constant after 800 μs under continuous wave optical pumping at a high excitation density of 0.5 kWcm ⁻² . ³⁰

In summary, the use of oxygen as a triplet actuator is proving that it is a promising route to develop continuous wave organic semiconductor laser technology. The gain medium used in this first order organic DFB laser is based on a solvent-free liquid carbazole host doped with a blue fluorescent heptafluorene derivative. By bubbling the liquid molecular semiconductor blend with molecular oxygen, the DFB laser threshold in the quasi-continuous wave region was reduced and found to be almost independent of the repetition rate. The oxygenated DFB device actually exhibited a laser operating threshold of 2 μJcm ⁻² even at a high repetition rate of 4 MHz. This improvement in quasi-continuous wave laser operation performance is due to the selective quenching of triplets in the gain medium by molecular oxygen.

References

, B.J. Schwartz, M. Andersson, Q. Pei, A.J. Heeger, Science 273, 1833 (1996). ¹ F. Hide, MA

, B. J. Schwartz, M. Andersson, Q. Pei, A. J. Heeger, Science 273, 1833 (1996).

² N. Tessler, G. J. Denton, RH Friend, Nature 382, 695 (1996).

³ IDW Samuel, G. Turnbull, Chem. Rev. 107, 1272 (2007).

, S. Forget, Polym. Int. 61, 390 (2012). ⁴ S.

, S. Forget, Polym. Int. 61, 390 (2012).

, Nature 434, 876 (2005). ⁵ A. Rose, ZG Zhu, CF Madigan, TM Swager, V.

, Nature 434, 876 (2005).

⁶ Y. Yang, G. Turnbull, IDW Samuel, Appl. Phys. Lett. 92, 163306 (2008).

⁷ OG Peterson, SA Tuccio, BB Snavely, Appl. Phys. Lett. 17, 245 (1970).

⁸ Y. Zhang, S. R. Forrest, Phys. Rev. B 84, 241301 (2011).

⁹ M. Inoue, T. Matsushima, H. Nakanotani, C. Adachi, Chem. Phys. Lett. 624, 43 (2015).

¹⁰ L. Zhao, M. Inoue, K. Yoshida, ASD Sandanayaka, JH Kim, JC Ribierre, C. Adachi, IEEE J. Select. Top. Quant. Electr. 22, 1300209 (2016).

¹¹ S. Schols, A. Kadashchuk, P. Heremans, A. Helfer, U. Scherf, Chem. Phys. Chem. 10, 1071 (2009).

¹² T. Rabe, K. Gerlach, T. Riedl, HH Johannes, W. Kowalsky, J. Niederhofer, W. Gries, J. Wang, T. Weimann, P. Hinze, F. Galbrecht, U. Scherf, Appl. Phys. Lett. 89, 081115 (2006).

¹³ ASD Sandanayaka, K. Yoshida, M. Inoue, C. Qin, K. Goushi, JC Ribierre, T. Matsushima, C. Adachi, Adv. Opt. Mater. 2016, DOI: 10.1002/adom.201600006

, Opt. Expr., 19, 26382 (2011). ¹⁴ R. Bornemann, E. Thiel, P.H.

, Opt. Expr., 19, 26382 (2011).

¹⁵ SS Babu, T. Nakanishi, Chem. Comm. 49, 9373 (2013).

¹⁶ BA Kamino, TP Bender, RA Klenker, J. Phys. Chem. Lett., 3, 1002 (2012).

¹⁷ JC Ribierre, T. Aoyama, T. Muto, Y. Imase, T. Wada, Org. Electron. 9, 396 (2008).

¹⁸ . JC Ribierre, T. Aoyama, T. Kobayashi, T. Sassa, T. Muto, T. Wada, J. App. Phys. 102, 033106 (2007).

¹⁹ JC Ribierre, T. Aoyama, T. Muto, P. Andre', Org. Electron. 12, 1800 (2011).

²⁰ D. Xu, C. Adachi, Appl. Phys. Lett. 95, 053304 (2009).

²¹ EY Choi, L. Mager, TT Cham, KD Dorkenoo, A. Fort, JW Wu, A. Barsella, JC Ribierre, Opt. Expr. 21, 11368 (2013).

²² JH Kim, M. Inoue, L. Zhao, T. Komino, SM Seo, JC Ribierre, C. Adachi, Appl. Phys. Lett. 106, 053302 (2015).

²³ JC Ribierre, L. Zhao, M. Inoue, PO Schwartz, JH Kim, K. Yoshida, ASD Sandanayaka, H. Nakanotani, L. Mager, S. Me'ry, C. Adachi, Chem. Comm. 52, 3103 (2016).

²⁴ EY Choi, L. Mazur, L. Mager, M. Gwon, D. Pitrat, JC Mulatier, C. Monnereau, A. Fort, AJ Attias, K. Dorkenoo, JE Kwon, Y. Xiao, K. Matczyszyn, M Samoc, DW Kim, A. Nakao, B. Heinrich, D. Hashizume, M. Uchiyama, SY Park, F. Mathevet, T. Aoyama, C. Andraud, JW Wu, A. Barsella, JC Ribierre, JC Phys. Chem. Chem. Phys. 16, 16941 (2014).

²⁵ R. Anemian, JC Mulatier, C. Andraud, O. Stephan, JC Vial, Chem. Comm. 1608 (2002).

²⁶ SJ Choi, PJ Yoo, SJ Baek, TW Kim, H,H, Lee, J. Am. Chem. Soc. 126, 7744 (2004).

²⁷ JH Kim, MJ Han, S. Seo, J. Polym. Sci. Part B 53, 453 (2015).

²⁸ L. Zhao, T. Komino, M. Inoue, JH Kim, JC Ribierre, C. Adachi, Appl. Phys. Lett. 106, 063301 (2015).

²⁹ JC Ribierre, G. Tsiminis, S. Richardson, GA Turnbull, IDW Samuel, HS Barcena, PL Burn, Appl. Phys. Lett. 91, 081108 (2007).

³⁰ Material for a characterization of the triplet losses and the determination of the threshold in the quasi-cw regime.

³¹ C. Kamutsch, C. Gyrtner, V. Haug, U. Lemmer, T. Farrell, BS Nehls, U. Scherf, J. Wang, T. Weimann, G. Heliotis, C. Pflumm, JC deMello, DDC Bradley, Appl. Phys. Lett. 89, 201108. (2006).

³² G. Tsiminis, JC Ribierre, A. Ruseckas, HS Barcena, GJ Richards, GA Turnbull, PL Burn, IDW Samuel, Adv. Mater. 20, 1940 (2008).

³³ G. Tsiminis, Y. Wang, PE Shaw, AL Kanibolotsky, IF Perepichka, MD Dawson, PJ Skabara, GA Turnbull, IDW Samuel, Appl. Phys. Lett. 94, 243304 (2009).

³⁴ AE Vasdekis, G. Tsiminis, JC Ribierre, L. O'Faolain, TF Krauss, GA Turnbull, IDW Samuel, Opt. Expr. 14, 9211 (2006).

³⁵ HL Chen, YF Huang, CP Hsu, TS Lim, LC Kuo, MK Leung, TC Chao, KT Wong, SA Chen, W. Fann, J. Phys. Chem. A, 111, 9424 (2007)

³⁶ T. Galllavardin, C. Armagnat, O. Maury, PL Baldeck, M. Lindgren, C. Monnereau, C. Andraud, Chem. Comm. 48, 1689 (2012).

[3] Continuous wave operation of organic semiconductor lasers

Abstract: Demonstration of continuous wave laser operation from organic semiconductor films remains a highly desirable but still challenging goal for practical applications in spectroscopy, data communication and sensing. We report a low-threshold surface-emitting organic dispersive feedback laser operating in the quasi-continuous wave region at 80 MHz and under continuous-wave optical excitation of 30 ms. These outstanding performances are achieved using organic semiconductor thin films with high optical gain, high photoluminescence quantum yield and no triplet absorption loss at laser operating wavelengths in combination with mixed-order dispersive feedback grating to achieve low laser operating thresholds. made it The simple encapsulation technique greatly reduced laser-induced thermal degradation and suppressed the gain medium removal that occurs differently under strong continuous wave optical excitation. Overall, this study will provide evidence that the development of true continuous wave organic semiconductor laser technology is possible through the engineering of gain media and device architectures.

Short conclusion: An optically pumped organic semiconductor laser operating under continuous wave optical excitation of 30 ms in the quasi-continuous wave region at 80 MHz is demonstrated.

introduction

Organic semiconductor materials are generally considered suitable for applications in photonics due to their ability to emit, modulate, and detect light (1). In particular, significant research efforts have been made over the past two decades for use in optically pumped solid-state laser light sources due to their excellent properties in terms of low-cost manufacturing, ease of processing, chemical diversity, mechanical flexibility, and wavelength tunability over the total visible range (2- 6). Since the first demonstration of optically pumped organic semiconductor lasers (OSLs) (2), significant advances in high-gain organic semiconductor materials and device design have resulted in significant performance improvements (7-15). Recent advances in low-threshold distributed feedback (DFB) OSL have demonstrated direct optical pumping by electrically driven nanosecond pulsed inorganic light emitting diodes, providing a pathway for new compact, low-cost visible laser technologies (12). , 13). Applications based on these OSLs, including the development of spectroscopy tools, data communication devices, medical diagnostics, and chemical sensors, are currently emerging (16, 20). Nevertheless, OSLs are still optically pumped by pulsed light excitation (typically with pulse widths varying from 100 fs to 10 ns) and driven with repetition rates (f) ranging from 10 Hz to 10 kHz. In this context, further breakthroughs are still needed to demonstrate optically pumped OSL operating in the continuous wave domain and ultimately to realize electrically pumped organic laser diodes (21, 22).

It has been demonstrated that the operation of OSL in the continuous wave region is difficult (23, 24). Thermal degradation of organic gain media under strong long-pulse optical pumping is a serious problem for long-term laser operation (25). Another important problem to be overcome relates to the loss caused by generation of long-lived triplet excitons through intercalation (26–29). When organic films are optically pumped in the long pulse region, the accumulation of triplet excitons usually occurs, resulting in increased absorption at the laser operating wavelength due to triplet absorption (TA) and due to singlet-triplet exciton annihilation (STA). Quenching of singlet excitons results. To overcome this obstacle, it has been proposed to incorporate triplet partners such as oxygen (30, 31), cyclooctatetraene (32) and anthracene derivatives (33) into the organic film. Another method to significantly reduce the triplet loss is based on using an emitter that exhibits a high photoluminescence quantum yield (PLQY) between the absorption band of the triplet excited state and the emission band of the singlet excited state, and there is no spectral overlap. (34-36). Two methods of suppressing triplet loss in OSL have been successfully used to improve device performance in the quasi-continuous wave region (31, 35). At the same time, continuous-wave laser operation durations of nearly 100 μs could be achieved in OSLs containing anthracene derivatives such as triplet actuators (33). We propose an improved DFB OSL architecture that enables quasi-continuous-wave laser operation (at a very high repetition rate of 80 MHz) and continuous-wave surface-emitting laser operation with excellent and unprecedented performance. These results represent a significant advance in the field of organic photonics and offer new prospects for the development of reliable and cost-effective organic-based continuous wave solid-state laser technology.

result

The surface-emitting OSL fabricated in this study used 4,4'-bis[(N-carbazole)styryl]biphenyl (BSBCz) in FIG. 12 as an emitter (34). Incorporation of triplet anchors into BSBCz films is not necessary because of the extremely weak generation of triplets through intercalation and negligible triplet absorption at the laser operating wavelength of this material (35). The fabrication method and structure of the organic semiconductor DFB laser fabricated in this study are schematically shown in Figs. 12 and 13A, respectively. To achieve a low laser motion threshold where the laser motion is emitted in the direction perpendicular to the substrate plane, we design a mixed-order DFB grating architecture with a secondary Bragg scattering region surrounded by a primary scattering region causing strong feedback to generate the laser radiation. provided an effective vertical outcoupling of (8). In the DFB structure, laser oscillation occurs when the Bragg condition mλ _Bragg = 2 n _eff Λ is satisfied (5), where m is the diffraction order, λ _Bragg is the Bragg wavelength, n _eff is the effective refractive index of the gain medium, and Λ is the grating. is the cycle of Using the reported values of n _eff and λ _Bragg for BSBCz (37–39), the grating periods of the mixed order (m = 1, 2) DFB laser device are calculated to be 140 and 280 nm, respectively. These gratings were directly etched into the silicon dioxide surface using electron beam lithography and reactive ion etching for an area of 5 X 5 mm ² . It should be noted that the optical simulation and experimental data reported in Figs. 16-17 and Tables S1-S3 (see Supplementary Material section A) were considered to select the parameters used in the resonator design.

According to the present specification, the DFB gratings prepared in this study were 140 ± 5 and 280 ± 5 nm with a grating depth of about 65 ± 5 nm, as shown by the scanning electron microscopy (SEM) images of Figs. 13B-C. has a grating period of The length of each primary and secondary DFB grating was about 15.12 and 10.08 μm, respectively. A BSBCz neat film having a thickness of 200 nm and a BSBCz:CBP (6:94 wt% and 20:80 wt%) blend film were prepared on the grating by vacuum deposition. 13D-E, the surface morphology of the organic layer exhibits a grating structure with a surface modulation depth of 20 to 30 nm. To significantly improve the efficiency and stability of DFB lasers operating in the quasi-continuous-wave and long-pulse regions, the device was encapsulated in a nitrogen-filled glove box (40). For this purpose, 0.05 ml of CYTOP (a chemically robust and optically transparent fluoropolymer with a refractive index of about 1.35) is spin-coated directly on top of the organic layer, and then the polymer film has good thermal conductivity at the BSBCz laser operating wavelength. (TC ~ 25 Wm ^-1 K ^-1 at 300 K) and the organic laser device was sealed by covering with a transparent sapphire lead, which was chosen for its good transparency. It was found that the CYTOP film generally had a thickness of about 2 μm and did not affect the optical properties of the BSBCz film (Fig. 18).

The laser operation characteristics of an encapsulated mixed-order DFB device using a BSBCz neat film or a BSBCz:CBP (6:94 wt%) blend film as a gain medium were first investigated by delivering a pulse of 800 ps at a repetition rate of 20 Hz and a wavelength of 337 nm. A nitrogen gas laser was used to irradiate under pulsed optical pumping (see Supplementary Material section B and FIG. 19 ). For the CBP blend film, the excitation light is mainly absorbed by the CBP host, but the large spectral overlap between the CBP emission and the BSBCz absorption ensures efficient Forster-type energy transfer from the host to the guest molecule. This was confirmed by the absence of CBP emission under 337 nm light excitation. According to the results shown in FIG. 19, the neat film and the blend film device showed low laser operation thresholds of 0.22 and 0.09 μJcm ^-2 in the 800 ps pulse region, respectively. In both cases these values were above thresholds previously reported for amplified spontaneously amplified emission (ASE) (0.30 μJcm ⁻² ) (39) and secondary DFB lasers (0.22 μJcm ⁻² ) (35) in the BSBCz:CBP blend. low (35–39), supporting the potential of mixed-order gratings for high-performance organic solid-state lasers (8). Importantly, it has been found that device encapsulation in this pulsed optical pumping region does not modify the critical and laser operating wavelengths of mixed-order DFB lasers.

Quasi-continuous-wave laser operation in organic semiconductor DFB lasers

The laser operating characteristics of various BSBCz and BSBCz:CBP (6:94 wt%) DFB devices with different resonator structures were analyzed using quasi-continuous wave optical pulses with a wavelength of 365 nm and a width of 10 ps from a Ti sapphire laser for optical pumping. area was investigated. 14A-C show streak camera images of above-threshold laser oscillations and corresponding changes in emission intensity at different repetition rates in a representative encapsulated blend mixed-order DFB device. The excitation light intensity was fixed at about 0.5 μJcm ⁻² . When the repetition rate of the optical excitation was increased from 10 kHz to 80 MHz, the time interval between laser oscillations gradually decreased from 100 μs to 12.5 ns. For the highest repetition rate (> 1 MHz), the DFB laser output appears continuous in the 500 µs window, meaning that even at the highest repetition rate of 80 MHz, the device operates properly in the quasi-continuous wave region. The possibility of operating the DFB device at such high repetition rates is clearly associated with low TA loss and STA quenching due to the formation of negligible triplet excitons in the BSBCz:CBP blend (35).

Similar experiments were performed with unencapsulated mixed-order and second-order DFB devices based on BSBCz neat or blended membranes. For each device, the laser output intensities obtained at different repetition rates were measured according to the excitation intensity to determine the laser operation threshold, and the results for representative encapsulated blend mixed-order DFB devices at repetition rates of 10 kHz and 80 MHz are shown in FIG. is shown. The repetition rate dependence of the laser operation threshold in different devices is summarized in Fig. 14D. The laser operating threshold (E _th ) was always low in the 6 wt% blend DFB laser due to an essentially nearly 100% PLQY and suppression of concentration quenching in this gain medium (compared to a PLQY of 76% in the BSBCz neat film). (36). The results also show that the lowest threshold was obtained with the mixed-order DFB resonator structure. Remarkably, the laser operating thresholds of all devices increased only very slightly when the repetition rate was increased from 10 kHz to 8 MHz. Since there is no significant triplet accumulation in the BSBCz system (35), the slight increase in the threshold with repetition rate results from the microscopic degradation of the device under high-intensity quasi-continuous wave irradiation (see Fig. 21). Interestingly, the encapsulated blend mixed-order DFB laser exhibited the lowest threshold (changed from 0.06 μJcm ² at 10 kHz to 0.25 μJcm ² at 80 MHz) and was the only device that operated properly at 80 MHz. When the other device was optically pumped at 80 MHz, the emission intensity decreased very quickly and the FWHM values of the emission spectra detected with the streak camera before the rapid degradation of the organic film were generally larger around 7-8 nm (Fig. 22). This is necessary for encapsulation of the DFB device to significantly reduce degradation, presumably laser ablation of organic thin films under high-intensity 80 MHz optical excitation. This reduction in device degradation due to encapsulation is presumably responsible for the lowering of the laser operating threshold observed in FIG. 14D .

The operational stability of different blend DFB devices under quasi-continuous wave optical pumping at 8 MHz was investigated. A similar experiment was performed with an encapsulated mixed-order DFB laser at a repetition rate of 80 MHz. The temporal change of the different DFB laser output intensities was monitored for 20 min using a pumping intensity 1.5 times greater than the laser operating threshold of each device (Fig. 23). These results show that the operating stability is improved when the laser operating threshold is reduced through the choice of grating structure and encapsulation. Higher pumping intensities were required to obtain laser operation in devices with higher thresholds, which resulted in faster laser-induced thermal degradation. More importantly, the unencapsulated DFB device did not work well under quasi-continuous wave optical pumping at 80 MHz, but the emission power intensity from the encapsulated organic laser decreased to 96% of its initial value after 20 min. This excellent operational stability highlights the key role played by encapsulation in the performance of organic semiconductor DFB lasers operating in the quasi-continuous wave region.

organic semiconductor DFB True continuous wave laser operation in lasers

By using the variable stripe length method to investigate the spontaneous amplification emission (ASE) characteristics of a 200 nm thick BSBCz:CBP (20:80 wt%) film, an understanding of the optical gain and loss factor under long-pulse light irradiation was obtained. As shown in Fig. 24 (see Table S4 and section C of the Supplementary Material), the optically pumped membrane at 405 nm with a long pulse of 50 μs had a high net gain of 40 cm ⁻¹ for a pumping intensity of 1.5 kWcm ⁻² . coefficient and a loss coefficient of 3 cm ^-1 are shown. This clearly supports the opinion that BSBCz is an excellent candidate for organic semiconductor lasers operating under long-pulse light excitation. The laser operating characteristics of the DFB device in continuous wave mode were investigated using an inorganic laser diode emitting at 405 nm. Since the absorption of CBP is negligible at this excitation wavelength (30), the concentration of BSBCz in the blend was increased to 20 wt% to enhance harvesting of laser diode pumped emission. The PLQY of the 20 wt % blend was measured to be about 86%. 15A shows integrated streaks over 100 pulses of encapsulated 20 wt% blend mixed-order DFB laser emission measured at pumping intensities of 200 Wcm ^-2 and 2.0 kWcm ^-2 , respectively, for continuous wave excitation pulse widths of 800 μs and 30 ms. Shows camera images. The corresponding emission spectrum of Fig. 25 provides further evidence together with the diagram of Fig. 15B that the encapsulating DFB laser operates properly in the long pulse region with laser operating durations that can certainly extend beyond 30 ms. Other data in FIG. 26 provide further evidence of laser operation under long pulsed light excitation of 30 ms. As shown in Fig. 27, the DFB laser emission output intensity decreased when the number of successive 30 ms field excitation pulses was increased from 10 to 500, probably due to thermal degradation of the gain medium under intense irradiation. Although device encapsulation between highly thermally conductive silicon and sapphire has certainly improved the performance and stability of OSL to unprecedented levels, it can be seen that heat dissipation needs to be improved for future development of practical continuous wave organic laser technology. have. 27 also shows that quenching of singlet excitons by TA or STA does not occur in BSBCz (see Supplementary Material Section D). The results confirm that the overlap between the emission and triplet absorption of BSBCz is negligible, and there is no deleterious triplet loss in the gain medium even under strong continuous wave optical excitation (35). To substantiate claims for continuous wave laser operation, the convergence of the emission beam below and above the criticality, and its polarization were investigated. The results shown in FIGS. 28 to 29 confirm that the laser operation works properly in the BSBCz DFB device under long-pulse light irradiation.

Organic DFB laser output intensity and emission spectra were measured according to excitation intensity in devices with different structures and various long pulse durations from 0.1 to 1000 μs. An example of data obtained from an encapsulating blend mixing order device is shown in FIG. 30 . The abrupt change in the efficiency of the slope of the laser output intensity was again used to determine the laser operation threshold. Figure 15C summarizes the pulse duration dependence of laser thresholds measured in different devices. Similar to the trend observed in the quasi-continuous wave region, blending BSBCz into a CBP host using a mixed-order DFB resonator structure and encapsulating the device significantly lowers the laser operation threshold. While the BSBCz-nit film-based encapsulated mixed-order DFB device could operate properly in the long pulse region for times longer than 100 μs, the encapsulated blend mixed-order organic DFB laser achieved the lowest laser operation threshold (5 to 75 ^{Wcm −2} ). It was the only device capable of effectively generating laser motion for durations greater than 800 μs. To provide additional evidence for the key role played by the selection of high TC sapphire as the encapsulation lead for the performance of organic semiconductor lasers in the long pulse region, laser motion thresholds obtained from mixed-order blend DFB devices encapsulated with sapphire or glass leads. We compared the excitation duration dependence of Figure 31 clearly shows that the use of high TC leads made of sapphire lowers the threshold and improves operational stability.

The operational stability of mixed-order DFB lasers with and without encapsulation was characterized in the long pulse region by monitoring the laser emission output intensity of these devices above the laser operating threshold with a number of excitation pulses of 100 μs with a pump intensity of 200 W/cm ^-2 . became As shown in Fig. 15D, the emission intensity gradually decreased over time in all devices, and this decrease was irreversible and indicative of laser-induced thermal degradation of the organic gain medium. Notably, the operational stability was greatly improved by the encapsulation and was clearly the best for the encapsulation blend device. In the latter case, the laser output intensity decreased only 3% after 500 pulses. 32 shows laser microscopy images of an unencapsulated blend mixed-order DFB laser before and after irradiation with 100 incident pulses with a width of 1 ms and an excitation intensity of 200 Wcm ^-2 . Although no signs of laser-induced thermal degradation were observed in the encapsulating device, laser ablation occurred in the non-encapsulated device with an incision depth of about 125 nm. The possibility of greatly reducing laser ablation by the proposed encapsulation technique is of great importance for the future development of continuous wave organic semiconductor laser technology. In order to draw conclusions about how the device is limited to the actual mode of continuous wave operation, thermal simulations of the heat dissipation of the device were performed and reported in Figures 38 to 42 (see Supplementary Material Table S4 and Section E). These results show the effect of the pump pulse width on the thermal properties of the device and the role of encapsulation. In particular, although encapsulation was found to be an important factor in this study, simulations suggest that CYTOP should be replaced with other materials with better thermal conductivity in future studies.

Argument

The first demonstrations of inorganic continuous-wave solid-state lasers were reported about 40 years ago (41), and subsequent developments have proven very successful, especially at wavelengths in the near-infrared and ultraviolet/blue regions of the electromagnetic spectrum (42–45). Although these devices generally require sophisticated microfabrication techniques under high vacuum and high temperature conditions, it has recently been demonstrated that continuous wave laser operation can be achieved using solution-processed inorganic quantum wells (46). On the other hand, the performance of organic semiconductor lasers in the quasi-continuous wave and long pulse regions has so far been much lower than that of inorganic semiconductors (33, 35).

The demonstration of an organic semiconductor laser operating at 80 MHz in the quasi-continuous wave region and still operating in the long pulse region after 500 continuous pulses of 300 ms represents an important step for the development of actual continuous-wave organic solid-state laser technology. This study shows that an organic laser-operated material with high PLQY, high optical gain and no spectral overlap between the laser-operated emission peak and TA band is highly desirable for suppressing triplet loss, and when combined with mixed-order DFB grating, low criticality There is strong support for the fact that continuous wave lasers can be achieved. The results also demonstrate that the use of silicon and sapphire encapsulated leads with higher thermal conductivity (47) than those of conventional glass and fused silica clearly improved the efficiency and stability of organic DFB lasers, but under strong continuous wave optical pumping. Laser-induced thermal degradation of organic gain media remains the most serious problem that needs to be overcome in the near future. In addition, further work to significantly improve the operational stability of continuous wave organic semiconductor lasers should focus on the development of organic semiconductor gain media with low continuous wave laser operating threshold and improved thermal stability, as well as integration into devices of efficient heat dissipation systems, Previous approaches developed to improve thermal management in continuous wave inorganic solid-state lasers should be considered (48, 49). Furthermore, in addition to finding better and more efficient gain materials, further optimization of the resonator geometry and laser structure should lower the laser operation threshold and present important future directions for the development of continuous wave organic laser technology and the implementation of electrically pumped organic laser diodes. do.

materials and methods

device manufacturing

A silicon substrate covered with a 1 μm-thick thermally grown silicon dioxide layer was ultrasonically cleaned using neutral detergent, pure water, acetone and isopropanol, followed by UV-ozone treatment. The silicon dioxide surface was treated by spin coating with hexamethyldisilazane (HMDS) at 4000 rpm for 15 seconds and annealed at 120° C. for 120 seconds. A resist layer with a thickness of about 70 nm was spin-coated from a ZEP520A-7 solution (ZEON Co.) at 4000 rpm for 30 seconds on the substrate and baked at 180° C. for 240 seconds. Electron beam lithography was performed to derive a grating pattern on the resist layer using a JBX-5500SC system (JEOL) with an optimized dose of 0.1 nCcm ^-2 . After electron beam irradiation, the pattern was developed in a developer (ZED-N50, Zeon Co.) at room temperature. The patterned resist layer was used as an etch mask and the substrate was plasma etched with CHF ₃ using an EIS-200ERT etch system (ELIONIX). To completely remove the resist layer from the substrate, the substrate was plasma etched with O ₂ using a FA-1EA etching system (SAMCO). The grating formed on the silicon dioxide surface was observed by SEM (SU8000, Hitachi). To complete the laser device, a 200 nm thick 6 or 20 wt% BSBCz:CBP blend film and a BSBCz neat film were grated by thermal evaporation with a total evaporation rate of 0.1 to 0.2 nms ^-1 under a pressure of 2.0 x 10 ^-4 Pa. provided above. Finally, 0.05 ml of CYTOP (Asahi Glass Co. Ltd., Japan) was directly spin-coated on a DFB laser device at 1000 rpm for 30 seconds, then sandwiched between sapphire leads to seal the top of the laser device and dried under vacuum overnight.

Spectroscopy Measurements

For the characterization of the pulsed organic laser, pulsed excitation light from a nitrogen gas laser (USHO, KEN-2020) was focused through a lens and a slit into a device with an area of 6 X 10 ^-3 cm ² . The excitation wavelength was 337 nm, the pulse width was 0.8 ns, and the repetition rate was 20 Hz. The excitation light was incident on the device at about 20 degrees to a direction normal to the plane of the device. Light emitted perpendicular to the device surface was collected with an optical fiber connected to a multichannel spectrometer (PMA-50, Hamamatsu Photonics) located 3 cm away from the device. The excitation intensity was controlled using a series of neutral density filters. For quasi-continuous wave operation, excitation light with an excitation wavelength of 365 nm, a pulse width of 10 ps, and a repetition rate of 0.01 to 8 MHz was generated using a mode-fixed frequency dual Ti sapphire laser (Millennia Prime, Spectra physics). The excitation light was focused through a lens and a slit to a device with an area of 1.9 × 10 ⁻⁴ cm ² , and the emitted light was focused on a streak scope (C9300, Hamamatsu Photonics) connected to a digital camera (C9300, Hamamatsu Photonics) with a temporal resolution of 15 ps. C10627, Hamamatsu Photonics). For true continuous-wave operation, a continuous-wave laser diode (NICHIYA, NDV7375E, maximum output 1400 mW) was used to generate excitation light with an excitation wavelength of 405 nm. In these measurements the pulses were delivered using an acoustic-optical modulator (AOM, Gooch & Housego) operated as a pulse generator (WF 1974, NF Co.). The excitation light was focused on an area of 4.5 X 10 ^-5 cm ² of the device through a lens and a slit, and the emitted light had a temporal resolution of 100 ps and a streak scope (C7700, Hamamatsu Photonics) connected to a digital camera (C9300, Hamamatsu Photonics). was collected using Hamamatsu Photonics). Emission intensity was recorded using a photomultiplier tube (PMT) (C9525-02, Hamamatsu Photonics). The PMT response and driving square wave signal were monitored on a multi-channel oscilloscope (Agilent Technologies, MSO6104A). The same irradiation and detection angles were used in this measurement as previously described. The size of the excitation area was carefully checked using a beam profiler (WimCamD-LCM, DataRay). All measurements were performed in a nitrogen atmosphere to prevent degradation due to moisture and oxygen. A solution containing BSBCz in CH ₂ Cl ₂ was prepared and bubbled with argon before use. Third harmonic laser light of wavelength 355 nm and FWHM 5 ns from Nd:YAG laser (Quanta-Ray GCR-130, Spectra-Physics) was used as pump light, and pulsed white light from Xe lamp was used as a streak camera (C7700, Hamamatsu Photonics) was used as a probe for triplet absorption measurements in solution phase.

References and related literature

1. J. Clark, G. Lanzani, Organic photonics for communications. Nature Photon. 4, 438-446 (2010).

2. D. Moses, High quantum efficiency luminescence from a conducting polymer in solution: A novel polymer laser dye. Appl. Phys. Lett. 60, 3215-3216 (1992).

3. N. Tessler, G. J. Denton, R. H. Friend, Lasing from conjugated-polymer microcavities. Nature 382, 695-697 (1996).

, B. J. Schwartz, M. Andersson, Q. Pei, A. J. Heeger, Semiconducting polymers: a new class of solid-state laser materials. Science 273, 1833-1836 (1996).4. F. Hide, MA

, BJ Schwartz, M. Andersson, Q. Pei, AJ Heeger, Semiconducting polymers: a new class of solid-state laser materials. Science 273, 1833-1836 (1996).

5. I. D. W. Samuel, G. A. Turnbull, Organic semiconductor lasers. Chem. Rev. 107, 1272-1295 (2007).

, S. Forget, Recent advances in solid-state organic lasers. Polym. Int. 61, 390-406 (2012).6. S.

, S. Forget, Recent advances in solid-state organic lasers. Polym. Int. 61, 390-406 (2012).

7. M. D. Mcgehee, A. J. Heeger, Semiconducting (conjugated) polymers as materials for solid-state lasers. Adv. Mater. 12, 1655-1668 (2000).

, U. Lemmer, Improved organic semiconductor lasers based on a mixed-order distributed feedback resonator design. Appl. Phys. Lett. 90 131104 (2007).8. C. Karnutsch, C. Pflumm, G. Heliotis, JC deMello, DDC Bradley, J. Wang, T. Weimann, V. Haug, C.

, U. Lemmer, Improved organic semiconductor lasers based on a mixed-order distributed feedback resonator design. Appl. Phys. Lett. 90 131104 (2007).

9. G. Heliotis, R. Xia, D. D. C. Bradley, G. A Turnbull, I. D. W. Samuel, P. Andrew, W. L. Barnes, Two-dimensional distributed feedback lasers using a broadband, red polyfluorene gain medium. J. Appl. Phys. 96, 6959-6965 (2004).

10. A. E. Vasdekis, G. Tsiminis, J. C. Ribierre, L. O'Faolain, T. Krauss, G. A. Turnbull, I. D. W. Samuel, Diode pumped distributed Bragg reflector lasers based on a dye-to-polymer energy transfer blend. Opt. Expr. 14, 9211-9216 (2006).

11. J. C. Ribierre, G. Tsiminis, S. Richardson, G. A. Turnbull, I. D. W. Samuel, H. S. Barcena, P. L. Burn, Amplified spontaneous emission and lasing properties of bisfluorene-cored dendrimers. Appl. Phys. Lett. 91, 081108 (2007).

12. Y. Yang, G. A. Turnbull, I. D. W. Samuel, Hybrid optoelectronics: A polymer laser pumped by a nitride light-emitting diode. Appl. Phys. Lett. 92, 163306 (2008).

13. G. Tsiminis, Y. Wang, A. L. Kanibolotsky, A. R. Inigo, P. J. Skabara, I. D. W. Samuel, G. A. Turnbull, Nanoimprinted organic semiconductor lasers pumped by a light-emitting diode. Adv. Mater. 25, 2826-2830 (2013).

14. E. R. Martins, Y. Wang, A. L. Kanibolotsky, P. J. Skabara, G. A. Turnbull, I. D. W. Samuel, Low-threshold nanoimprinted lasers using substructured gratings for control of distributed feedback. Adv. Opt. Mater. 1, 563-566 (2013).

15. J. Herrnsdorf, Y. Wang, J. J. D. McKendry, Z. Gong, D. Massoubre, B. Guilhabert, G. Tsiminis, G. A. Tunrbull, I. D. W. Samuel, N. Laurand, E. Gu, M. D. Dawson, Micro-LED pumped polymer laser: A discussion of future pump sources for organic lasers. Laser & Photon. Laser & Photon. Rev. 7, 1065-1078 (2013).

16. C. Grivas, M. Pollnau, Organic solid-state integrated amplifiers and lasers. Laser & Photon. Rev. 6, 419-462 (2012).

17. C. Vannahme, S. Klinkhammer, U. Lemmer, T. Mappes, Plastic lab-on-a-chip for fluorescence excitation with integrated organic semiconductor lasers. Opt. Expr. 19, 8179-8186 (2011).

18. W. Zheng, L. He, Label-free, real-time multiplexed DNA detection using fluorescent conjugated polymers. J. Am. Chem. Soc. 131, 3432-3433 (2009).

19. Y. Wang, P. O. Morawska, A. L. Kanibolotsky, P. J. Skabara, G. A. Turnbull, I. D. W. Samuel, LED pumped polymer laser sensor for explosives. Laser & Photon. Rev. 7, L71-L76 (2013).

, Sensitivity gains in chemosensing by lasing action in organic polymers. Nature 434, 876-879 (2005).20. A. Rose, Z. Zhu, CF Madigan, TM Swager, V.

, Sensitivity gains in chemosensing by lasing action in organic polymers. Nature 434, 876-879 (2005).

21. I. D. W. Samuel, E. B. Namdas, G. A. Turnbull, How to recognize lasing. Nature Photon. 3, 546-549 (2009).

22. S. Z. Bisri, T. Takenobu, Y. Iwasa, The pursuit of electrically-driven organic semiconductor lasers. J. Mater. Chem. C 2, 2827-2836 (2014).

23. E. Bornemann, E. Thiel, P. Haring Bolivar, High-power solid-state cw dye laser. Opt. Expr. 19, 26382-26393 (2011).

24. E. Bornemann, U. Lemmer, E. Thiel, continuous-wave solidstate dye laser. Opt. Lett. 31, 1669-1671 (2006).

, Thermal effects in thin-film organic solid-state lasers. Opt. Expr. 22, 30092-30107 (2014).25. Z. Zhao, O. Mhibik, T. Leang, S. Forget, S.

, Thermal effects in thin-film organic solid-state lasers. Opt. Expr. 22, 30092-30107 (2014).

26. N. C. Giebink, S. R. Forrest, Temporal response of optically pumped organic semiconductor lasers and its implication for reaching threshold under electrical excitation. Phys. Rev. B 79, 073302 (2009).

27. M. A. Baldo, R. J. Holmes, S. R. Forrest, Prospects for electrically pumped organic lasers. Phys. Rev. B 66, 035321 (2002).

28. M. Lehnhardt, T. Riedl, T. Weimann, W. Kowalsky, Impact of triplet absorption and triplet-singlet annihilation on the dynamics of optically pumped organic solid-state lasers. Phys. Rev. B 81, 165206 (2010).

29. M. A. Stevens, C. Silva, D. M. Russell, R. H. Friend, Exciton dissociation mechanisms in the polymeric semiconductors poly(9,9-dioctylfluorene) and poly(9,9-dioctylfluorene-co-benzothiadiazole). Phys. Rev. B 63, 165213 (2001).

30. L. Zhao, M. Inoue, K. Yoshida, A. S. D. Sandanayaka, J. H. Kim, J. C. Ribierre, C. Adachi, Singlet-triplet exciton annihilation nearly suppressed in organic semiconductor laser materials using oxygen as a triplet quencher. IEEE J. Sel. Top. Quant. Electron. 22, 1300209 (2016).

31. A. S. D. Sandanayaka, L. Zhao, D. Pitrat, J. C. Mulatier, T. Matsushima, C. Andraud, J. H. Kim, J. C. Ribierre, C. Adachi, Improvement of the quasi-continuous-wave lasing properties in organic semiconductor lasers using oxygen as triplet quencher. Appl. Phys. Lett. 108, 223301 (2016).

32. S. Schols, A. Kadashchuk, P. Heremans, A. Helfer, U. Scherf, Triplet excitation scavenging in films of conjugated polymers. Chem. Phys. Chem. 10, 1071-1076 (2009).

33. Y. F. Zhang, S. R. Forrest, Existence of continuous-wave threshold for organic semiconductor lasers. Phys. Rev. B 84, 241301 (2011).

34. T. Rabe, K. Gerlach, T. Riedl, H. H. Johannes, W. Kowalsky, J. Niederhofer, W. Gries, J. Wang, T. Weimann, P. Hinze, F. Galbrecht, U. Scherf, Quasi -continuous-wave operation of an organic thin-film distributed feedback laser. Appl. Phys. Lett. 89, 081115 (2006).

35. A. S. D. Sandanayaka, K. Yoshida, M. Inoue, K. Goushi, J. C. Ribierre, T. Matsushima, C. Adachi, Quasi-continuous-wave organic thin film distributed feedback laser. Adv. Opt. Mater. 4, 834-839 (2016).

36. H. Nakanotani, C. Adachi, S. Watanabe, R. Katoh, Spectrally narrow emission from organic films under continuous-wave excitation. Appl. Phys. Lett. 90, 231109 (2007).

37. D. Yokoyama, M. Moriwake, C. Adachi, Spectrally narrow emissions at cutoff wavelength from edges of optically and electrically pumped anisotropic organic films. J. Appl. Phys. 103, 123104 (2008).

38. H. Yamamoto, T. Oyamada, H. Sasabe, C. Adachi, Amplified spontaneous emission under optical pumping from an organic semiconductor laser structure equipped with transparent carrier injection electrodes. Appl. Phys. Lett. 84, 1401 (2004).

39. T. Aimono, Y. Kawamura, K. Goushi, H. Yamamoto, H. Sasabe, C. Adachi, 100% fluorescence efficiency of 4,4'-bis[(N-carbazole)styryl]biphenyl in a solid film and the very low amplified spontaneous emission threshold. Appl. Phys. Lett. 86, 071110 (2005).

40. S. Richardson, O. P. M. Gaudin, G. A. Turnbull, I. D. W. Samuel, Improved operational lifetime of semiconducting polymer lasers by encapsulation. Appl. Phys. Lett. 91, 261104 (2007).

41. Z. I. Alzerov, V. M. Andreed, D. Z. Garbuzov, Y. V. Zhilyaev, E. P. Morozov, E. L. Portnoi, V. G. Trofim, Investigation of the influence of the AlAs-GaAs heterostructure parameters on the laser threshold current and the realization of continuous emission at room temperature. Sov. Phys. Semicond. 4, 1573-1575 (1971).

42. M. Beck, D. Hofstetter, T. Aellen, J. Faist, U. Oesterle, M. Ilegems, E. Gini, H. Melchior, Continuous wave operation of a mid-infrared semiconductor laser at room temperature. Science 295, 301-305 (2002).

43. H. Rong, R. Jones, A. Liu, O. Cohen, D. Hak, A. Fang, M. A. Paniccia, A continuous-wave Raman silicon laser. Nature 433, 725-728 (2005).

44. T. Someya, Room temperature lasing at blue wavelengths in gallium nitride microcavities. Science 285, 1905-1906 (1999).

45. A. C. Tamboli, E. D. Haberer, R. Sharma, K. H. Lee, S. Nakamura, E. L. Hu, Room temperature continuous-wave lasing in GaN/InGaN microdisks. Nature Photon. 1, 61-64 (2007).

46. J. Q. Grim, S. Christodoulou, F. Di Stasio, R. Krahne, R. Cingolani, L. Manna, I. Moreels, Continuous-wave biexciton lasing at room temperature using solution-processed quantum wells. Nature Nanotech. 9, 891-895 (2014).

47. S. H. Choi, T. I. Lee, H. K. Baik, H. H. Roh, O. Kwon, D. H. Suh, The effect of electrode heat sink in organic-electronic devices. Appl. Phys. Lett. 93, 183301 (2008).

48. Y. Bai, S. R. Darvish, S. Slivken, W. Zhang, A. Evans, J. Nguyen, M. Razeghi, Room temperature continuous wave operation of quantum cascade lasers with watt-level optical power. Appl. Phys. Lett. 92, 101105 (2008).

49. V. Spagnolo, A. Lops, G. Scamarcio, M. S. Vitiello, C. Di Franco, Improved thermal management of mid-IR quantum cascade lasers. J. Appl. Phys. 103, 043103 (2008).

[4] Supplementary material

Section A. Optical Simulation

1. Introduction

Recently, organic semiconductor lasers (OSLs) have attracted much attention because of their advantageous properties such as wavelength tunability in the visible range, low cost, flexibility, and large-area fabrication [1]. These properties make them good candidates for many applications including sensing, display applications, data storage and xerography. However, only optically pumped organic lasers have been realized so far. Much effort has been focused on reducing the energy threshold of optically pumped organic lasers [4] [5], [6] by improving the gain medium properties [2], [3] and optimizing the resonant cavity. Further optimization is needed to further reduce the energy threshold in terms of achieving an electrically pumped organic laser that has not yet been realized. With respect to resonant cavities, distributed feedback (DFB) resonators [7], [8], distributed Bragg resonators (DBR) [9], microrings [10], microdisc [11] and There are several forms that can be combined with organic gain media containing microsphere cavities [12]. The role of the resonator is to provide positive optical feedback in addition to the optical amplification provided by the gain medium.

The laser architecture used in modern organic lasers is based on DFB resonators [5], [4], [13]. These resonators do not use conventional cavity mirrors, but instead use periodic nanostructures responsible for Bragg diffraction. DFB resonators are compact and can be easily integrated into planar organic thin films. It can also provide a high level of spectral selection.

The structure of the laser studied in this study consists of an organic thin film deposited on the secondary DFB grating. In such a grating, the light generated by the gain medium is guided along the high refractive index organic film and then scattered by periodic structuring. Optical feedback occurs due to the coupling between the forward and backward evolution wavelengths [14]. This coupling is maximum for a particular wavelength that satisfies the Bragg condition:

m is the diffraction order, λ _Bragg is the resonance wavelength in the cavity, n _eff is the effective refractive index of the uniform waveguide, and Λ is the grating period. For the second-order grating (m = 2), the first-order diffracted light is vertically outcoupled at the surface of the film and the in-plane feedback is provided by the second-order diffraction. According to the coupling mode theory, a wavelength that satisfies the Bragg condition (1) cannot propagate in the film [15]. This is due to the periodic modulation of the refractive index leading to the appearance of an optical stopband centered on the Bragg wavelength. Therefore, a decrease in emission is observed at λ _Bragg and the laser oscillation occurs at a pair of wavelengths located at the edge of the stopband. In the secondary grating, the laser oscillates (at the highest wavelength) only at one end of the stopband. At this wavelength, the threshold is low due to the low radiation loss [16].

Resonant cavities affect laser performance through two parameters: a limiting factor Γ and a quality factor Q. The exciton density at the laser threshold is inversely proportional to both Γ and Q [17]. Therefore, optimization of the geometry of the DFB resonant cavity is important to reduce losses that can be quantified by Γ and Q.

The goal of this study is to investigate the effect of organic film thickness on laser performance including energy threshold and laser operating wavelength. First, the laser design is fixed. This is done by calculating the effective index of the waveguide structure to infer the grating period required to obtain laser operation at the ASE wavelength. The thickness of the organic film was changed from 100 nm to 300 nm, and the effective index at each thickness was calculated. Second, optical simulation is performed to obtain a physical understanding of the change in laser critical energy with thickness. The quality factor and limiting factor of the resonant cavity are calculated according to the film thickness and compared with the experimental energy threshold of the organic laser device.

2. Device Structure and Simulation Details

The geometry of the grating coupling waveguide constituting the secondary DFB organic laser studied in this study is shown in Fig. 33. The waveguide structure consists of a gain medium (6 wt% BSBCz:CBP) consisting of a low water SiO ₂ grating and a high water layer surrounded by air. The gain medium consisted of a 6 wt% BSBCz:CBP blend film vacuum deposited over the secondary DFB grating. The grating is made on the SiO ₂ substrate by electron beam lithography. Fabrication of DFB lasers has been described elsewhere [4].

The input parameters used in the simulation are the thickness and refractive index of the layers. Air (n _a = 1) and SiO ₂ substrate (n _s = 1.46) are considered semi-infinite layers. The refractive index n _f of the 6 wt % BSBCz:CBP blend is considered equal to that of CBP reported in [18] (n _f of about 1.8). The thickness of the organic film is changed from 100 nm to 300 nm. The structure of the laser is designed so that the laser oscillates at the natural amplified emission (ASE) wavelength of BSBCz near 477 nm [19], [20].

Simulation software:

Effective refractive index calculation and fan-fitting are performed using a domestic script using python 3.5 software.

The quality factors and limiting factors are extracted from the calculation of the eigenvalues of the resonant cavity mode using the finite element method in the RF module of Comsol 5.2a software.

3. Results and Discussion

3.1 Waveguide Characterization (Calculation of Effective Refractive Index)

To calculate the grating period using the Bragg condition (Equation 1), the effective refractive index n _eff of a uniform waveguide (without grating) is required. In this model, the grating is ignored, so the thickness of the waveguide is the thickness of the organic film. The effective refractive index n _eff value is calculated by solving the propagation wave equation [21] at a wavelength of 477 nm as a function of the organic film thickness.

An asymmetric waveguide without a grating is considered in this calculation (Fig. 34(a)). For an asymmetric three-layer slab waveguide, the electric field in each region is expressed as

Equation 2 and

k ₀ k ₀ is the vacuum propagation constant mode ( k _o = n _eff k _o ) and β is the propagation constant of the induction mode ( β = n _eff k _o ). The effective refractive index of the waveguide mode is calculated from the transcendental equation obtained after applying the following boundary conditions:

Fig. 34(b) provides waveguide dispersion curves derived from Equations 6 and 7, and shows the change in the effective refractive index according to the thickness of the organic film at the laser operation wavelength of 477 nm. From these curves one can infer the number of propagation modes at a given thickness and the cutoff thickness for a particular propagation mode. In this work, the thickness is chosen to vary from 100 nm to 300 nm. At a thickness of 280 nm or less, only the fundamental mode TE ₀ can oscillate. When the thickness increases beyond 280 nm, the order increases (TE ₁ , TE ₂ ).

Once the effective refractive index is calculated, the value of the grating period can be inferred at λ _ASE of 477 nm using the Bragg condition (Equation 1) at different film thicknesses. For a 200 nm film thickness, n _eff is 1.7. The value of the grating period Λ satisfying the Bragg condition (Equation 1) is 280 nm. Next, the grating period is fixed at 280 nm and the grating depth is fixed at 70 nm thickness. Only the thickness of the organic film is changed from 100 nm to 300 nm.

3.2 DFB Resonant cavity optimization

The resonant cavity is described by the photon lifetime and limiting factor. The photon lifetime τ represents the time a photon spends in the cavity (the rate at which photons are lost in the cavity). Photons can either escape the cavity or be absorbed by the material and be lost. This photon lifetime τ is related to the cavity quality factor Q as

ω ₀ is the resonance angular frequency.

The Q-factor of the optical cavity is calculated in two different ways.

(1) unique mode Calculation

In the first method, the quality factor is extracted from the eigenvalue calculation of the resonant cavity mode using the finite element method in the RF module of Comsol software. The computational area is limited to one period unit cell of the grating. The Floquet periodic boundary condition is applied for the lateral boundary and the scattering boundary condition is used for the upper and lower regions [22], [23]. An eigenfrequency solver is used to find the propagating eigenmode of the resonant cavity. A Q-factor is derived from the real and imaginary parts of the eigenvalues.

α is the attenuation (|α| = 1/τ). Also, the limiting factor of the eigenmode is calculated using the following equation.

E _norm E _norm is the normalized field strength distribution of the eigenmode.

(2) Fano of the reflection spectrum fitting

The second method used to extract the quality factor is to compute the reflection spectrum with an electric field parallel to the grating for a normally incident TE polarized plane wave using a scattering matrix implemented in Comsol software [ref]. Then, the Q-factor (Equation 8) is obtained by fitting the resonance line width present in the simulated reflection spectrum with the following Fano resonance equation [24].

w ₀ is the center frequency, t is the lifetime of the resonance, r and t are the amplitude reflection and transmission coefficients of a uniform slab with a thickness equal to the grating and effective refractive index n _eff,g . For binary grating, the effective refractive index can be described using the following effective medium theory [25].

ff is defined as the ratio of the width w of the grating to the period L.

35 shows the calculated reflection spectrum according to the wavelength and film thickness and the corresponding fitting Fano-resonance curve using Equation 11; For cavities with film thicknesses of 100, 150, 200, 250 and 300 nm, reflection peaks are observed at wavelengths of 448, 462, 472, 478 and 483 nm, respectively. At these wavelengths, resonance occurs due to the phase matching between the grating-induced diffraction wave and the leakage waveguide mode [26], [27]. Thus, multiple reflections occur in the waveguide and the wavelength of the incident light is selected by the resonance of the waveguide grating.

As confirmed by the calculations presented in section 3.1 and by previously reported studies [28], an increase in d _f causes an increase in the mode n _eff (Fig. 34(b)) leading to the tuning of the laser operating wavelength. As can be seen in Fig. 36(a), an increase in d _f results in a spectral red shift of the laser emission. A comparison of the experimental laser operating wavelength and the laser wavelength calculated from the Fano model and the "model d _f + h _g " presented in section 3.1 is presented in Fig. 36(b), where h _g represents the depth of the grating. Both models give almost identical results and are close to the experimental values, but at small d _f (< 200 nm) the difference between the experimental and calculated wavelengths is still large (Δλ > 10 nm). When the ratio of h _g / d _f exceeds 0.3 [28], index coupling is reported to be the dominant mechanism when d _f is 200 nm or less. When index coupling dominates over gain coupling, no laser motion occurs near λ _Bragg as described above. Therefore, the discrepancy between the experimental laser operating wavelength and the calculated laser wavelength can be explained by the _predominance of index coupling for df below 200 nm.

Figure 37 (a) shows the calculated Q -factor and Γ value. Both methods used to calculate the Q -factor give the same results. It can be seen that Γ increases with d _f , which represents a good optical limiting factor. This is because n _eff of the basic mode TE ₀ increases. However, the Q -factor of the resonant cavity _is highest at a df value of 200 nm. The measured energy threshold E _th for different d _f is presented in Fig. 37(a). It can be seen that the Q -factor is inversely proportional to E _th . Also, as d _f increases from 100 to 200 nm, E _th decreases. This is due to an increase in both the Q -factor and Γ. At a d _f value of 200 nm, E _th shows a minimum value and then increases with d _f . The high E _th for large d _f is due to the low Q -factor of the resonant cavity.

Finally, the full width at half maximum (FWHM) of the peak reflection extracted from the calculation and Fano fitting is compared with the FWHM of the experimental laser emission (Fig. 37(b)). Both the experimental FWHM and the calculated FWHM values show the same trend with the minimum obtained for d _f equal to 200 nm.

3.3 Encapsulation DFB Laser Optimization

In this section, we use CYTOP to calculate the Γ and Q -factors for an encapsulated DFB laser. The input parameters used in the optical simulation are the organic film thickness and the refractive index of the layer. CYTOP ( n _CYTOP = 1.35) and SiO ₂ substrates ( n _{SiO 2} = 1.46) are considered semi-infinite layers. The refractive index n _f of the 6 wt% BSBCz:CBP film is considered equal to the reported refractive index of CBP ( n _f = 1.85) ( 1 ). The thickness d ₀ of the BSBCz:CBP film is changed from 100 to 300 nm. Due to the top surface structuring, a thin layer with a thickness of ( h _g - h _{g (top)} )/2 = 30 nm is applied with a thin grating with a depth of h _{g (top)} = 5 nm.

3.3. Act 1 thickness change

First, the effect of changing the film thickness d ₀ is investigated while maintaining the grating depth h _g constant ( h _g = 65 nm) by calculating the G and Q -factors. Table S1 shows the calculation results.

Table S1. Film thickness, resonance wavelength, quality factor and limiting factor.

As the thickness increases, the G and Q -factors increase, but the resonant wavelength λ _o shifts at the ASE wavelength of the gain material so that d ₀ of 200 nm remains the optimal thickness for device operation.

3.3.2 grating depth change

Second, by calculating the G and Q -factors, we investigate the effect of the h _g change while keeping d ₀ constant ( d = 200 nm). Table S2 below shows the calculation results.

Table S2. Grating depth, resonance wavelength, quality factor and limiting factor.

By reducing the grating depth, the Q -factor increases, but Γ remains almost the same. However, fabricating shallow gratings is difficult because small changes in depth dramatically affect the optical response of the grating. Although this aspect can certainly be improved in future studies, it is considered most appropriate to choose a depth of 65 nm for this study.

3.3.3 Encapsulation and unencapsulated Comparison of devices

Calculate using the same geometry. For encapsulation, the top layer is CYTOP with a refractive index of 1.35. For non-encapsulation, CYTOP is replaced by air ( n = 1). In this case, the Q -factor and Γ increase and the resonance wavelength shifts slightly to blue as in Table S3.

Table S3. Comparison of resonant wavelengths, quality factors and limiting factors between encapsulated and non-encapsulated devices

However, based on the experimental results, the encapsulated device showed better performance (FWHM) than the non-encapsulated device. This could be due to changes in the top surface when encapsulating the device or protection from moisture.

3.3. 4 secondary grating of the realm dimensional effect

The laser operating threshold of the BSBCz:CBP (6:94 wt%) blend mixed-order DFB laser was determined experimentally using different dimensions for the secondary region. The results are shown in FIG. 17 . It can be seen that the DFB architecture used in this study (corresponding to 36 cycles) is not fully optimized, meaning that further improvements in device performance should be possible by simple processing on the resonator structure.

References

[1] C. Ge, M. Lu, X. Jian, Y. Tan, and B. T. Cunningham, “Large-area organic distributed feedback laser fabricated by nanoreplica molding and horizontal dipping,” vol. 18, no. 12, pp. 12980-12991, 2010.

[2] H. Nakanotani, S. Akiyama, D. Ohnishi, M. Moriwake, M. Yahiro, T. Yoshihara, S. Tobita, and C. Adachi, "Extremely low-threshold amplified spontaneous emission of 9,9-disubstituted -spirobifluorene derivatives and electroluminescence from field-effect transistor structure," Adv . Funct . Mater. , vol. 17, no. 14, pp. 2328-2335, 2007.

[3] G. Tsiminis, Y. Wang, P. E. Shaw, A. L. Kanibolotsky, I. F. Perepichka, M. D. Dawson, P. J. Skabara, G. A. Turnbull, and I. D. W. Samuel, "Low-threshold organic laser losses," "Low-threshold organic laser based on an oligofluorene truxene with low optical losses," pp. 3-5, 2009.

[4] ASD Sandanayaka, K. Yoshida, M. Inoue, C. Qin, K. Goushi, J.-C. Ribierre, T. Matsushima, and C. Adachi, "Quasi-Continuous-Wave Organic Thin-Film Distributed Feedback Laser," Adv . Opt. Mater. , vol. 4, no. 6, pp. 834-839, Jun. 2016.

[5] ER Martins, Y. Wang, AL Kanibolotsky, PJ Skabara, GA Turnbull, and IDW Samuel, “Low-Threshold Nanoimprinted Lasers Using Substructured Gratings for Control of Distributed Feedback,” Adv . Opt. Mater. , vol. 1, no. 8, pp. 563-566, 2013.

[6] V. Qaradaghi, V. Ahmadi, and G. Abaeiani, “Design of organic vertical-cavity surface-emitting laser for electrical pumping,” IEEE Electron Device Lett . , vol. 33, no. 11, pp. 1616-1618, 2012.

, F. Hide, R. Gupta, E. K. Miller, D. Moses, and A. J. Heeger, "Semiconducting polymer distributed feedback lasers," Appl. Phys. Lett ., vol. 72, no. 13, pp. 1536-1538, 1998.[7] MD McGehee, MA

, F. Hide, R. Gupta, EK Miller, D. Moses, and AJ Heeger, “Semiconducting polymer distributed feedback lasers,” Appl. Phys. Lett . , vol. 72, no. 13, pp. 1536-1538, 1998.

[8] G. Heliotis, R. Xia, DDC Bradley, GA Turnbull, IDW Samuel, P. Andrew, and WL Barnes, "Blue, surface-emitting, distributed feedback polyfluorene lasers," Appl . Phys. Lett . , vol. 83, no. 11, pp. 2118-2120, 2003.

[9] A. E. Vasdekis, G. Tsiminis, J.-C. Ribierre, L. O' Faolain, TF Krauss, GA Turnbull, and IDW Samuel, "Diode pumped distributed Bragg reflector lasers based on a dye-to-polymer energy transfer blend.," Opt. Express, vol. 14, no. 20, pp. 9211-9216, 2006.

[10] R. Osterbacka, M. Wohlgenannt, M. Shkunov, D. Chinn, and ZV Vardeny, "Excitons, polarons, and laser action in poly(p-phenylene vinylene) films," J. Chem . Phys. , vol. 118, no. 19, pp. 8905-8916, 2003.

[11] CX Sheng, RC Polson, ZV Vardeny, and DA Chinn, “Studies of pi-conjugated polymer coupled microlasers,” Synth . Met. , vol. 135-136, no. April, pp. 147-149, 2003.

[12] M. Berggren, A. Dodabalapur, ZN Bao, and RE Slusher, "Solid-state droplet laser made from an organic blend with a conjugated polymer emitter," Adv . Mater. , vol. 9, no. 12, pp. 968-971, 1997.

[13] G. Tsiminis, Y. Wang, AL Kanibolotsky, AR Inigo, PJ Skabara, IDW Samuel, and GA Turnbull, “Nanoimprinted organic semiconductor laser pumped by a light-emitting diode,” Adv . Mater. , vol. 25, no. 20, pp. 2826-2830, 2013.

[14] IDW Samuel and G. a Turnbull, "Organic semiconductor lasers.," Chem. Rev. , vol. 107, no. 4, pp. 1272-1295, 2007.

[15] H. Kogelnik and CV Shank, “Coupled-wave theory of distributed feedback lasers,” J. Appl . Phys. , vol. 43, no. 5, pp. 2327-2335, 1972.

[16] RF Kazarinov and CH Henry, “Second-Order Distributed Feedback Lasers with Mode Selection Provided by First-Order Radiation Losses,” IEEE J. Quantum Electron. , vol. 21, no. 2, pp. 144-150, 1985.

[17] S. Schols, Device Architecture and Materials for Organic Light-Emitting Devices . Springer, 2011.

[18] D. Yokoyama, A. Sakaguchi, M. Suzuki, and C. Adachi, "Horizontal orientation of linear-shaped organic molecules having bulky substituents in neat and doped vacuum-deposited amorphous films," Org . Electron. , vol. 10, no. 1, pp. 127-137, 2009.

[19] J. Chang, Y. Huang, P. Chen, R. Kao, X. Lai, C. Chen, and C. Lee, "Reduced threshold of optically pumped amplified spontaneous emission and narrow line-width electroluminescence at cutoff wavelength. from bilayer organic waveguide devices," vol. 23, no. 11, pp. 67-74, 2015.

[20] M. Inoue, T. Matsushima, and C. Adachi, "Low amplified spontaneous emission threshold and suppression of electroluminescence efficiency roll-off in layers doped with ter(9,9'-spirobifluorene)," Appl . Phys. Lett . , vol. 108, no. 13, 2016.

[21] A. K. Sheridan, G. A. Turnbull, A. N. Safonov, and I. D. W. Samuel, “Tuneability of amplified spontaneous emission through control of the waveguide-mode structure in conjugated polymer films,” vol. 62, no. 18, pp. 929-932, 2000.

[22] J.-H. Hu, Y.-Q. Huang, X.-M. Ren, X.-F. Duan, Y.-H. Li, Q. Wang, X. Zhang, and J. Wang, "Modeling of Fano Resonance in High-Contrast Resonant Grating Structures," Chinese Phys. Lett . , vol. 31, no. 6, p. 64205, Jun. 2014.

[23] T. Zhai, X. Zhang, and Z. Pang, "Polymer laser based on active waveguide grating structures.," Opt. Express, vol. 19, no. 7, pp. 6487-6492, 2011.

[24] A. E. Miroshnichenko and Y. S. Kivshar, “Fano resonances in nanoscale structures,” vol. 82, no. September, pp. 2257-2298, 2010.

[25] M. Foldyna, R. Ossikovski, A. De Martino, B. Drevillon, E. Polytechnique, and P. Cedex, "Effective medium approximation of anisotropic lamellar nanogratings based on Fourier factorization," vol. 14, no. 8, pp. 3055-3067, 2006.

[26] SS Wang and R. Magnusson, “Theory and applications of guided-mode resonance filters.,” Appl . Opt. , vol. 32, no. 14, pp. 2606-2613, 1993.

[27] T. Khaleque and R. Magnusson, “Light management through guided-mode resonances in thin-film silicon solar cells,” J. Nanophotonics , vol. 8, no. 1, p. 83995, 2014.

[28] V. Navarro-Fuster, I. Vragovic, E. M. Calzado, P. G. Boj, J. A. Quintana, J. M. Villalvilla, A. Retolaza, A. Juarros, D. Otaduy, S. Merino and M. A. Diaz-Garcia, “Film thickness and grating depth variation in organic second-order distributed feedback lasers," J. Appl. Phys. Vol.112, no.4, pp. 43104, 2012.

Section B. mixed order DFB Laser operating characteristics of the device

The laser operation characteristics of the encapsulated mixed-order DFB device using a BSBCz neat film or a BSBCz:CBP (6:94 wt%) blend film as a gain medium were evaluated using a nitrogen laser delivering 800 ps pulses at a repetition rate of 20 Hz and a wavelength of 337 nm. irradiated under pulsed optical pumping. In the case of the CBP blend film, the excitation light was mainly absorbed by the CBP host. However, the large spectral overlap between CBP emission and BSBCz absorption ensures efficient Forster-type energy transfer from host to guest molecules ( 2-6 ). This was confirmed by the absence of CBP emission under 337 nm light excitation. 19A-19E show emission spectra collected perpendicular to the surface of BSBCz and BSBCz:CBP (6:94 wt%) films at different excitation intensities below and above the threshold. Bragg dips ( 2 ), corresponding to the optical stopband of the DFB grating at low excitation intensity, were observed at 480 and 483 nm in the neat and blended films, respectively. The small change in the Bragg dip position is probably due to the slightly different refractive indices of the blend film and the knit film ( 2-6 ). When the pumping intensity was increased above the critical threshold, a narrow emission peak appeared in both the knit device and the blend device, indicating the onset of laser action. It can be seen that the laser peak increases more rapidly than the photoluminescent background in intensity, providing evidence of the nonlinearity associated with stimulated emission. The laser operating wavelength was found to be 484 nm for the blend film and 481 nm for the neat film. 19C and 19D show the output emission intensity and full width at full width at half maximum (FWHM) as a function of pumping intensity for two DFB devices. The FWHM was found to be lower than 0.2 nm at high excitation intensities. The laser operating threshold of the DFB laser was determined from the abrupt change in output intensity. Devices based on the neat and blended membranes were shown to exhibit laser operating thresholds of 0.22 and 0.09 μJcm ⁻² , respectively. In both cases, these values were lower than previously reported thresholds for spontaneously amplified emission (ASE) and secondary DFB lasers in the BSBCz:CBP blend ( 2–6 ), suggesting the potential of mixed-order grating for high-performance organic solid-state lasers. backs up

Section C. Optical Gain

It was possible to determine the net gain and loss factors from these ASE experimental data, and the values are shown in Table S4.

Table S4. Pulse width, excitation power, net gain and loss factor

These ASE results provide clear evidence that a large net optical gain can be obtained from the BSBCz-based film in the continuous wave region. Thus, this clearly supports the statement that BSBCz is one of the best candidates for continuous wave and quasi-continuous wave laser operation.

Section D. Transient Absorption

The results in Fig. 27A show that the PL intensity remains constant after several μs of irradiation. This means that there is no quenching of singlet excitons by the STA in the device. 27C also shows that there is no significant spectral overlap between the laser operating spectrum and the triplet absorption spectrum. The results provide clear evidence that there is no adverse triplet loss in the gain medium used in this study.

From this data, the stimulated emission cross-section σ _em and the triplet excited-state cross-section σ _TT were evaluated as in previous reports ( 3 , 9 ). The σ _em at 480 nm is 2.2 x 10 ^-16 cm ² , which is significantly larger than the σ _TT of 3.0 x 10 ^-19 cm ² , meaning that the triplet absorption has little effect on the long pulse region.

The triplet lifetime (τ _TT ), triplet absorption cross-section (σ _TT ) and inter-periodic cross-yield (Φ _ISC ) in solution are respectively τ _TT = 5.7 x 10 ³ s ^-1 , σ _TT = 3.89 x 10 ^-17 cm ² ( At 630 nm, Fig. 27D) and φ _ISC = 0.04. ϕ _ISC was evaluated based on the excitation force dependence of transient absorption ( FIG. 27E ) compared to benzophenone ( 9 ). However, it should be emphasized that the contribution of triplets in thin films cannot be observed using transient absorption measurement systems. For example, intercalation in the blend film can be neglected due to the ? _PL value of nearly 100%.

Overall, the emission spectrum measured above E _th does not overlap significantly with the triplet absorption spectrum, so the net gain for optical amplification in the long pulse region is large. Therefore, BSBCz is considered to be one of the best candidates for continuous-wave and quasi-continuous-wave laser operation.

Section E. Thermal Simulation

A transient 2D heat transfer simulation is performed using COMSOL 5.2a to investigate the temperature distribution within the device. 38 shows a schematic diagram of the geometry of the laser device. It should be noted that the grating is ignored in this simulation.

The governing partial differential equation for the temperature distribution is expressed as

ρ is the material density, C _p C _p is the specific heat capacity, T is the temperature, t is the time, k is the thermal conductivity, and Q is the laser heat source term. The laser pump beam has a Gaussian shape. Because of the annular symmetry of the laser beam, the heat transfer equation is solved in cylindrical coordinates. For a pulsed Gaussian laser beam, the heat source is written as ( 10 ).

α is the absorption coefficient, R is the reflection of the pump beam at the bottom of the device, P is the incident pump power reaching the gain region, r and z are spatial coordinates, r ₀ is the 1/2 e ² radius of the pump laser beam, r = 0 is the center of the laser beam, z _g is the z-coordinate of the interface between the gain region and the upper layer (see Fig. 38), H ( t ) is a rectangular pulse function with a pulse width t _p , and h _g is the laser It is the fraction of pump power absorbed in the gain region that is converted to heat in the absence of a field ( 11 ).

f _PLφPL is the fluorescence quantum yield ( f _PL (BSBCz:CBP) = 86%), l _pump is the pump laser wavelength, and _{λlaser /laser} is the extracted laser wavelength. Regarding the boundary condition, in the radial direction, a symmetric boundary condition is used for the axis of rotation. Adiabatic boundary conditions are applied to the bottom, top and edge surfaces (air convection is neglected). The radius of the device is set to 2.5 mm. The power density is 2 kW/cm ² . Table S5 presents the thermophysical and geometric parameters used for the simulations obtained from the COMSOL database. For the BSBCz:CBP layer, the same thermal parameters were chosen for the organic material as in Ref (11).

Table S5. Thermophysical and geometric parameters of materials

After absorption of the pump laser energy, the BSBCz layer acts as a heat source. The generated heat is transferred towards the upper and lower layers by conduction.

1.1 Pulse Width Variation

39 and 40 show pulse widths of 10, 30, and 40 ms, respectively. The maximum temperature rise and temperature rise at the interface of the BSBCz/CYTOP layer after each pump with t _p are shown.

These simulation results show that the temperature rise due to long pulse pumping irradiation increases with pulse duration, but this effect tends to saturate for pulses longer than 30 ms. It can also be seen from these calculations that the temperature rise is not predicted to increase significantly with the number of incident pulses.

1.2 10 ms Effect of encapsulation in the case of pulse width

The simulation results in FIG. 41 provide clear evidence for the importance of encapsulation used in the present device to improve thermal management of devices operating under long pulse light irradiation.

1.3 CYTOP thickness change

As can be seen from FIG. 42 , when the thickness of the CYTOP is increased, the thermal conductivity of the CYTOP is lowered and the temperature of the gain region is increased. Although encapsulation of DFB lasers by CYTOPs has been shown to be important for improving the performance of devices under long-pulse light excitation, the weak thermal conductivity of CYTOPs is clearly a limiting factor, and this aspect will be explored in future studies to demonstrate actual continuous wave organic semiconductor technology. should be addressed through the selection of a more suitable encapsulating material.

References

1. D. Yokoyama, A. Sakaguchi, M. Suzuki, C. Adachi, Horizontal orientation of linear-shaped organic molecules having bulky substituents in neat and doped vacuum-deposited amorphous films. Org . Electron. 10(1), 127-137 (2009).

2. ASD Sandanayaka, K. Yoshida, M. Inoue, K. Goushi, JC Ribierre, T. Matsushima, C. Adachi, Quasi-continuous-wave organic thin film distributed feedback laser. Adv . Opt. Mater. 4, 834-839 (2016).

3. H. Nakanotani, C. Adachi, S. Watanabe, R. Katoh, Spectrally narrow emission from organic films under continuous-wave excitation. Appl . Phys. Lett . 90, 231109 (2007).

4. D. Yokoyama, M. Moriwake, C. Adachi, Spectrally narrow emissions at cutoff wavelength from edges of optically and electrically pumped anisotropic organic films. J. Appl . Phys. 103, 123104 (2008).

5. H. Yamamoto, T. Oyamada, H. Sasabe, C. Adachi, Amplified spontaneous emission under optical pumping from an organic semiconductor laser structure equipped with transparent carrier injection electrodes. Appl . Phys. Lett . 84, 1401 (2004).

6. T. Aimono, Y. Kawamura, K. Goushi, H. Yamamoto, H. Sasabe, C. Adachi, 100% fluorescence efficiency of 4,4'-bis[( N -carbazole)styryl]biphenyl in a solid film and the very low amplified spontaneous emission threshold. Appl . Phys. Lett . 86, 071110 (2005).

7. MD Mcgehee, AJ Heeger, Semiconducting (conjugated) polymers as materials for solid-state lasers. Adv . Mater. 12, 1655-1668 (2000).

8. JC Ribierre, G. Tsiminis, S. Richardson, GA Turnbull, IDW Samuel, HS Barcena, PL Burn, Amplified spontaneous emission and lasing properties of bisfluorene-cored dendrimers. Appl . Phys. Lett . 91, 081108 (2007).

9. S. Hirata, K Totani, T. Yamashita, C. Adachi, M. Vacha, Large reverse saturable absorption under weak continuous incoherent light. Nature Materials. 13, 938-946 (2014).

, Thermal effects in thin-film organic solid-state lasers. Opt. Express. 22, 30092-30107 (2014).10. Z. Zhao, O. Mhibik, T. Leang, S. Forget, S.

, Thermal effects in thin-film organic solid-state lasers. Opt. Express. 22, 30092-30107 (2014).

, F. Druon, S. Forget, F. Balembois, P. Georges, On thermal effects in solid-state lasers: The case of ytterbium doped materials, Prog. Quantum Electron. 30(4), 89-153 (2006).11. S.

, F. Druon, S. Forget, F. Balembois, P. Georges, On thermal effects in solid-state lasers: The case of ytterbium doped materials, Prog. Quantum Electron. 30(4), 89-153 (2006).

[5] Electrically driven organic semiconductor laser diodes

Summary of the invention

Despite major advances in the performance of optically pumped organic semiconductor lasers and their applications, electrically driven organic laser diodes have not yet been realized. Here, we report the first demonstration of an organic semiconductor laser diode. The reported device incorporates a mixed-order distributed feedback SiO ₂ grating in an organic light-emitting diode structure. A high current density of 3.30 kAcm ^-2 could be injected into the device, and the blue laser operation was observed to be above the critical level of about 0.54 kAcm ^-2 . The implementation of organic semiconductor laser diodes is mainly due to the selection of high gain organic semiconductors that show no triplet absorption loss at the laser operating wavelength and suppress the electroluminescence efficiency roll-off at high current densities. This is a major advance in the field of organic electronics and represents the first step towards a novel cost-effective organic laser diode technology that enables the seamless integration of organic optoelectronic circuits.

DETAILED DESCRIPTION OF THE INVENTION

The properties of optically pumped organic semiconductor lasers ( ^{OSLs) have improved significantly over the past two decades as a result of major advances both in the development of high-gain organic semiconductor materials and in the design of high-quality element resonator structures 1-5 .} Advantages of organic semiconductors as a gain medium for lasers include high photoluminescence quantum yield (PLQY) and large stimulated emission cross-section, its chemical harmonization potential, its broad emission spectrum in the visible range, and its ease of fabrication. . Recent advances in low-threshold distributed feedback (DFB) OSLs have demonstrated optical pumping by electrically driven nanosecond pulsed inorganic light-emitting diodes and paved the way for novel compact, low-cost visible laser technologies. This type of small organic laser is particularly promising for lab-on-a-chip applications, chemical sensing and bioanalysis. However, to achieve full integration of organic photons and optoelectronic circuits, electrically driven organic semiconductor laser diodes (OSLDs) are required and remain a hitherto unrealized scientific challenge. The problems that have prevented laser operation from direct electrical pumping of organic semiconductor devices stem mainly from optical losses from electrical contacts and additional triplet and polaron losses occurring ^{at high current densities 4,5,7-9 .} Different approaches have already been proposed to solve these problems, for example, using triplet quenchers to suppress singlet quenching by ^{10 -12} triplet absorption loss and singlet-triplet exciton annihilation. as well as reducing the device active region to spatially separate exciton formation from the ¹³ exciton radiation attenuating region and to minimize the polaron quenching process. Given the current state-of-the-art performance of optically pumped organic semiconductor DFB lasers ⁵ , careful combination of these approaches along with optimization of device structures can lead to electrically driven laser-driven emission from organic thin films.

Previous studies have suggested that current densities of several kA/cm ² or higher would be necessary to achieve laser operation in OSL if the additional losses associated with electrical pumping were completely suppressed ¹⁴ . Among the various organic semiconductor thin films that exhibit spontaneously amplified emission (ASE) thresholds lower than 0.5 μJ/cm ^{2 5} , one of the most promising molecules for observing laser-driven emission under electric pumping is 4,4′-bis[(N-carr bazole)styryl]biphenyl (BSBCz) (see chemical structure in FIG. 43 ) ¹⁵ . The ASE threshold of BSBCz-based thin films was reported to be as low as 0.30 μJcm ⁻² under 800 ps pulsed light excitation ¹⁶ . At the same time, another study showed that it was possible to inject a current density of 2.8 kAcm ^-2 into BSBCz-based organic light-emitting diodes (OLEDs) under pulsed operation with a pulse width of 5 μs ¹³ . These devices exhibited maximum electroluminescent external quantum efficiency (EQE) values of more than 2%. In addition, the efficiency roll-off at high current densities due to singlet-column and singlet-polaron dissipation was substantially reduced by narrowing one of the dimensions of the current injection/transport region to 50 nm. More recently, quasi-continuous-wave laser operation at 80 MHz and true continuous-wave laser operation lasting longer than 30 ms have been demonstrated in optically pumped BSBCz-based organic DFB lasers ¹⁷ . This unprecedented performance could be achieved because the PLQY of BSBCz was close to 100% in the 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) blend, and the laser operating wavelength of the BSBCz film It can be obtained that there is no significant triplet absorption loss in By combining an inverted OLED structure with a mixed-order DFB SiO ₂ grating integrated into the active region of the device, we demonstrate electrically driven laser-driven emission from BSBCz thin films, making it the first provide clear evidence.

The fabrication method and architecture of the OSLDs developed in this study are schematically illustrated in Figures 43 to 45 (see the Materials and Methods section for a detailed description of the experimental procedure). A 100 nm thick dielectric SiO ₂ layer was first sputtered onto a pre-cleaned patterned indium tin oxide (ITO) glass substrate. We design a mixed-order DFB grating in which the first-order Bragg scattering region is surrounded by the second-order Bragg scattering region, which generates strong optical feedback and provides efficient vertical outcoupling of laser emission ^, respectively ^17,18 . In DFB lasers ^, it is well known that laser oscillation occurs when the Bragg condition ^4,19 mλ _Bragg = 2 n _eff Λ is satisfied, m is the diffraction order, l _Bragg is the Bragg wavelength, n _eff is the effective refractive index of the gain medium, Λ is the period of grating. Using the reported values of n _eff and l _Bragg for BSBCz, the grating periods of ^{20 and 21} mixed-order (m = 1, 2) DFB laser devices are calculated to be 140 and 280 nm, respectively. Electron beam lithography and reactive ion etching were used to engraving a mixed-order DFB grating in a SiO ₂ layer over an area of 140×200 μm ( FIG. 46A ). As shown by the scanning electron microscopy (SEM) image of FIG. 46B, the DFB grating prepared in this study was approximately 65 ± 5 nm with periods of 140 ± 5 nm and 280 ± 5 nm, perfectly meeting the present specifications provided above. has a grating depth of The length of the primary and secondary DFB gratings, respectively, was about 10 and 15.1 μm, respectively. Energy dispersive X-ray spectroscopy (EDX) analysis was then performed to confirm that the ITO layer was not damaged during grating fabrication and that the SiO ₂ layer was completely removed from the etched area. The EDX results shown in Figures 46C and 46D provide evidence that charge can be injected from ITO into the organic semiconductor layer deposited on top of the DFB grating in the etched region where the ITO contacts are located. It has also been proposed that the DFB resonator can be prepared at low cost using a simple nanoimprint lithography process (FIG. 45). As illustrated by the schematic shown in Figure 47A, the OSLD fabricated in this study has the following simple inverted OLED structure vacuum deposited on top of a DFB grating: ITO (100 nm)/20 wt% Cs:BSBCz (60 nm)/BSBCz (150 nm)/MoO ₃ (10 nm)/Ag (10 nm)/Al (90 nm). In this inversion device structure, doping the BSBCz film with Cs in the region close to the ITO improves electron injection into the organic layer while MoO ₃ is used as the hole injection layer ( FIGS. 48 to 49 ). As shown in Fig. 50, the surface morphology of all layers shows a grating structure having a surface modulation depth of 20 to 30 nm. Although the most efficient OLEDs generally use multilayer architectures to optimize charge balance ^{22,23 at} high current densities, charge accumulation can occur at the organic heterointerface and can be detrimental to device performance and stability ²⁴ . The OSLD fabricated in this study contains only BSBCz as an organic semiconductor and is specially designed to minimize the number of organic heterointerfaces. It should be noted that a device without a SiO ₂ DFB grating was also fabricated and used as a reference to obtain additional information on the effect of the grating on the electroluminescent properties. There is also a desire to fabricate organic semiconductor laser diodes with different one-dimensional DFB resonator structures in SiO ₂ , ITO and polymers or on top of active layers ( FIGS. 51A-D ). As shown in Fig. 52, organic semiconductor laser diodes with a two-dimensional DFB resonator structure are also promising for low-threshold 2D-DFB lasers.

Figures 47B and 53A-53D show optical microscopy images of OSLD, and Figure 47C shows optical microscopy images of a reference OLED without grating, both under direct current operation at 4.0 V. The electroluminescence is carried out uniformly from the active area of the reference OLED. For OSLDs, stronger emission can be seen from the secondary DFB grating regions of the OSLDs specifically designed to promote vertical light outcoupling. The measured current-voltage (JV) and EQE-J curves for representative devices with and without DFB grating are shown in FIGS. 47D-47E. The device was characterized under DC and pulse (voltage pulse width 500 ns and repetition rate 100 Hz) conditions. The active area of the OSLD was evaluated from SEM and laser microscopy images required to calculate the current density injected into the device. The reference devices under direct current and pulsed operation showed maximum current densities before device failure (J _max ) of 70 and 850 Acm ⁻² , respectively. Due to the reduction of Joule heat with a smaller active device area, ^{13,25 OSLDs} exhibited significantly higher J _max of 80 and 3220 Acm ^-2 under DC and pulsed operation, respectively. All BSBCz devices were found to exhibit maximum EQE values greater than 2% at low current densities. However, at current densities higher than 15 Acm ^-2 under direct current operation, which could be attributed to the thermal degradation of the organic gain medium, significant efficiency roll-offs were observed in OSLD and reference devices. Under pulsed operation, the reference device showed an efficiency roll-off at current densities higher than 410 Acm ^-2 , consistent with the results of previous reports ¹³ . More importantly, the efficiency roll-off was suppressed in OSLD under pulsed operation, and the EQE appeared to increase substantially above 800 Acm ^-2 , reaching a maximum of 3.3%. When the current density is increased above 3200 Acm ^-2 , the EQE can be drastically reduced due to the thermal degradation of the organic semiconductor.

As shown in Fig. 54, the electroluminescence spectrum of the reference device is similar to the steady-state PL spectrum of the BSBCz neat film and does not change with the current density. 53E, 55A, 55C and 56A show the evolution of the electroluminescence spectra of several OSLDs under pulsed operation at different current densities. These spectra were measured perpendicular to the substrate plane from the ITO side of the OSLD. It can be clearly seen that a strong spectral line narrowing effect occurs at 456.8 nm when J becomes larger than 800 Acm ^-2 . To gain a further understanding, the output intensity and full width at half maximum (FWHM) as a function of current density are shown in Figures 53F, 55B, 55D and 56B. The FWHM of the steady-state PL spectrum of the BSBCz neat film is about 35 nm, but decreases to a value lower than 0.2 nm at the highest current density. At the same time, a sharp change in the slope efficiency of the output intensity is observed, which is consistent with the response of the EQE-J curve and can be used to determine the threshold of 960 Acm ^-2 . Similar to what is shown in the EQE-J curve, when J is greater than 3.2 kAcm ^-2 , the output intensity decreases with J due to device failure due to thermal degradation. However, it is important to note that the emission spectrum of OSLDs is very sharp in this region. The observed response clearly indicates that light amplification occurs at high current densities and that the OSLD exhibits laser action emission above the laser threshold.

The quest for the first organic semiconductor laser diodes involves several controversial reports in the past, suggesting that considerable caution should be exercised before claiming that the OSLDs fabricated in this study provide electrically driven laser emission ⁹ . First ^, several studies ^20,26,27 have shown that edge emission of waveguide ^modes from organic light emitting devices can lead to very strong line narrowing effects without laser amplification. Unlike previous studies, the emission from this OSLD is sensed perpendicular to the substrate plane and shows a clear critical response. It should also be emphasized that the ASE linewidth of organic thin films is generally in the range of a few nm, but the FWHM of organic DFB lasers may be desirable at 1 nm or less ⁵ . When the FWHM is less than 0.2 nm, the emission spectrum of this OSLD cannot be attributed to ASE alone and is consistent with that commonly obtained with optically pumped organic DFB lasers. Second, a previous report showed a very narrow emission spectrum by accidentally exciting the transition in ITO ²⁸ . The atomic spectral lines of ITO include those at 410.3, 451.3 and 468.5 nm ²⁹ . The emission peak wavelength of the OSLD in Figure 55a is 456.8 nm and cannot be attributed to emission from ITO. It should also be emphasized that the emission of the OSLD should be characteristic of the resonator mode, and therefore the output should be very sensitive to the deformation of the laser cavity. A simple way to tune the emission wavelength in an optically pumped organic DFB laser is to change the grating period ^{4 , 5 , 30 , 31} . 55C-55D show emission spectra at different current densities and output intensity as a function of current density for OSLDs with grating periods of 300 nm (for secondary scattering) and 150 nm (for primary scattering), respectively. do. This device shows a laser action peak at 475.5 nm with a FWHM of 0.16 nm and a threshold of 1.07 kAcm ^-2 (Fig. 57).

Also provided is an organic DFB laser based on a thin film of BSBCz, which adopts an improved resonator design (Fig. 58). Since laser irradiation coupled by secondary gratings exhibit lossy channels, these lasers generally exhibit a higher threshold than their primary counterparts. To investigate the exchange between outcoupling and criticality, gratings with different widths of the primary and secondary regions were fabricated. The threshold was inferred from the laser input-output curve and is plotted as a function of secondary region width in FIG. 59 . It can be seen that the vibration threshold increases linearly with the secondary region size. It can be understood that the laser threshold is proportional to the waveguide loss that increases linearly with the increasing period. Thus, the threshold of the mixed-order resonator increases as the proportion of outcoupled light increases, but in the case of strong outcoupling, the threshold remains low. By changing the grating parameters, the organic solid-state laser can have optimized properties (low threshold and high outcoupling).

56B shows emission spectra at different current densities, with grating periods of 300 nm (for secondary scattering) and 150 nm (for primary scattering) with primary and secondary periods of 4 and 12, respectively. The output intensity as a function of current density for the OSLD is plotted (FIG. 60). This device exhibits a laser operating peak at 500.5 nm with a FWHM of 0.18 nm and a threshold of 540 Acm ^-2 . This clearly provides evidence that the laser motion emission of this OSLD is strongly influenced by the DFB resonator structure and can be used to tune laser wavelengths in a range of wavelengths. Laser emission from OSLDs must conform to several criteria in terms of output beam polarization, output beam shape, and temporal coherence ⁹ . As shown in Figure 61, the output beam of the OSLD is strongly linearly polarized along the grating pattern, providing clear evidence of true one-dimensional DFB laser action in an electrically driven device.

Another important issue to be clarified is to look at the comparison of the laser operating threshold of an electrically driven OSLD with that obtained by optical pumping. 62 shows the laser operating characteristics of an OSLD optically pumped through an ITO plane at an excitation wavelength of 405 nm by a laser diode delivering a 500 ns pulse. The laser action emission occurred at 481 nm and coincided with the electrically driven laser action wavelength. The laser operating threshold under optical pumping was measured to be about 450 Wcm ^-2 , which is higher than the 36 Wcm ^-2 value obtained with an optically pumped BSBCz-based DFB laser without two electrodes. It should be noted that the thicknesses of the different layers used for OSLD were optimized in this work to minimize optical losses due to the presence of this electrode. Assuming no additional loss mechanisms in BSBCz OSLDs operating at high current densities, the measured thresholds in optically pumped devices indicate that electrically driven laser action emission should be achieved for current densities higher than 1125 Acm ⁻² . hints at Similar thresholds for optical and electrical pumping indicate additional losses typically encountered in organic electroluminescent devices at high current densities (including high electric fields, Joule heat-induced exciton annihilation, triplet and polaron absorption, and quenching) ³² This suggests that it is almost suppressed. This is fully consistent with the fact that no electroluminescent efficiency roll-off was observed in OSLD under strong pulsed electrical excitation. To explain these results, it should be remembered that the BSBCz film does not show significant triplet absorption at the laser operation/ASE wavelength, and exhibits very weak quenching of singlets by singlet-triplet annihilation. Importantly, previous studies have shown that reduction of the device active area can be used to separate exciton formation from radiative decay of excitons and to substantially reduce the polaron/thermal quenching process.

In addition, as shown in FIG. 63, a device with 9 DFB on one chip is fabricated to provide efficient laser emission of power. For low-threshold organic semiconductor laser diodes, annular DFB resonators have also been successfully fabricated (Figs. 64-65).

In conclusion, this study demonstrates the first implementation of an electrically driven organic semiconductor laser diode implementing a mixed-order distributed feedback SiO ₂ resonator in the active region of an organic light-emitting diode structure. In particular, the device exhibited blue laser action emission with a low critical current density of 540 Acm ^-2 . In terms of emission linewidth, polarization, and criticality, the different criteria that can be used to distinguish laser motion emission from other phenomena have been carefully validated and are sufficient to argue that it is the first observation of electrically driven laser motion in organic semiconductors. back up This report opens up new opportunities and prospects in organic photonics and future developments in organic semiconductor laser diode technology supporting the advantages of simple, inexpensive and tunable laser sources and their suitability for complete and direct integration of organic-based optoelectronic platforms. It will certainly serve as a strong foundation for

Materials and Methods

Device fabrication and characterization

Indium tin oxide (ITO) coated glass substrates (100 nm ITO, Atsugi Micro Co.) were ultrasonically cleaned using neutral detergent, pure water, acetone and isopropanol, followed by UV-ozone treatment. DFB was engraved on the ITO substrate by sputtering 100 nm thick SiO ₂ on the 100 nm ITO-coated glass substrate at room temperature. The argon pressure during sputtering was 0.2 Pa. The RF power was set to 100 W ( FIGS. 43 and 44 ). The substrate was again ultrasonically cleaned using isopropanol followed by UV-ozone treatment. The silicon dioxide surface was treated by spin coating with hexamethyldisilazane (HMDS) at 4000 rpm for 15 seconds and annealed at 120° C. for 120 seconds. A resist layer with a thickness of about 70 nm was spin-coated from a ZEP520A-7 solution (ZEON Co.) at 4000 rpm for 30 seconds on the substrate and baked at 180° C. for 240 seconds. Electron beam lithography was performed to derive a grating pattern on the resist layer using a JBX-5500SC system (JEOL) with an optimized dose of 0.1 nCcm ^-2 . After electron beam irradiation, the pattern was developed in a developer (ZED-N50, Zeon Co.) at room temperature. The patterned resist layer was used as an etch mask and the substrate was plasma etched with CHF ₃ using an EIS-200ERT etch system (ELIONIX). To completely remove the resist layer from the substrate, the substrate was plasma etched with O ₂ using a FA-1EA etching system (SAMCO). Etching conditions were optimized to completely remove SiO ₂ from the pitch-modulate in the DFB until the ITO was contacted. The grating formed on the silicon dioxide surface was observed by SEM (SU8000, Hitachi) (Fig. 46B). EDX (at 6.0 kV, SU8000, Hitachi) analysis was performed to confirm that SiO ₂ was completely removed from the ditch of DFB ( FIGS. 46C and 46D ).

The DFB substrate was cleaned by conventional sonication. Indium tin oxide (ITO) (100 nm) by vacuum deposition of organic layers and metal electrodes by thermal evaporation with a total evaporation rate of 0.1 to 0.2 nms ^-1 under a pressure of 2.0 - 10 ^-4 Pa on a DFB substrate with a SiO ₂ insulator An i-OLED having a structure of /20 wt% Cs:BSBCz (60 nm)/BSBCz (150 nm)/MoO ₃ (10 nm)/Ag (10 nm)/Al (90 nm) was prepared. The SiO ₂ layer remaining on the ITO surface served as an insulator. Therefore, the current flow region of the OLED was limited to the DFB region where BSBCz was in direct contact with the ITO. A reference OLED with an active area of 140 x 200 μm was also prepared with the same current flow area. Current density-voltage-EQE (JV-EQE) characteristics (DC) of OLEDs were measured at room temperature using an integrating sphere system (A10094, Hamamatsu Photonics). JVL characteristics under pulse driving were measured using a pulse generator (NF, WF1945), an amplifier (NF, HSA4101) and a photomultiplier tube (PMT) (C9525-02, Hamamatsu Photonics). The PMT response and driving square wave signal were monitored on a multichannel oscilloscope (Agilent Technologies, MSO6104A). In the device with varying peak current, rectangular pulses with a pulse width of 500 ns, a pulse period of 5 μs, and a repetition frequency of 100 Hz were applied.

Spectroscopy Measurements

The laser light emitted perpendicular to the device surface was collected with an optical fiber connected to a multi-channel spectrometer (PMA-50, Hamamatsu Photonics) located 3 cm away from the device. For continuous wave operation, a continuous wave laser diode (NICHIYA, NDV7375E, maximum output 1400 mW) was used to generate excitation light with an excitation wavelength of 405 nm. In these measurements the pulses were delivered using an acoustic-optical modulator (AOM, Gooch & Housego) operated as a pulse generator (WF 1974, NF Co.). The excitation light was focused on an area of 4.5 X 10 ^-5 cm ² of the device through a lens and a slit, and the emitted light had a temporal resolution of 100 ps and a streak scope (C7700, Hamamatsu Photonics) connected to a digital camera (C9300, Hamamatsu Photonics). was collected using Hamamatsu Photonics). Emission intensity was recorded using a photomultiplier tube (PMT) (C9525-02, Hamamatsu Photonics). The PMT response and driving square wave signal were monitored on a multi-channel oscilloscope (Agilent Technologies, MSO6104A). The same irradiation and detection angles were used in this measurement as previously described. All measurements were performed in a nitrogen atmosphere to prevent degradation due to moisture and oxygen.

References

1. Tessler, N., Denton, GJ & Friend, RH Lasing from conjugated-polymer microcavities. Nature 382, 695-697 (1996).

, V., Burrows, P. E. & Forrest, S. R. Laser action in organic semiconductor waveguide and double-heterostructure devices. Nature 389, 362-364 (1997).2. Kozlov, VG,

, V., Burrows, PE & Forrest, SR Laser action in organic semiconductor waveguide and double-heterostructure devices. Nature 389, 362-364 (1997).

3. Hide, F. et al. Semiconducting polymers: a new class of solid-state laser materials. Science 273, 1833 (1996).

4. Samuel, IDW & Turnbull, GA Organic semiconductor lasers. Chem. Rev. 107, 1272-1295 (2007).

5. Kuehne, AJC & Gather MC Organic lasers: recent development on materials, device geometries and fabrication techniques. Chem . Rev. in press.

6. Tsiminis, G. et al . Nanoimprinted organic semiconductor lasers pumped by a light-emitting diode. Adv . Mater. 25, 2826-2830 (2013).

7. Baldo, MA, Holmes, RJ & Forrest, SR Prospects for electrically pumped organic lasers. Phys. Rev. B 66, 035321 (2002).

8. Bisri, SZ, Takenobu, T. & Iwasa, Y. The pursuit of electrically-driven organic semiconductor lasers. J. Mater. Chem . C 2, 2827-2836 (2014).

9. Samuel, IDW, Namdas, EB & Turnbull, GA How to recognize lasing. Nature Photon. 3, 546-549 (2009).

10. Sandanayaka, ASD et al . Improvement of the quasicontinuous-wave lasing properties in organic semiconductor lasers using oxygen as triplet quencher. Appl . Phys. Lett . 108, 223301 (2016).

11. Zhang, YF & Forrest, S.R. Existence of continuous-wave threshold for organic semiconductor lasers. Phys. Rev. B 84, 241301 (2011).

12. Schols, S. et al . Triplet excitation scavenging in films of conjugated polymers. Chem . Phys. Chem . 10, 1071-1076 (2009).

13. Hayashi, K. et al . Suppression of roll-off characteristics of organic light-emitting diodes by narrowing current injection/transport area to 50 nm. Appl . Phys. Lett . 106, 093301 (2015).

, C. et al. The influence of annihilation processes on the threshold current density of organic laser diodes. J. Appl. Phys. 101, 023107 (2007).14.

, C. et al . The influence of annihilation processes on the threshold current density of organic laser diodes. J. Appl. Phys. 101, 023107 (2007).

15. Sandanayaka, ASD et al . Quasi-continuous-wave organic thin film distributed feedback laser. Adv . Opt. Mater. 4, 834-839 (2016).

16. Aimono, T. et al. 100% fluorescence efficiency of 4,4'-bis[(N-carbazole)styryl]biphenyl in a solid film and the very low amplified spontaneous emission threshold. Appl . Phys. Lett . 86, 71110 (2005).

17. Sandanayaka, ASD et al . Continuous-wave operation of organic semiconductor lasers. Submitted.

18. Karnutsch, C. et al. Improved organic semiconductor lasers based on a mixed-order distributed feedback resonator design. Appl . Phys. Lett . 90 131104 (2007).

, S. & Forget, S. Recent advances in solid-state organic lasers. Polym. Int. 61, 390-406 (2012).19.

, S. & Forget, S. Recent advances in solid-state organic lasers. Polym. Int. 61, 390-406 (2012).

20. Yokoyama, D. et al . Spectrally narrow emissions at cutoff wavelength from edges of optically and electrically pumped anisotropic organic films. J. Appl. Phys. 103, 123104 (2008).

21. Yamamoto, H. et al. Amplified spontaneous emission under optical pumping from an organic semiconductor laser structure equipped with transparent carrier injection electrodes. Appl . Phys. Lett . 84, 1401 (2004).

22. Kim, S.Y. et al. Organic light-emitting diodes with 30% external quantum efficiency based on horizontally oriented emitter. Adv . Funct . Mater. 23, 3896-3900 (2013).

23. Uoyama, H. et al. Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 492, 234-238 (2012).

24. Matsushima, T. & Adachi, C. Suppression of exciton annihilation at high current densities in organic light-emitting diode resulting from energy-level alignments of carrier transport layers. Appl . Phys. Lett . 92, 063306 (2008).

25. Kuwae, H. et al. Suppression of external quantum efficiency roll-off of nanopatterned organic light-emitting diodes at high current densities. J. Appl. Phys. 118, 155501 (2015).

26. Bisri, SZ et al. High mobility and luminescent efficiency in organic single-crystal light-emitting transistors. Adv . Funct . Mater. 19, 1728-1735 (2009).

27. Tian, Y. et al . Spectrally narrowed edge emission from organic light-emitting diodes. Appl . Phys. Lett . 91, 143504 (2007).

28. El-Nadi, L. et al. Organic thin film materials producing novel blue laser. Chem . Phys. Lett . 286, 9-14 (1998).

29. Wang, X., Wolfe, B. & Andrews, L. Emission spectra of group 13 metal atoms and indium hybrids in solid H ₂ and D ₂ . J. Phys. Chem . A 108, 5169-5174 (2004).

30. Ribierre, JC et al. Amplified spontaneous emission and lasing properties of bisfluorene-cored dendrimers. Appl . Phys. Lett . 91, 081108 (2007).

31. Schneider, D. et al. Ultrawide tuning range in doped organic solid-state lasers. Appl . Phys. Lett . 85, 1886 (2004).

32. Murawski, C., Leo, K. & Gather, MC Efficiency roll-off in organic light-emitting diodes. Adv . Mater. 25, 6801-6827 (2013).

Electrical Simulation of Distributed Feedback Electrically Driven Organic Lasers

1. Device model and parameters

In this study, charge transport in organic light-emitting diodes (OLEDs) is described using a so-called “first-generation model”. In this model, a two-dimensional, time-independent drift-diffusion model is used by self-coherently solving for electron density n , hole density p , and electrostatic potential ψ . Poisson's equation relates the electrostatic potential ψ to the space charge density as follows:

F is the vector electric field, q is the elementary charge, ε _r is the relative permittivity of the material, and ε ₀ is the vacuum permittivity, n ( p ) is the electron (hole) concentration, and n _t ( p _t ) is the filling electron It is the concentration of the (hole) trap state. Assuming the state of parabolic density (DOS) and Maxwell-Boltzmann statistics, the concentrations of electrons and holes are expressed as

N _LUMO and N _HOMO are the density of states of carriers in the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO), and E _LUMO and E _HOMO are the energy levels of LUMO and HOMO, E _fn and E _fp are pseudo-Fermi levels for electrons and holes, k _B is the Boltzmann constant, and T is the device temperature.

The presence of charge carrier traps in organic semiconductors is due to structural defects and/or impurities. The injected charge will first fill this trap before setting the current. This region is known as the trap limiting current (TLC) ^{1 , 2} . Exponential or Gaussian distributions are used to model trap distributions in organic semiconductors ³ . In this study, a Gaussian distribution for the hole trap state is used ^4,5 .

N _TP is the total density of the trap, E _TP is the energy trap depth above the HOMO level, and σ _TP is the dispersion width. The density of trap holes is evaluated by integrating the Gaussian density of the state-time Fermi-Dirac distribution.

Charge transport is governed by the drift of electric field F and diffusion due to charge density gradient. The steady-state continuity equations for electrons and holes in the drift-diffusion approximation are given as

μ _n ( μ _p ) is the electron (hole) mobility, D _n ( D _p ) is the electron (hole) diffusion constant, and R is the recombination rate. The charge carrier mobility is assumed to be field-dependent and has a Pool-Frenkel shape ^6,7 .

μ _no ( μ _po ) is the zero field mobility, and F _no ( F _po ) is the characteristic field for electrons (holes). Since energy disturbances are not considered in this model, we assume the validity of Einstein's relation to calculate the diffusion constant from the charge mobility. The recombination rate R is given by the Langevin model ⁸ .

When electrons and holes recombine, an exciton is formed. The generated excitons may migrate with a characteristic diffusion constant D _s prior to radioactive or non-radioactive attenuation. The continuity equation for singlet excitons is:

S is the exciton density. The first term is the rate of singlet exciton generation in electron-hole recombination of 1/4, the second term represents exciton diffusion, the third term is the radioactive decay constant k _r and the non-radiative decay constant k _nr , and the final term is The field-dependent dissociation rate R _diss represents the dissociation of excitons by an electric field, and R _diss is given by the Onsager-Braun model ^9,10 .

r _s is the exciton radius, E _b = q ² /( 4πεε ₀ r _s ) is the exciton binding energy, J ₁ is the first-order Bessel function, and b = q ³ F /( 8πεε ₀ ( k _B T ) ² ) is the field-dependent parameter. Within this model, the effect of electric field quenching (EFQ) depends on the exciton binding energy E _b .

1. Comparison with simulation results and experiments

1.1. unipolar and a bipolar reference device

Before simulating the bipolar device, hole-only and electron-only devices were considered to test the electrical model, simulation parameters, and charge carrier mobility. The electron-only device consists of a 190 nm BSBCz layer sandwiched between Cs (10 nm)/Al and 20 wt% Cs:BSBCz (10 nm)/ITO electrodes. The hole-only device is obtained by inserting a 10 nm MoO3 layer between _BSBCz (200 nm) and ITO and Al. The bipolar OLED device comprises an ITO/20 wt % Cs:BSBCz (10 nm)/BSBCz (190 nm)/MoO ₃ (10 nm)/Al structure. The work function of the cathode (ITO/20 wt% Cs:BSBCz) is taken to be 2.6 eV and the work function of the anode (MoO ₃ /Al) is taken to be 5.7 eV. An energy level diagram of this device structure is shown in FIG. 48 .

The reported charge carrier mobility (measured as time-of-flight) for BSBCz was used [11]. 49A shows the measured reported mobility for electrons and holes for BSBCz and its correspondence with the Pool-Frenkel field dependence model. The values of the fitted mobility parameters are presented in Table 1 along with other values of the input parameters required for the electrical simulation. The hole and electron mobility of BSBCz is about the same size, indicating that BSBCz can transport two kinds of charge carriers.

Table 1. Electrical simulation parameters

Experimental and simulated J(V) curves for unipolar and bipolar devices are presented in Figure 49b. The experimental value of 18 V or less is measured under DC driving, and 18 V The above experimental values were measured under pulse driving. The hole-only device current is greatly limited by the trap at V below 20 V. Table 1 shows the values of N _tp , E _tp , and σ _tp obtained by optimizing the experimental data for simulation. The results show good agreement between experiments and simulations for unipolar devices. For bipolar devices, the small deviation in low current density between measurements and simulations is due to the presence of experimental leakage currents. The simulation model predicts similar current densities for hole-only devices and electron-only devices at high voltage and shows good electron and hole transport balance. Bipolar devices show one order of magnitude higher current density than unipolar current densities.

2.1. bipolar DFB Device

The use of a DFB grating resonator affects the electrical properties as well as the optical properties of ^12-14 organic lasers by providing positive optical feedback for optical amplification. The effect of the nanostructured cathode on the electrical properties of the DFB OLED is calculated and compared to a reference OLED (without grating). The structure of DFB OLED is similar to bipolar OLED, but the difference lies in the nanostructured cathode composed of periodic grating SiO ₂ -Cs:BSBCz deposited on ITO. The grating period is 280 nm and the grating depth is 60 nm as shown in Fig. 66a. The thickness of BSBCz in this structure is 150 nm. For comparison a reference OLED (ITO/20 wt% Cs:BSBCz (60 nm)/BSBCz (150 nm)/MoO ₃ (10 nm)/Al) is prepared with the same thickness and without grating. All parameters and conditions used for the bipolar device were maintained for the DFB and reference OLED without additional fitting parameters.

Experimental and simulated J ( V ) curves for the DFB grating and reference OLED are shown in Fig. 66b. Electrical simulations predict J ( V ) curves in good agreement with the experimental results for both direct current ( V < 18 V ) and pulsed operation ( V ≥ 18 V ).

In addition to predicting J ( V ) curves, electrical simulations allow access to physical parameters that are difficult to determine experimentally, such as spatial distribution of charge carrier density, electric field, and location of recombination regions.

First, consider the reference OLED. Figures 67a and 67b show the charge carrier distribution and electric field profile for a reference OLED at 10 and 70 V. Free electrons are injected from the ITO/CS:BSBCz cathode into the BSBCZ (at x = 0 μm, and free holes from the Al/MoO ₃ anode (x = 0.215 μm)). Carrier density decreases with out-of-contact due to carrier recombination. When n is p, the electric field increases and reaches a maximum at the center. At 10 V, the electric field is blocked by the high charge carrier density near the cathode and anode. At higher voltages (70 V), electrons penetrate deeper and the electric field remains high near the anode.

For the DFB grating OLED, the physical parameters are extracted at 70 V. 68A-68B show the spatial distribution of charge carrier densities n and P. Since electrons are not uniformly injected due to the periodic nanostructure of the cathode, its spatial distribution follows periodic injection as clearly shown in Figs. 68b, 68c. Holes are injected at the uniform anode and extend relatively uniformly in the bulk (FIGS. 68a, 68c). When the hole reaches the cathode, it is attenuated as in the reference OLED (FIG. 67(b)). However, due to the presence of the SiO ₂ grating, holes accumulate at the SiO ₂ /BSBCz interface, resulting in a high density (Fig. 68a).

69a shows the periodic profile of a slightly modulated electric field in the BSBCz layer that is high in the insulator and maintains the same intensity as the reference OLED (about 3.5 X10 ⁶ V/cm). The current density profile shown in FIG. 69b is strongly modulated and exhibits high values near the SiO ₂ /Cs:BSBCz interface.

To clarify the reason for the high current density values near the SiO ₂ /Cs:BSBCz interface, the recombination rate profile R is shown in Fig. 70a. R shows that there is a periodic change inside the device. In the region defined by the cathode/anode, the profile is the same as the reference OLED, and decreases in the region defined by the SiO ₂ /anode (see FIG. 70c ). At the Cs:BSBCz/SiO ₂ interface, R shows a maximum value due to hole and electron accumulation as in FIG. 70d .

The electric field inside the device is on the order of MV/cm ² as shown in FIG. 69a. Therefore, the dissociation of excitons by electric field cannot be ignored and strongly affects the device performance. The singlet exciton binding energies of organic semiconductors are in the range of 0.3 to 1.6 eV, ^{15 -18} . At low electric fields, the dominant deactivation process is radioactive and non-radiative attenuation. At high electric fields, the probability of exciton dissociation increases dramatically and is dependent on the exciton binding energy. To account for the field-induced exciton dissociation, the field-dependent dissociation rate given by Equation 10 is included in the singlet exciton continuity, Equation 9. 71A shows the calculated exciton density S for a reference device comprising EFQs with different exciton binding energies E _b (0.2 to 0.6 eV). EFQ becomes a serious loss mechanism when E _b is reduced. The use of molecules with high exciton binding energies is required to overcome EFQ. The exciton binding energy of BSBCz was evaluated using a PL quenched yield experiment and has a lower limit of 0.6 eV.

Figure 71b shows the S(J) characteristics with and without field-induced exciton dissociation for reference and DFB devices. In the absence of EFQ, S increases with I and at J = 3 KA/cm ² shows high values for the DFB device of 9 X 10 ¹⁷ cm ^-3 versus 2 X 10 ¹⁷ cm ^-3 for the reference device. This difference in S stems from the various device architectures resulting in different R distributions inside the device as shown in Figures 70a, 70b. By considering the EFQ model and E _b = 0.6 eV of BSBCz, S is increased for both devices and decreased due to electric field dissociation of excitons until J = 0.5 KA/cm ² . The EFQ of the DFB device is slightly lower than that of the reference device, which may explain the lower experimental EQE roll-off of the DFB device compared to the reference device shown in FIG. 47E.

The one-dimensional distribution of excitons inside a reference device with and without EFQ is shown in FIG. 72A to gain a further physical understanding of why EQE is improved in the DFB device. For the DFB device, the two-dimensional exciton distributions with and without EFQ are shown in Figs. 72b and 72c, respectively.

Comparison of the exciton density distribution (Fig. 73, lower right) with the optical mode distribution of the device (Fig. 74, lower) indicates that there is a large overlap between them in the second grating region, contributing to the light amplification. The large overlap certainly contributes to the low laser operating threshold.

In the reference device, in the absence of EFQ, S is uniformly distributed. In the presence of EFQ, S is reduced in the bulk due to the high electric field reaching 3.5 MV/cm in the bulk (see Fig. 79b). Near the Cs:BSBCz/ITO boundary, the electric field is low, so that the EFQ of excitons can be avoided. In the case of the DFB apparatus, excitons are generated at two recombination sites (site 1 and site 2), as shown in Figure 71a. Accumulation of charge near the SiO ₂ grating creates a recombination region with high exciton density (S = 6 X 10 ¹⁷ cm ^-3 ) named site 1 . Site 2 has an S equal to the reference device (S = 1 X 10 ¹⁷ cm ^{- 3} ). In the absence of EFQ, the maximum S is provided by site 1 and explains the value of the high DFB device compared to the reference device at low electric fields (see Fig. 71b). When EFQ is considered, excitons at site 1 are quenched by the high electric field at this site (F = 3.5 MV/cm), and the maximum S is at site 2 where the electric field is low near the boundary (F = 1.2 MV/cm). given by and gradually increased in the bulk region relative to the reference device. Therefore, some excitons near the boundary may survive unless quenched by other mechanisms (not included in this simulation).

References

[1] G. Malliaras, J. Salem, P. Brock, and C. Scott, “Electrical characteristics and efficiency of single-layer organic light-emitting diodes,” Phys. Rev. B , vol. 58, no. 20, pp. R13411-R13414, 1998.

[2] J. Staudigel, M. Stoⓢel, F. Steuber, and J. Simmerer, "A quantitative numerical model of multilayer vapor-deposited organic light emitting diodes," J. Appl. Phys. , vol. 86, no. 7, p. 3895, 1999.

[3] V. Kumar, SC Jain, AK Kapoor, J. Poortmans, and R. Mertens, "Trap density in conducting organic semiconductors determined from temperature dependence of JV characteristics," J. Appl . Phys. , vol. 94, no. 2, pp. 1281-1285, 2003.

, "Charge carrier mobility in doped disordered organic semiconductors," J. Non. Cryst. Solids, vol. 338-340, no. 1 SPEC. ISS., pp. 603-606, 2004.[4] VI Arkhipov, P. Heremans, EV Emelianova, GJ Adriaenssens, and H.

, “Charge carrier mobility in doped disordered organic semiconductors,” J. Non. Crystal. Solids, vol. 338-340, no. 1 SPEC. ISS., pp. 603-606, 2004.

[5] HT Nicolai, MM Mandoc, and PWM Blom, “Electron traps in semiconducting polymers: Exponential versus Gaussian trap distribution,” Phys. Rev. B - Condens. Matter Mater. Phys. , vol. 83, no. 19, pp. 1-5, 2011.

[6] GAN Connell, DL Camphausen, and W. Paul, “Theory of Poole-Frenkel conduction in low-mobility semiconductors,” Philos . Mag. , vol. 26, no. 3, pp. 541-551, 1972.

, "Poole-Frenkel behavior of charge transport in organic solids with off-diagonal disorder studied by Monte Carlo simulation," Synth. Met., vol. 37, no. 1-3, pp. 271-281, 1990.[7] L. Pautmeier, R. Richert, and H.

, "Poole-Frenkel behavior of charge transport in organic solids with off-diagonal disorder studied by Monte Carlo simulation," Synth. Met. , vol. 37, no. 1-3, pp. 271-281, 1990.

[8] M. POPE and CE SWENBERG, Electronic Processes in Organic Crystals and Polymers. New York: Oxford University. Press, 1999.

[9] L. Onsager, "Initial recombination of ions," Phys. Rev. , vol. 54, no. 8, pp. 554-557, 1938.

[10] CL Braun, “Electric Field Assisted Dissociation of Charge Transfer States as a Mechanism of Photocarrier Production,” J. Chem . Phys. , vol. 80, no. 9, pp. 4157-4161, 1984.

[11] Y. Setoguchi and C. Adachi, "Suppression of roll-off characteristics of electroluminescence at high current densities in organic light emitting diodes by introducing reduced carrier injection barriers," J. Appl. Phys. , vol. 108, no. 6, p. 64516, 2010.

[12] RF Kazarinov and CH Henry, “Second-Order Distributed Feedback Lasers with Mode Selection Provided by First-Order Radiation Losses,” IEEE J. Quantum Electron. , vol. 21, no. 2, pp. 144-150, 1985.

, and U. Lemmer, "Improved organic semiconductor lasers based on a mixed-order distributed feedback resonator design," Appl. Phys. Lett ., vol. 90, no. 13, pp. 2005-2008, 2007.[13] C. Karnutsch, C. Pflumm, G. Heliotis, JC DeMello, DDC Bradley, J. Wang, T. Weimann, V. Haug, C.

, and U. Lemmer, "Improved organic semiconductor lasers based on a mixed-order distributed feedback resonator design," Appl. Phys. Lett . , vol. 90, no. 13, pp. 2005-2008, 2007.

[14] ASD Sandanayaka, K. Yoshida, M. Inoue, C. Qin, K. Goushi, J.-C. Ribierre, T. Matsushima, and C. Adachi, "Quasi-Continuous-Wave Organic Thin-Film Distributed Feedback Laser," Adv . Opt. Mater. , vol. 4, no. 6, pp. 834-839, 2016.

[15] JL Bredas, J. Cornil, AJ Heeger, and BJ Briidas, "The exciton binding energy in luminescent conjugated polymers," Adv . Mater. , vol. 8, no. 5, p. 447--{&}, 1996.

, S. Krause, F. Reinert, D. Rauh, and V. Dyakonov, "Energetics of excited states in the conjugated polymer poly(3-hexylthiophene)," Phys. Rev. B - Condens. Matter Mater. Phys., vol. 81, no. 8, pp. 1-5, 2010.[16] C. Deibel, D. MacK, J. Gorenflot, A.

, S. Krause, F. Reinert, D. Rauh, and V. Dyakonov, "Energetics of excited states in the conjugated polymer poly(3-hexylthiophene)," Phys. Rev. B - Condens. Matter Mater. Phys. , vol. 81, no. 8, pp. 1-5, 2010.

[17] PK Nayak, “Exciton binding energy in small organic conjugated molecule,” Synth. Met. , vol. 174, pp. 42-45, 2013.

[18] S. Kraner, R. Scholz, F. Plasser, C. Koerner, and K. Leo, "Exciton size and binding energy limitations in one-dimensional organic materials," J. Chem. Phys. , vol. 143, no. 24, pp. 1-8, 2015.

[6] Very low amplified emission threshold and spin coated Octafluorene from knit membrane blue electroluminescence

Organic semiconductor lasers have been intensively studied over the past two decades and have made great strides in terms of laser operation threshold and actuator stability. ^{1-3 These} devices are currently being considered for a variety of applications including spectroscopic tools, data communication devices, medical diagnostics, and chemical sensors. ^{1 -5} However, demonstrations of electrically driven organic laser diodes have not yet been made, and a breakthrough is still needed to develop truly continuous wave optically pumped organic semiconductor laser technology. The challenges for realizing ^{1 -3,6-8} electrically pumped organic laser diodes are now well recognized, and include (i) additional absorption losses at the laser operating wavelength due to polarons and long-lived triplets, ( ii) quenching of singlet excitons due to singlet-triplet, singlet-polaron and singlet-thermal dissipation, and (iii) stability of organic materials in electroluminescent devices operating at high current densities. are doing Approaches to reduce triplet and polaron losses have already been proposed and include the use of triplet antennas and reduction of the organic light emitting diode (OLED) active area to spatially separate the exciton forming region and the exciton decay region. it should be noted that ^9,10 It is necessary to completely overcome these problems ^and further studies are needed, but at the same time, it is also important to significantly lower the threshold of spontaneous amplified emission (ASE) and laser operation of organic semiconductor thin films. ³ For this purpose, there is a need for the development of novel high laser gain organic materials as well as improved resonator structures that can be integrated into electro-pumped organic light-emitting devices.

The radiation decay rate ( k _R ) is directly related to the Einstein B coefficient expressed by the equation B ∝ ( c / 8πhν ₀ ³ ) k _R . ν ₀ is the frequency of light, h is the Planck constant, and c is the speed of light. The ASE threshold is inversely proportional to the B factor, which generally means that a large k _R is desirable to achieve a low ASE threshold. ^11,12 As summarized in ^a recent review article on organic lasers ³ , the lowest ASE threshold reported for small-molecule-based organic thin films is 110 nJ/cm ² , obtained using 9,9'-spirofluorene derivatives. lost. ¹³ Two other good organic semiconductor laser materials with low ASE thresholds in thin films of about 300 to 400 nJ/cm ² are 4,4′-bis[( N -carbazole)styryl]biphenyl (BSBCz) and heptafluorene is a derivative. ^12,14 It is important to note that ^the ASE thresholds generally depend on the nature of the light source used for optical pumping, but the ASE thresholds described above are determined using a similar nitrogen laser for optical excitation. Fluorene derivatives have been found to be very promising for achieving low ASE thresholds, some of which show radiation decay rates higher than 1 x 10 ⁹ s ^-1 . ^13-19 It is noteworthy that in previous studies, the photophysical properties of terfluorene, pentafluorene and heptafluorene derivatives functionalized with hexyl side chains were investigated in detail. ¹⁸ The results showed that when the length of the oligofluorene molecule was increased, the radiation attenuation rate was increased while the ASE threshold was decreased. In this context, it was important to verify that the ASE/laser operating properties could still be further improved by increasing the oligomer length.

There are reports of octafluorene derivatives that do not show concentration quenching in spin-coated neat films with a PLQY of 87% and a fluorescence lifetime of about 600 ps. The chemical structure of this molecule is shown in Figure 75a. The large PLQY value and the short PL lifetime of the octafluorene neat membrane entails an ASE threshold of about 90 nJ/cm ² , reaching unprecedented levels of ASE performance in organic non-polymeric gain media. The performance of organic distributed feedback (DFB) lasers and OLEDs based on ³ -octafluorene neat films provides further evidence that these fluorene derivatives are very promising for further research into organic semiconductor laser operating devices and their applications. .

The experimental procedures used in these works are described in the Supplementary Material. ¹⁹ Absorption and steady-state PL spectra of a neat octafluorene film spin-coated on a fused silica substrate are shown in Fig. 75b. The film is almost transparent at wavelengths in the visible range and shows one major absorption band in the ultraviolet region with a maximum absorption peak wavelength of 375 nm. Previously, this absorption peak was due to exciton bonding between the fluorene monomers. ¹⁸ The optical energy gap was calculated to be about 2.9 eV from the long-wavelength absorption edge. The PL spectrum and photograph shown in the inset of FIG. 75B show that the octafluorene neat film emits blue fluorescence. The spectrum shows a sharp oscillatory structure with two peaks that can be assigned to the (0,0) and (0,1) transitions and a shoulder at the longer wavelength associated with the (0,2) transition. The maximum PL peak wavelength was found to be about 423 nm. Absorption and steady-state PL spectra measured on spin coating blends containing 10 and 20 wt% of octafluorene dispersed in 4,4′-bis( N -carbazolyl)-1,10-biphenyl (CBP) host This is shown in Figure 76 (see Supplementary Material). The CBP host was chosen for this study because it is known that efficient Forster-type energy transfer from CBP to most of the oligofluorene derivatives occurs. It can be seen that the absorption spectrum of the blend ¹⁴ is dominated by the CBP absorption, but the PL spectrum is not significantly different from the PL spectrum of the octafluorene neat film. PLQY and PL lifetimes were then measured on neat membranes and CBP blends. The 10 wt% and 20 wt% blends exhibited PLQY values of 88% and 87%, respectively, close to those found in the neat membrane. The neat membrane and the 10 wt % and 20 wt % blends show similar single exponential fluorescence decay with characteristic PL lifetimes of 609, 570 and 611 ps, respectively (see FIG. 77 in Supplementary Material). This provides evidence that the octafluorene neat film does not exhibit PL concentration quenching, contrary to what has already been reported for similar terfluorene, pentafluorene and heptafluorene derivatives. Considering the large radiation attenuation rate of about 1.7 x 10 ⁹ s ^-1 measured in the ¹⁸ -octafluorene neat film, the oligofluorene derivative can be expected to show excellent ASE properties. The optical constants of ^{the 11,12 octafluorene} neat films were measured using variable incidence angle spectroscopic ellipticity and are shown in Fig. 75c (elliptically polarized reflection data from which the optical constants are calculated can be found in Fig. 78 of the Supplementary Material). The small optical anisotropy of the neat film indicates that the octafluorene molecules are almost randomly oriented, which is consistent with the elliptically polarized light reflection results reported previously in the heptafluorene neat film. ²⁰

As schematically shown in Fig. 79a, the ASE properties of the octafluorene neat film were characterized by optically pumping the sample at 337 nm with a nitrogen laser delivering 800 ps pulses at a repetition rate of 10 Hz. The excitation beam was focused on a 0.5 cm × 0.08 cm stripe and PL was collected from the edge of the organic thin film. 79b shows the PL spectrum measured from the edge of a 260 nm thick octafluorene neat film at various pumping intensities. The spectral line narrowing effect is clearly visible at high excitation densities of full width at half maximum (FWHM) dropping to 5 nm, providing evidence that ASE occurs in this sample. Light amplification occurs at about 450 nm due to spontaneous emission photons that are guided in the organic film and amplified by stimulated emission. ²¹ The ASE threshold was then determined for the excitation intensity from a plot of the output intensity emitted from the edge of the membrane. The abrupt change in the gradient efficiency shown in FIG. 79c results in an ASE threshold of about 90 nJ/cm ² . It should be noted that the ASE properties were measured on octafluorene neat films with different film thicknesses ranging from 53 to 540 nm. The data shown in FIGS. 79d and 80 (see Supplementary Information) indicate that the ASE threshold is lowest for a 260 nm thick film. A similar thickness dependence of the ASE threshold has already been reported for poly(9,9-dioctylfluorene) films. ²² This response was due to the interaction between the increase of the mode limit and the decrease of the pump mode overlap when increasing the thickness. Of note, the ASE threshold measured for the 260 nm thick octafluorene neat film is lower than the lowest value reported for the low molecular-based organic thin film. ³ This excellent performance also means that the octafluorene thin film exhibits a very low loss factor value. For this purpose, the ASE intensity was measured as a function of the distance between the edge of the octafluorene film and the pump stripe. In the result of FIG. 81 (see Supplementary Information), the loss factor of the 260 nm thick octafluorene neat film is 5.1 cm ⁻¹ . This low value is close to the value reported for the poly(9,9-dioctylfluorene) thin film and provides evidence of the excellent optical waveguide properties of the ²³ octafluorene thin film. It should be emphasized that, unlike most polyfluorene systems, most low molecular weight fluorene and octafluorene do not show significant deterioration in photophysical properties under intense light irradiation in the atmosphere due to the formation of ^{24-26 fluorene} . There is a need. In addition, the results shown in Fig. 82 (see Supplementary Information) show that the octafluorene neat film exhibits excellent light stability at high pumping intensities above the ASE threshold in both atmospheric and nitrogen-based layers. This may be related to the high radiation decay rate of the film, possibly reducing the photobleaching of the material under high-intensity irradiation. Comparing with the ASE threshold measured at shorter oligofluorenes, ^{14 ,18} results show that increasing oligomer length improves ASE performance. However, preliminary experiments performed on decafluorene films exhibit higher ASE thresholds than those obtained on octafluorene films and show that octafluorene derivatives are clearly the most promising candidates for organic semiconductor lasers in the oligomeric family.

A mixed-order DFB grating structure composed of a secondary Bragg scattering region surrounded by a primary scattering region was designed and fabricated. ¹⁷ This grating architecture was chosen to achieve a low laser motion threshold with laser motion emission in the direction perpendicular to the substrate. Laser emission from a DFB laser occurs near the Bragg wavelength (λ _Bragg ) defined as: mλ _Bragg = 2 n _eff Λ. n _eff is the effective refractive index of the laser gain medium, m is the Bragg order, and L is the grating period. Using ^{the 1-3 elliptically} polarized light reflection (Fig. 75c) and the refractive index of the octafluorene neat film determined by the ASE wavelength measured in this study, the diffraction periods for m = 1 and 2 were selected to be 260 and 130 nm, respectively. 83A and 83B show schematics and scanning electron microscopy (SEM) images of DFB SiO ₂ gratings prepared using electron beam lithography and reactive ion etching techniques. It should be noted that the depth of the DFB grating is about 70 nm. To complete the laser device, a 260 nm thick octafluorene knit film was spin-coated on top of the DFB structure. 83c shows emission spectra detected perpendicular to the substrate plane at several excitation densities below and above the laser operating threshold. Below the threshold, a Bragg dip due to the optical stopband of the DFB grating can be observed. When the laser operating threshold is exceeded, a sharp laser emission peak at the laser operating wavelength of about 452 nm is evident. The output emission intensity and FWHM of this DFB laser are plotted as a function of excitation intensity in FIG. 81d . The FWHM of the laser emission peak was found to be lower than 0.3 nm at high excitation densities. At the same time, the laser operating threshold determined from the change in the slope of the output intensity curve was found to be about 84 nJ/cm ² , which is slightly lower than the previously reported ASE threshold. Overall, the very low ASE and laser operating threshold measured in this study, along with the excellent optical stability of the film under high optical excitation intensity, demonstrate that octafluorene derivatives are very promising gain media materials for organic semiconductor laser applications.

In order to fully evaluate the potential of this octafluorene derivative for organic laser diodes, it is important to investigate the electroluminescent (EL) properties of this compound in a neat film and CBP blend using standard OLED structures. A schematic diagram of the OLED fabricated in this study is provided in Figure 82a. The architecture of this device is as follows: indium tin oxide (ITO) (100 nm)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (45 nm)/EML ( ~ 40 nm)/2,8-bis(diphenylphosphoryl)dibenzo[b,d]thiophene (PPT) (10 nm)/2,2',2"-(1,3,5-benzinetri yl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) (55 nm)/LiF (1 nm)/Al (100 nm) The emission layer (EML) is an octafluorene neat film or octaflu This corresponds to the orene:CBP blend.In these devices, PEDOT:PSS serves as the hole injection layer, while PPT and TPBi are used as hole blocking and electron transport layers, respectively.PEDOT:PSS, PPT and TPBi in Figure 84a. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy values are taken from the literature The ionization potential of a ²⁰ -octafluorene neat film was determined to be 5.9 eV by atmospheric photoelectron spectroscopy (Supplementary Data). 85).Using the optical band gap value of 2.9 eV determined from the absorption spectrum of the neat film, the electron affinity of octafluorene can be estimated to be about 3 eV. As can be seen in Fig. 86a (see Supplementary Information), these OLEDs The EL spectra measured at 10 mA/cm ² are similar to the PL spectra measured in the octafluorene neat film and the CBP blend, and it can be seen that the blue EL emitted from these devices comes only from the octafluorene chromophore. The current-density-voltage-brightness ( JVL ) curve of the device is shown in Figure 86b (see Supplementary Information), OLEDs based on octafluorene neat film, 10 wt% CBP blend and 20 wt% CBP blend were 1 cd/m ² shows the driving voltages of 5.0, 4.9 and 4.5 V, respectively. The highest luminance values obtained for these OLEDs were 4580 cd/m ² (at 12.6 V) for the neat film and 852 for the 20 wt% blend. 0 cd/m ² (at 10.4 V), and 8370 cd/m ² (at 11.2 V) for the 10 wt % blend. The external quantum efficiency (η _ext ) of the device is plotted as a function of the current density in Fig. 84b. Their maximum was found to be 3.9% for the neat membrane, 4.3% for the 20 wt% blend, and 4.4% for the 10 wt% blend. The difference in efficiency cannot be explained by the PLQY values of the three films, and these values are almost identical. Indeed, in a recent study on the molecular orientation of oligofluorene molecules in spin-coated thin films, 20 wt% and 10 wt% octafluorene:CBP blends showed relatively good octafluorene molecules even if randomly oriented in the neat film. It was verified that the fluorene molecule shows horizontal orientation. ²⁶ This horizontal molecular orientation of the emission dipole tends to improve the light outcoupling efficiency, which may explain the slightly higher η _ext values measured in OLEDs based on the CBP blend. ^20,26 With respect to organic laser diodes, the maximum η _ext values obtained in these OLEDs are clearly promising. However, before seriously considering these octafluorene derivatives as candidates for electrically driven organic laser-operated devices, much higher current densities must be injected into the device and the efficiency roll-off that occurs at current densities higher than 100 mA/cm ² Improvements in the device architecture need to be restrained in future work.

In summary, this study demonstrates that the ASE threshold is an unprecedentedly low 90 nJ/cm ² in non-polymeric organic thin films. This achievement was achieved by using an octafluorene derivative showing a PLQY of 87% and a large radiation attenuation rate of 1.7 x 10 ⁹ s ^-1 in the spin-coated neat film. This blue light-emitting material has been used in low-threshold organic semiconductor DFB lasers and in fluorescent OLEDs with high external quantum efficiencies of 4.4% and maximum luminance values approaching 10,000 cd/m ² . Overall, this study provides evidence that octafluorene derivatives are excellent organic materials for organic semiconductor lasers.

For all experimental procedures used in this study, absorption and fluorescence spectra of CBP blends, elliptically polarized reflection data, additional ASE characterization results, and HOMO and LUMO determinations in octafluorene films, see See Supplementary Material.

References

1. I. D. W. Samuel and G. A. Turnbull, Chem. Rev. 107, 1272 (2007).

and S. Forget, Polym. Int. 61, 390 (2012).2. S.

and S. Forget, Polym. Int. 61, 390 (2012).

3. A. J. C. Kuehne and M. C. Gather, Chem. Rev. in press, DOI: 10.1021/acs.chemrev.6b00172.

4. Y. Wang, P. O. Morawska, A. L. Kanibolotsky, P. J. Skabara, G. A. Turnbull and I. D. W. Samuel, Laser & Photon. Rev. 7, L71-L76 (2013).

5. A. Rose, Z. G. Zhu, C. F. Madigan, T. M. Swager and V. Bulovic, Nature 434, 876 (2005).

6. I. D. W. Samuel, E. B. Namdas and G. A. Turnbull, Nature Photon. 3, 546 (2009).

7. S. Z. Bisri, T. Takenobu and Y. Iwasa, J. Mater. Chem. C 2, 2827 (2014).

8. A. S. D. Sandanayaka, T. Matsushima, K. Yoshida, M. Inoue, T. Fujihara, K. Goushi, J. C. Ribierre and C. Adachi, Adv. Opt. Mater. 4, 834 (2016).

9. A. S. D. Sandanayaka, L. Zhao, D. Pitrat, J. C. Mulatier, T. Matsushima, J. H. Kim, J. C. Ribierre and C. Adachi, Appl. Phys. Lett. 108, 223301 (2016).

10. K. Hayashi, H. Nakanotani, M. Inoue, K. Yoshida, O. Mikhnenko, T. Q. Nguyen and C. Adachi, Appl. Phys. Lett. 106, 093301 (2015).

11. F.-H. Kan and F. Gan, Laser Materials (World Scientific, Singapore, 1995).

12. T. Aimono, Y. Kawamura, K. Goushi, H. Yamamoto, H. Sasabe and C. Adachi, Appl. Phys. Lett. 86, 071110 (2005).

13. H. Nakanotani, S. Akiyama, D. Ohnishi, M. Moriwake, M. Yahiro, T. Yoshihara, S. Tobita and C. Adachi, Adv. Funct. Mater. 17, 2328 (2007).

14. L. Zhao, M. Inoue, K. Yoshida, A. S. D. Sandanayaka, J. H. Kim, J. C. Ribierre and C. Adachi, IEEE J. Sel. Top. Quant. Electron. 22, 1300209 (2016).

15. J. C. Ribierre, G. Tsiminis, S. Richardson, G. A. Turnbull, I. D. W. Samuel, H. S. Barcena and P. L. Burn, Appl. Phys. Lett. 91, 081108 (2007).

16. T. W. Lee, O. O. Park, D. H. Choi, H. N. Cho and Y. C. Kim, Appl. Phys. Lett. 81, 424 (2002).

and U. Lemmer, Appl. Phys. Lett. 90, 131104 (2007).17. C. Karnutsch, C. Pflumm, G. Heliotis, JC DeMello, DDC Bradley, J. Wang, T. Weimann, V. Haug, C.

and U. Lemmer, Appl. Phys. Lett. 90, 131104 (2007).

18. E. Y. Choi, L. Mazur, L. Mager, M. Gwon, D. Pitrat, J. C. Mulatier, C. Monnereau, A. Fort, A. J. Attias, K. Dorkenoo, J. E. Kwon, Y. Xiao, K. Matczyszyn, M. Samoc, D.-W. Kim, A. Nakao, B. Heinrich, D. Hashizume, M. Uchiyama, S. Y. Park, F. Mathevet, T. Aoyama, C. Andraud, J. W. Wu, A. Barsella and J. C. Ribierre, Phys. Chem. Chem. Phys. 16, 16941 (2014).

19. J.-H. Kim, M. Inoue, L. Zhao, T. Komino, S. Seo, J. C. Ribierre and C. Adachi, Appl. Phys. Lett. 106, 053302 (2015).

20. L. Zhao, T. Komino, M. Inoue, J. H. Kim, J. C. Ribierre and C. Adachi, Appl. Phys. Lett. 106, 063301 (2015).

21. M. D. McGehee and A. J. Heeger, Adv. Mater. 12, 1655 (2000).

22. M. Anni, A. Perulli and G. Monti, J. Appl. Phys. 111, 093109 (2012).

23. G. Heliotis, D. D. C. Bradley, G. A. Turnbull, and I. D. W. Samuel, Appl. Phys. Lett. 81, 415 (2002).

23. J. C. Ribierre, A. Ruseckas, I. D. W. Samuel, H. S. Barcena and P. L. Burn, J. Chem. Phys. 128, 204703 (2008).

and C. Adachi, Chem. Comm. 52, 3103 (2016).24. JC Ribierre, L. Zhao, M. Inoue, PO Schwartz, JH Kim, K. Yoshida, ASD Sandayanaka, H. Nakanotani, L. Mager, S.

and C. Adachi, Chem. Comm. 52, 3103 (2016).

25. J. C. Ribierre, A. Ruseckas, O. Gaudin, I. D. W. Samuel, H. S. Barcena, S. V. Staton and P. L. Burn, Org. Electron. 10, 803 (2009).

26. L. Zhao, T. Komino, D. H. Kim, M. H. Sazzad, D. Pitrat, J. C. Mulatier, C. Andraud, J. C. Ribierre and C. Adachi, J. Mater. Chem. C 4, 11557 (2016).

experimental procedure

mineral physics and ASE measurements

The octafluorene derivative was synthesized according to a method known in the literature. ^{1 The} fused silica substrate was ultrasonically cleaned in detergent, pure water, acetone and isopropanol, followed by UV-ozone treatment. The octafluorene neat film and the CBP:octafluorene blend film were deposited on the fused silica substrate by spin coating with a chloroform solution in a nitrogen-filled glove box. It should be noted that the concentration of the solution and the spin speed were varied to control the thickness of the octafluorene knit film. Absorption and steady-state emission spectra were measured using a UV-vis spectrophotometer (Perkin-Elmer Lambda 950-PKA) and a spectrofluorometer (Jasco FP-6500), respectively. The PLQY of the film was measured using a xenon lamp with an excitation wavelength of 340 nm and an integrating sphere (C11347-11 Quantaurus QY, Hamamatsu Photonics). PL attenuation was measured using a streak camera and a Ti-sapphire laser system (Millenia Prime, Spectra Physics) delivering optical pulses with a width of 10 ps and a wavelength of 365 nm.

Variable incident angle spectral ellipticity (VASE) (J.A. Wollam, M-2000U) measurements were performed in steps of 5 degrees at different angles from 45 degrees to 75 degrees on a 75 nm thick octafluorene knit film. The elliptically polarized reflection data were then analyzed using analysis software (J.A. Woollam, WVASE32) to determine the anisotropic extinction coefficient and refractive index of the film.

For characterization of ASE properties, samples were optically pumped with a pulsed nitrogen laser (KEN2020, Usho) emitting at 337 nm. The laser delivers pulses with a pulse duration of 800 ps at a repetition rate of 10 Hz. The pump beam intensity was varied using a set of neutral density filters. The pump beam was focused on a 0.5 cm x 0.08 cm stripe. Emission spectra from the edges of the organic thin film were collected using an optical fiber connected to a charge-coupled device spectrometer (PMA-11, Hamamatsu Photonics).

abandonment DFB Fabrication and Characterization of Lasers

A silicon substrate having a thermally grown 1 μm thick SiO ₂ layer was cleaned according to the same cleaning procedure as above. Hexamethyldisilazane (HMDS) was spin coated on top of the SiO ₂ surface and the sample was annealed at 120° C. for 2 minutes. Then, a 70 nm thick resist layer was spin-coated on the substrate from the ZEP520A-7 solution (Zeon Co.) and annealed at 180° C. for 4 minutes. The DFB grating on the resist layer was then patterned using a JBX-5500SC system (JEOL) using electron beam lithography. After electron beam irradiation, the pattern was developed in a developing solution (ZED-N50, Zeon Co.). In the next step, the patterned resist layer acts as an etch mask. The substrate was plasma etched with CHF ₃ using an EIS-200ERT etch system (ELIONIX). Finally, the substrate was plasma etched with O ₂ using a FA-1EA etching system (SAMCO) to completely remove the resist layer. The quality of the DFB grating was checked using SEM (SU8000, Hitachi). In order to complete the organic laser device, a 260 nm thick octafluorene neat film was finally spin coated on the DFB grating from a chloroform solution.

For laser actuation, pulsed excitation light from a nitrogen gas laser (SRS, NL-100) was focused through a lens and a slit to an area of 6 X 10 ^-3 cm ² of the device. The excitation wavelength was 337 nm, the pulse width was 3.5 ns, and the repetition rate was 20 Hz. The excitation light was incident on the device at about 20 degrees perpendicular to the plane of the device. The emitted light was collected perpendicular to the device surface using an optical fiber 6 cm away from the device connected to a multichannel spectrometer (PMA-50, Hamamatsu Photonics). Excitation intensity was controlled using a set of neutral density filters.

Fabrication and Characterization of OLEDs

OLEDs were fabricated by depositing an organic layer and a cathode on a pre-cleaned ITO glass substrate. The structures of the OLEDs fabricated in this study were as follows: ITO (100 nm)/PEDOT:PSS (45 nm)/EML (~ 40 nm)/PPT (10 nm)/TPBi (55 nm)/LiF (1 nm) )/Al (100 nm). The light emitting layer (EML) corresponds to an octafluorene neat film or an octafluorene:CBP blend. A PEDOT:PSS layer was spin coated onto ITO and annealed at 130° C. for 30 min. Octafluorene neat membranes and blend membranes were spin coated from chloroform solution on top of the PEDOT:PSS layer in a glove box environment. The thickness of the EML layer was typically about 40 nm. Then, a 10 nm thick PPT layer and a 40 nm thick TPBi layer were deposited by thermal evaporation. Finally, a cathode made of a thin LiF layer and a 100 nm thick Al layer was prepared by thermal evaporation through a shadow mask. The active area of the device was 4 mm ² . Prior to characterization, the device was encapsulated in a nitrogen gas layer to prevent oxygen and moisture related degradation effects.

Current-density-voltage-luminance ( JVL ) characteristics under DC driving were measured using a source meter (Keithley 2400, Keithley Instruments Inc.) and an absolute external quantum efficiency measurement system (C9920-12, Hamamatsu Photonics). EL spectra were measured using an optical fiber connected to a spectrometer (PMA-12, Hamamatsu Photonics).

[1] R. Anemian, J. C. Mulatier, C. Andraud, O. Stephan, J. C. Vial, Chem. Comm. 1608 (2002).

[7] Continuous Wave Natural Amplified Emission (ASE) Experiment

Continuous wave spontaneously amplified emission (ASE) experiments were performed on bisfluorene cored dendrimers and octafluorene spin-coated knit films. The film was deposited on a pre-cleaned planar fused silica substrate and was not encapsulated. The film thickness was about 250 nm.

To study the continuous wave ASE properties, the film was optically pumped at 355 nm with a continuous wave laser diode. An acoustic-optical modulator (AOM, Gooch & Housego) operated as a pulse generator (WF 1974, NF Co.) was used to transmit pulses with different widths. Emission light was collected at the membrane edge using a streak scope (C7700, Hamamatsu Photonics) with a temporal resolution of 100 ps coupled with a digital camera (C9300, Hamamatsu Photonics). Emission intensity was recorded using a photomultiplier tube (PMT) (C9525-02, Hamamatsu Photonics). Both the PMT response and the driven square wave signal were monitored on a multichannel oscilloscope (Agilent Technologies, MSO6104A).

In both materials, streak camera images and emission spectra at various pumping intensities show a sharp above-threshold narrowing effect and are due to stimulated emission and can be assigned to an ASE. The ASE threshold was measured from output intensity versus excitation intensity curves for different pulse widths. The results show that the ASE threshold hardly changed for pulse widths varying from 100 μs to 5 ms. It can also be seen that these ASE thresholds are consistent with those measured in these materials using a pulsed nitrogen laser (pulse width 800 ps and repetition rate 10 Hz). The possibility of obtaining continuous wave laser operation in both octafluorene and bisfluorene cored dendrimers means that the triplet loss is negligible. This is consistent with the fact that both materials exhibit very high photoluminescence quantum yields (PLQY) (92% PLQY for bisfluorene cored dendrimers and 82% PLQY for octafluorene neat films). In addition, transient absorption measurements were performed in octafluorene and bisfluorene cored dendrimer solutions to investigate triplet-triplet absorption spectra. It can be seen that there is no overlap between the ASE and triplet absorption spectra, providing clear evidence that triplet absorption does not play any detrimental role in the continuous wave laser operation in both materials.

Claims (38)

A laser having an organic layer over a distributed feedback grating structure, the laser comprising:
the organic layer comprises an organic gain medium;
The laser dispersive feedback grating structure is comprised of an insulating material formed on the surface of the transparent conductive layer. The laser according to claim 1, wherein the distributed feedback grating structure is a mixed-order distributed feedback grating structure in which a secondary Bragg scattering region is surrounded by a primary scattering region. The laser according to claim 1, wherein the distributed feedback grating structure is a mixed-order distributed feedback grating structure in which secondary Bragg scattering regions and primary scattering regions are alternately formed. The laser of claim 1 , wherein the transparent conductive layer consists of ITO. The laser of claim 1 , wherein there is no substantial spectral overlap between excited state absorption and laser action emission. The laser of claim 1 , wherein the stimulated emission cross section (σ _em ) is at least 100 times greater, or at least 400 times larger, or at least 700 times larger than the triplet excited state cross section (σ _TT ). The laser of claim 1 , wherein the organic layer comprises a host material and at least one dopant. The laser of claim 1 , wherein the organic gain layer comprises 4,4′-bis[( N -carbazole)styryl]biphenyl (BSBCz). 9. The method of claim 8, wherein the organic gain layer is 4,4'-bis[( N -carbazole)styryl]biphenyl (BSBCz) and 4,4'-bis( N -carbazolyl)-1,1' -Laser containing biphenyl (CBP). The laser of claim 1 , wherein the organic gain layer comprises a compound having at least two fluorene structures, such as heptafluorene, octafluorene and bisfluorene cored dendrimers. The laser according to claim 1, wherein the organic layer is directly or indirectly covered with sapphire or silicon. The laser according to claim 1 , wherein the organic layer is directly or indirectly covered with a fluoropolymer. 13. The laser of claim 12, wherein the insulating material is removed in the valleys of the distributed feedback grating structure. 14. The laser of claim 13, wherein the organic layer covered with the fluoropolymer is further covered with sapphire. The laser according to claim 1 , wherein the organic layer has a thickness of 100 to 300 nm. The laser of claim 1 , wherein the grating structure has a depth of less than 75 nm. The laser according to claim 1 , wherein the grating structure consists of SiO ₂ . The laser of claim 1 , wherein there is no triplet occupancy. The laser of claim 1 , which is a continuous wave laser. The laser of claim 1 , which is a quasi-continuous wave laser. The laser of claim 1 , which is an electrically driven semiconductor laser diode. The laser of claim 1 , wherein the organic gain medium is a non-polymeric gain medium. An electrically driven organic semiconductor laser comprising two electrodes, an organic optical gain layer and an optical resonator structure, comprising:
An organic optical gain layer and an optical resonator structure are formed between the two electrodes,
The optical resonator is an electrically driven organic semiconductor laser having a grating structure. 24. The organic semiconductor laser of claim 23, wherein at least one of the two electrodes is transparent. 24. The organic semiconductor laser of claim 23, wherein the optical resonator structure is comprised of a distributed feedback structure. 26. The organic semiconductor laser of claim 25, wherein the distributed feedback structure is made of an insulating material and formed on a transparent electrode. 27. The organic semiconductor laser of claim 26, wherein the insulating material is removed in the valleys of the distributed feedback grating structure. 24. The organic semiconductor laser of claim 23, wherein the optical resonator structure consists of a secondary Bragg scattering region surrounded by a primary Bragg scattering region. 24. The electrically driven organic semiconductor laser of claim 23, having no organic layer other than the organic optical gain layer. 24. The organic semiconductor laser of claim 23, wherein the organic optical gain layer comprises a non-polymeric gain medium. 24. The organic semiconductor laser of claim 23, wherein the organic optical gain layer comprises an organic compound having at least one stilbene unit. 24. The organic semiconductor laser of claim 23, wherein the organic optical gain layer comprises an organic compound having at least one fluorene unit. 24. An electrically driven organic semiconductor laser according to claim 23, which is of the edge emission type. 24. The organic semiconductor laser of claim 23, wherein charges are injected into the organic optical gain layer and excitons are generated and feedback is provided to the organic optical gain layer. The method of claim 1,
wherein the distributed feedback grating structure has a curved structure. 26. The method of claim 25,
wherein the distributed feedback structure has a curved structure. The method of claim 1,
wherein the distributed feedback grating structure has an annular structure. 26. The method of claim 25,
wherein the distributed feedback structure has an annular structure.

Images