CN114268019A - Method for realizing singlet exciton gain in semiconductor nanocrystal by using circular polarization laser - Google Patents
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- 238000000034 method Methods 0.000 title claims abstract description 22
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- 230000005283 ground state Effects 0.000 claims abstract description 31
- 238000002835 absorbance Methods 0.000 claims abstract description 27
- 238000004061 bleaching Methods 0.000 claims abstract description 27
- 238000005086 pumping Methods 0.000 claims abstract description 25
- 238000010521 absorption reaction Methods 0.000 claims abstract description 15
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Abstract
The invention provides a method for realizing singlet exciton gain in a semiconductor nanocrystal by using circular polarization laser, and relates to the technical field of laser. The method comprises the following steps: preparing a film sample by adopting semiconductor nanocrystalline, so that sample molecules are tightly filled and the position of the sample molecules is fixed; measuring the absorbance of the film sample by using an ultraviolet-visible absorption spectrometer; generating a beam of circularly polarized light by using a femtosecond laser and a circular polarizer, and focusing the circularly polarized light as pump light onto a film sample; changing the light intensity of the pumping light, and detecting the intensity of the ground state bleaching signal of the film sample by using another beam of circularly polarized light as a detection light; and judging whether the intensity of the ground state bleaching signal is equal to the absorbance, and if so, determining that the critical light intensity of the pumping light is the gain threshold of the singlet exciton.
Description
Technical Field
The invention relates to the technical field of laser, in particular to a method for realizing singlet exciton gain in a semiconductor nanocrystal by using circular polarization laser.
Background
The laser technology has wide application in the fields of communication, medical treatment, scientific research and the like, and has great market demand. Laser threshold is one of the core problems in the research of the laser field, and how to reduce the laser threshold and search for new materials with low threshold is a subject of important attention in the laser technology.
At present, the research on the laser principle tends to mature, and the understanding of the laser threshold is also gradually deepened. The basis of laser generation is the occurrence of amplified spontaneous emission. The electrons in the excited state transition back to the valence band, photons are generated through spontaneous radiation, and the photons further induce the electrons in the excited state to generate stimulated radiation, so that the purpose of light amplification is achieved. When the photons generated by the stimulated emission are larger than the stimulated absorption, the rest photons will be emitted in the form of laser. The light intensity of the incident light just meeting the laser emitting condition is the threshold value.
In recent years, semiconductor nanocrystals have been the focus of research for optical gain materials due to their excellent luminescence properties such as continuously tunable wavelength, narrow-band emission, and high photoluminescence quantum yield. However, in a semiconductor nanocrystal, electrons in an excited state are combined with valence band holes to form excitons, and the excitons are combined to form multiple excitons through interaction between the excitons, and due to auger recombination, energy of the multiple excitons is lost in the form of thermal energy, so that excited radiation efficiency is low, and a threshold value is difficult to lower.
Disclosure of Invention
In view of the above-mentioned deficiencies of the prior art, the present invention provides a method for realizing the gain of a singlet exciton in a semiconductor nanocrystal by using a circularly polarized laser.
The invention provides a method for realizing singlet exciton gain in a semiconductor nanocrystal by using circular polarization laser, which comprises the following steps: preparing a film sample by adopting semiconductor nanocrystalline, so that sample molecules are tightly filled and the position of the sample molecules is fixed; measuring the absorbance of the film sample by using an ultraviolet-visible absorption spectrometer; generating a beam of circularly polarized light by using a femtosecond laser and a circular polarizer, and focusing the circularly polarized light as pump light onto a film sample; changing the light intensity of the pumping light, and detecting the intensity of the ground state bleaching signal of the film sample by using another beam of circularly polarized light as a detection light; and judging whether the intensity of the ground state bleaching signal is equal to the absorbance, and if so, determining that the critical light intensity of the pumping light is the gain threshold of the singlet exciton.
Go toStep by step, the semiconductor nanocrystal is a perovskite nanocrystal material, and the perovskite nanocrystal material comprises CsPbBr3、CsPbI3、MAPbI3Or MAPbBr3。
Further, the step of preparing the film sample comprises: and (3) spin-coating the solution sample on a glass sheet by using a spin-coating instrument, and tightly filling sample molecules and fixing the position after the solvent is volatilized.
Further, measuring the absorbance of the film sample, comprising:
measuring the absorbance of the film sample in the wavelength range of 400 nm-600 nm, wherein the absorbance is measured according to the following formula:
in the formula, alpha0Showing the absorbance of the film sample in the wavelength range of 400 nm-600 nm; i is0Representing the original intensity of the white light; i represents the intensity of white light after transmission through the film sample without pump light excitation.
Further, the wavelength range of the pump light is 450nm to 515 nm.
Further, the wavelength range of the detection light is 450nm to 550 nm.
Further, the intensity of the ground state bleaching signal is the variation of the absorption of the thin film sample on the detection light under the conditions of pumping light and no pumping light excitation; the ground state bleaching signal intensity is measured according to the following formula:
wherein Δ α represents the ground state bleaching signal intensity of the film sample; i represents the intensity of the detection light after transmitting the film sample under the condition of no pumping light excitation; i is1The intensity of the probe light after passing through the film sample when the pump light is excited is shown.
Further, determining whether the ground state bleaching signal intensity is greater than or equal to absorbance comprises:
when alpha is0When the + delta alpha is less than or equal to 0, the stimulated radiation compensates the absorption, optical gain is generated, and the critical light intensity of the pumping light is determined to be the gain threshold value of the singlet exciton, wherein the critical light intensity of the pumping light is I1I-the intensity of the pump light.
Compared with the prior art, the method for realizing the gain of the singlet excitons in the semiconductor nanocrystal by using the circularly polarized laser provided by the invention has the advantages that the change of the absorption and stimulated emission conditions of the film sample after excitation along with the incident power is detected by changing the polarization of the excitation light, and the comparison with the absorbance shows that the threshold value of the optical gain is lower when the circularly polarized light is excited, which indicates that the gain is dominated by the singlet excitons.
Drawings
The above and other objects, features and advantages of the present invention will become more apparent from the following description of the embodiments of the present invention with reference to the accompanying drawings, in which:
FIG. 1 schematically illustrates a flow chart of a method for achieving singlet gain in a semiconductor nanocrystal using a circularly polarized laser;
FIG. 2 schematically illustrates a simplified apparatus diagram of a method of achieving singlet exciton gain in a semiconductor nanocrystal;
FIG. 3 schematically shows a schematic diagram of circularly polarized light producing a mono-exciton gain;
fig. 4 is a graph schematically showing the test results when the pump light and the probe light have circular polarizations of the same/opposite rotation directions;
fig. 5 schematically shows a graph of gain thresholds under excitation of circularly/non-circularly polarized light.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to specific embodiments and the accompanying drawings. It is to be understood that the embodiments described are only a few embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The terms "comprises," "comprising," and the like, as used herein, specify the presence of stated features, steps, operations, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, or components.
In the present invention, unless otherwise expressly specified or limited, the terms "mounted," "connected," "secured," and the like are to be construed broadly and can, for example, be fixedly connected, detachably connected, or integrally formed; can be mechanically connected, electrically connected or can communicate with each other; either directly or indirectly through intervening media, either internally or in any other suitable relationship. The specific meanings of the above terms in the present invention can be understood by those skilled in the art according to specific situations.
All terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art unless otherwise defined. It is noted that the terms used herein should be interpreted as having a meaning that is consistent with the context of this specification and should not be interpreted in an idealized or overly formal sense.
Fig. 1 schematically shows a flow diagram of a method for achieving singlet exciton gain in a semiconductor nanocrystal using a circularly polarized laser. As shown in fig. 1, the method for realizing singlet exciton gain in a semiconductor nanocrystal by using a circularly polarized laser provided in an embodiment of the present invention includes the following steps:
step S1, preparing a film sample by adopting semiconductor nanocrystals, and tightly filling sample molecules and fixing the positions of the sample molecules;
step S2, measuring the absorbance of the film sample by using an ultraviolet-visible absorption spectrometer;
step S3, generating a beam of circularly polarized light by using a femtosecond laser and a circular polarizer, and focusing the circularly polarized light as pump light onto a film sample;
step S4, changing the light intensity of the pump light, using another beam of circularly polarized light as the detection light, detecting the intensity of the ground state bleaching signal of the film sample;
and step S5, judging whether the intensity of the ground state bleaching signal is equal to the absorbance, if so, determining that the critical light intensity of the pumping light is the gain threshold of the singlet exciton.
Experiments show that the threshold value under the excitation of circularly polarized light is obviously lower compared with the threshold value under the excitation of non-circularly polarized light and is dominated by the double excitons, which indicates that the gain is dominated by the single excitons.
In the embodiment of the invention, the semiconductor nanocrystal is a perovskite nanocrystal material, and the perovskite nanocrystal material comprises CsPbBr3、CsPbI3、MAPbI3Or MAPbBr3。
In an embodiment of the present invention, the step of preparing the film sample comprises: and (3) spin-coating the solution sample on a glass sheet by using a spin-coating instrument, wherein sample molecules are tightly filled and the position is fixed after the solvent is volatilized so as to generate laser emitted along a specific direction.
In an embodiment of the present invention, measuring the absorbance of a film sample comprises: measuring the absorbance of the film sample in the wavelength range of 400 nm-600 nm, wherein the absorbance is measured according to the following formula:
in the formula, alpha0Showing the absorbance of the film sample in the wavelength range of 400 nm-600 nm; i is0Representing the original intensity of the white light; i represents the intensity of white light after transmission through the film sample without pump light excitation.
In the embodiment of the invention, the wavelength range of the pump light is 450 nm-515 nm.
In the embodiment of the invention, the wavelength range of the detection light is 450 nm-550 nm.
In the embodiment of the invention, the intensity of the ground state bleaching signal is the variable quantity of the absorption of the film sample to the detection light under the conditions of pumping light and no pumping light excitation; the ground state bleaching signal intensity is measured according to the following formula:
wherein Δ α represents the ground state bleaching signal intensity of the film sample; i represents the intensity of the detection light after transmitting the film sample under the condition of no pumping light excitation; i is1The intensity of the probe light after passing through the film sample when the pump light is excited is shown.
In the embodiment of the present invention, determining whether the intensity of the ground state bleaching signal is greater than or equal to the absorbance includes: when alpha is0When the + delta alpha is less than or equal to 0, the stimulated radiation compensates the absorption, optical gain is generated, and the critical light intensity of the pumping light is determined to be the gain threshold value of the singlet exciton, wherein the critical light intensity of the pumping light is I1I-the intensity of the pump light.
By embodiments of the present invention, absorption spectra as well as transient absorption spectra are used to measure the gain produced by a sample. Firstly, an ultraviolet-visible absorption spectrometer is used for measuring the absorbance of a sample in the range of 400 nm-600 nm, and the proportion of the original intensity of the white light to the intensity of the sample after the white light penetrates through the sample can be obtained. Then, the intensity of the detection light transmitted through the sample when the pumping light exists is obtained through an experimental device of transient absorption spectrum. The gain threshold for circularly polarized excitation is significantly reduced compared to the case of excitation by excitons dominated by non-circularly polarized excitation, indicating that its gain is dominated by singlet excitons.
The invention adopts absorption spectrum and transient absorption spectrum to measure the gain generated by a sample, and the specific device is as follows: a femtosecond pulse of 1030nm generated by a femtosecond laser is divided into two beams after passing through a beam splitter, wherein the wavelength of one beam is continuously adjustable after passing through an optical parametric amplifier and is used as pumping light. The other strand passes through BBO crystal (beta-phase barium metaborate crystal, beta-BaB)2O4) The wavelength range of the sapphire crystal is expanded to 450-550 nm to be used as detection light. By changing the time difference between the arrival of the pump light and the arrival of the probe light at the sample, the change of the sample in a period of time after the pump light excites the sample can be obtained. After the pumping light excites part of electrons to conduction band, the absorption of sample to detecting light will be reduced, and at the same time, the intensity of detecting light penetrating through sample will be greater than that without pumping due to the existence of stimulated radiationThe intensity of the probe light transmitted through the sample upon photoexcitation is called the ground state bleaching signal.
Fig. 2 schematically illustrates a simplified apparatus diagram of a method of achieving singlet exciton gain in a semiconductor nanocrystal.
To measure the gain threshold for different polarization excitations, a simplified experimental setup for the measurement of the gain of a singlet exciton as shown in fig. 2 was used. After passing through the circular polarizer, the pump light generates a beam of left-handed circularly polarized light, and the beam of left-handed circularly polarized light is incident on the sample at a certain angle; then, another left-handed circularly polarized light beam generated by a linear polarizer and an 1/4 wave plate is focused on the sample, and transmitted light is collected after passing through the sample, so that the intensity of a ground state bleaching signal of the sample is detected. And combining the intensity of the ground state bleaching signal with the absorbance to calculate a gain threshold value.
In fig. 2, two achromatic prisms and an optical fiber collector are further disposed behind the sample to be measured. It will be appreciated that the achromatic prism is mainly used for chromatic aberration correction and light focusing. The optical fiber collector is mainly used for collecting light required by testing, and the light collection efficiency is effectively improved.
Based on the above disclosure, the principle of circularly polarized light realizing the singlet exciton gain will be described in detail below.
Fig. 3 schematically shows a schematic diagram of circularly polarized light generating a mono-exciton gain. As shown in fig. 3, the electron spin can take ± 1/2, taking into account the doubly degenerate valence and conduction bands. When the non-polarized light is used for excitation, the nanocrystals in the ground state can absorb light, so that the intensity of transmitted light is reduced, the nanocrystals in the singlet exciton state can absorb a part of photons and can also generate a part of photons due to excited radiation, and the number of absorbed and generated photons is equal, so that the light intensity cannot be changed when the exciting light penetrates through the nanocrystals, and the nanocrystals are transparent to the exciting light. However, when the nanocrystal is in a double exciton state, the conduction band population is already filled, population inversion is realized, so that the nanocrystal does not absorb exciting light, but additionally emits photons due to excited radiation, and thus gain is generated. Under the condition of non-polarized light excitation, when the number of the double-exciton-state nanocrystals generated after excitation is larger than that of the nanocrystals in the ground state, photons can be emitted. In the case of circularly polarized light excitation, only electrons satisfying the transition selection rule can be excited to the conduction band. Thus, only when the nanocrystal is in the ground state will the excitation light be absorbed. While gains can already be generated when the nanocrystals are in the singlet exciton state. Therefore, in the case of excitation of specific circularly polarized light, the generation condition of gain is that the number of singlet exciton state nanocrystals is larger than the number of nanocrystals in the ground state. Compared with the case of non-polarized light excitation, the threshold condition to be satisfied is lower when circularly polarized light excitation is used, and the gain is mainly dominated by singlet excitons.
The effect of using circularly polarized light excitation to achieve mono-exciton gain is demonstrated experimentally below.
Fig. 4 schematically shows a graph of test results when the pump light and the probe light have circular polarizations of the same/opposite handedness. Fig. 5 schematically shows a graph of gain thresholds under excitation of circularly/non-circularly polarized light.
Example one is a spin lifetime test, in which a ground state bleaching signal of a sample is detected using left-handed and right-handed circularly polarized light, respectively, under excitation of left-handed circularly polarized light, and an experimental result as shown in fig. 4 can be obtained. After the left-handed circularly polarized light excites a sample, a signal detected by the left-handed circularly polarized light rapidly rises to the maximum value, a signal detected by the right-handed circularly polarized light slowly rises, and the signal coincide after 7ps, which shows that pumping light generates electrons with angular momentum of-1/2, because of the transition selection rule, a corresponding ground state bleaching signal can be detected only by the left-handed circularly polarized light, and the right-handed circularly polarized light gradually detects the signal as the electron spin relaxes to 1/2, so that the polarization degree generated by exciting light can be 75% by the amplitude difference of the signals of the two. This demonstrates that within 7ps, polarization of electrons can be achieved using circular polarized light excitation, energy level degeneracy is reduced, only a single exciton is generated, and feasibility of gain of the single exciton is reflected.
Since the incident light polarization can only be maintained at 7ps, the gain data of 1.5ps after excitation is mainly detected. Adjusting incident light power, recordingChange curve of ground state bleaching signal with power as alpha0When + delta alpha is less than or equal to 0, gain is generated. In order to compare the situation with the gain of the bisexciton, fig. 5 shows the graph of the change of the ground state bleaching signal with the power under the excitation of circularly polarized/non-circularly polarized light, and it can be seen that the threshold value is 140 muj/cm when the excitation of the non-circularly polarized light is used, i.e. when the gain is dominated by the bisexciton2And when excited by circularly polarized light, the threshold value is 90 muJ/cm2Significantly smaller than in the case of non-circularly polarized excitation. Thus, it was experimentally demonstrated that using circularly polarized light can achieve a mono-exciton gain.
In summary, embodiments of the present invention provide a method for realizing singlet exciton gain in a semiconductor nanocrystal by using circularly polarized laser, wherein the polarization of excitation light is changed, the change of absorption and stimulated emission of a film sample after excitation along with incident power is detected, and the comparison with absorbance shows that the threshold of optical gain is lower when circularly polarized light is excited, indicating that the gain is dominated by singlet excitons.
It should also be noted that unless steps are specifically described or must occur in sequence, the order of the steps is not limited to that listed above and may be varied or rearranged as desired. The embodiments described above may be mixed and matched with each other or with other embodiments based on design and reliability considerations, i.e., technical features in different embodiments may be freely combined to form further embodiments.
In the description of the present invention, it is to be understood that the terms "upper", "lower", "front", "rear", "left", "right", and the like indicate orientations or positional relationships based on those shown in the drawings, and are only for convenience of description and simplicity of description, but do not indicate or imply that the referred device or element must have a specific orientation, be constructed in a specific orientation, and be operated, and thus, should not be construed as limiting the present invention.
Throughout the drawings, like elements are represented by like or similar reference numerals. And conventional structures or constructions will be omitted when they may obscure the understanding of the present invention. And the shapes, sizes and positional relationships of the components in the drawings do not reflect the actual sizes, proportions and actual positional relationships. Further, in the description of the present invention, "a plurality" means at least two, e.g., two, three, etc., unless specifically limited otherwise.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are only exemplary embodiments of the present invention, and are not intended to limit the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (8)
1. A method for realizing singlet exciton gain in a semiconductor nanocrystal by using circularly polarized laser is characterized by comprising the following steps:
preparing a film sample by adopting semiconductor nanocrystalline, so that sample molecules are tightly filled and the position of the sample molecules is fixed;
measuring the absorbance of the film sample using an ultraviolet-visible absorption spectrometer;
generating a beam of circularly polarized light by using a femtosecond laser and a circular polarizer, and focusing the circularly polarized light as pump light onto the film sample;
changing the light intensity of the pumping light, and detecting the intensity of the ground state bleaching signal of the film sample by using another beam of circularly polarized light as a detection light;
and judging whether the intensity of the ground state bleaching signal is equal to the absorbance, and if so, determining that the critical light intensity of the pumping light is the gain threshold of the singlet exciton.
2. The method for achieving singlet exciton gain in a semiconductor nanocrystal with a circularly polarized laser of claim 1, wherein the semiconductor nanocrystal is a perovskite nanocrystal material comprising CsPbBr3、CsPbI3、MAPbI3Or MAPbBr3。
3. The method for achieving singlet exciton gain in a semiconductor nanocrystal with a circularly polarized laser according to claim 1, wherein the step of preparing the thin film sample comprises:
and (3) spin-coating the solution sample on a glass sheet by using a spin-coating instrument, and tightly filling sample molecules and fixing the position after the solvent is volatilized.
4. The method for realizing singlet exciton gain in a semiconductor nanocrystal with a circularly polarized laser according to claim 1, wherein the measuring the absorbance of the thin film sample comprises:
measuring the absorbance of the film sample in the wavelength range of 400 nm-600 nm, wherein the absorbance is measured according to the following formula:
in the formula, alpha0Showing the absorbance of the film sample in the wavelength range of 400 nm-600 nm; i is0Representing the original intensity of the white light; i represents the intensity of white light after transmission through the film sample without pump light excitation.
5. The method for realizing the singlet gain in the semiconductor nanocrystal by using the circularly polarized laser according to claim 1, wherein the wavelength of the pump light is in a range of 450nm to 515 nm.
6. The method for realizing the singlet gain in the semiconductor nanocrystal by using the circularly polarized laser according to claim 1, wherein the wavelength of the probe light is in a range of 450nm to 550 nm.
7. The method for realizing the singlet gain in the semiconductor nanocrystal by using the circularly polarized laser according to claim 4, wherein the ground state bleaching signal intensity is the variation of the absorption of the probe light by the thin film sample under the excitation of the pump light and the non-pump light;
the ground state bleaching signal intensity is measured according to the following formula:
wherein Δ α represents the ground state bleaching signal intensity of the film sample; i represents the intensity of white light after the white light penetrates through the film sample under the condition of no pumping light excitation; i is1The intensity of the probe light after passing through the film sample when the pump light is excited is shown.
8. The method of claim 7, wherein the determining whether the ground state bleaching signal intensity is greater than or equal to the absorbance comprises:
when alpha is0When the + delta alpha is less than or equal to 0, the stimulated radiation compensates the absorption, optical gain is generated, and the critical light intensity of the pumping light is determined to be the gain threshold value of the singlet exciton, wherein the critical light intensity of the pumping light is I1I-the intensity of the pump light.
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Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
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| US20020196827A1 (en) * | 2001-05-15 | 2002-12-26 | Kabushhiki Kaisha Toshiba | Photon source and a method of operating a photon source |
| JP2006098742A (en) * | 2004-09-29 | 2006-04-13 | National Institute Of Information & Communication Technology | Method for controlling dephasing of exciton and method for analyzing feature of environmental system causing dephasing |
| CN111665222A (en) * | 2020-07-17 | 2020-09-15 | 中国科学院长春光学精密机械与物理研究所 | Femtosecond pumping detection system and method |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US20020196827A1 (en) * | 2001-05-15 | 2002-12-26 | Kabushhiki Kaisha Toshiba | Photon source and a method of operating a photon source |
| JP2006098742A (en) * | 2004-09-29 | 2006-04-13 | National Institute Of Information & Communication Technology | Method for controlling dephasing of exciton and method for analyzing feature of environmental system causing dephasing |
| CN111665222A (en) * | 2020-07-17 | 2020-09-15 | 中国科学院长春光学精密机械与物理研究所 | Femtosecond pumping detection system and method |
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