CN112284510A

CN112284510A - Coherent acoustic phonon echo induction and detection method in multilayer two-dimensional semiconductor

Info

Publication number: CN112284510A
Application number: CN202011154395.2A
Authority: CN
Inventors: 崔乾楠; 徐春祥
Original assignee: Southeast University
Current assignee: Southeast University
Priority date: 2020-10-26
Filing date: 2020-10-26
Publication date: 2021-01-29
Anticipated expiration: 2040-10-26
Also published as: CN112284510B

Abstract

The invention discloses a method for realizing all-optical induction and detection of coherent acoustic phonon echoes in a multilayer two-dimensional semiconductor film based on femtosecond laser pumping detection. Firstly, growing or transferring a multilayer two-dimensional semiconductor film on a substrate, and determining the wavelength of pumping and detection laser according to the optical absorption spectrum of the film; then measuring a time-resolved transient absorption signal of relaxation between bands of the photo-generated carriers as a time reference node; furthermore, periodic coherent acoustic phonon echo signals surging in the transient absorption signals of the photon-generated carriers are observed by improving the pump laser power and measuring the signal to noise ratio, and the extraction of the coherent acoustic phonon echo signals is realized by utilizing a curve fitting method. The invention is based on the ultrafast spectroscopy means to induce and measure the coherent acoustic phonon echo in the multilayer two-dimensional semiconductor, does not need electric devices such as an acoustic transducer and the like, and can obtain the complete waveform information of a GHz frequency coherent acoustic phonon wave packet.

Description

Coherent acoustic phonon echo induction and detection method in multilayer two-dimensional semiconductor

Technical Field

The invention relates to a femtosecond laser pumping detection technology, in particular to an all-optical induction and detection method of coherent acoustic phonon echoes in a multilayer two-dimensional semiconductor.

Background

The multilayer two-dimensional semiconductor film with indirect band gap, such as molybdenum disulfide, has good stability and photoelectric and mechanical properties, is a unique photoacoustic coupling system, has great potential for developing novel light-operated phonon information devices, and has started to attract the attention of scientists in recent years. When interacting with pulsed laser light, a traveling wave of high-frequency ultrasound can be radiated due to the photoacoustic effect. The high-frequency ultrasonic traveling wave is essentially a coherent acoustic phonon wave packet induced and directionally transmitted by lattice orientation resonance, the frequency reaches GHz magnitude, and is far higher than the upper frequency detection limit (MHz) of the traditional ultrasonic transducer based on piezoelectric effect and the like. In the traditional technology, direct observation on information such as the phase of a coherent acoustic phonon wave packet cannot be realized. Due to the lack of direct observation technology for high-frequency coherent acoustic phonon wave packets in the multilayer two-dimensional semiconductor, the understanding of complex transient coupling processes of light, sound, electricity, force and the like in the multilayer two-dimensional semiconductor is not deep, and the development of the light-operated phonon information device based on the multilayer two-dimensional semiconductor film is severely restricted.

Disclosure of Invention

The purpose of the invention is as follows: aiming at the defects of the prior art, the invention provides an all-optical induction and detection method of coherent acoustic phonon echoes in a multilayer two-dimensional semiconductor, which achieves the aim of directly observing the multiple reflection process of coherent acoustic phonon packets on the surface of a thin film.

The technical scheme is as follows: the invention discloses an all-optical induction and detection method of coherent acoustic phonon echoes in a multilayer two-dimensional semiconductor, which comprises the following steps of: (1) growing or transferring a multi-layer two-dimensional semiconductor sample on a substrate; (2) measuring the optical absorption spectrum of a sample, and selecting pumping and detecting laser wavelength according to the exciton resonance peak position; (3) pumping detection pulses are vertically incident and focused on the surface of the multilayer two-dimensional semiconductor through a microscope objective, and the space collimation characteristics of pumping detection laser spots are controlled through a lens to optimize time resolution signals of photon-generated carrier transient absorption; (4) improving the power of pumping laser, inducing coherent acoustic phonon packets, and observing the emergence of coherent acoustic phonon echo signals; (5) the signal-to-noise ratio of the detection laser differential reflection signal is improved by adopting balanced detection, and a coherent acoustic phonon echo signal is measured; (6) and fitting the measured transient absorption time-resolved signal by adopting a double-exponential decay function, and subtracting the fitted signal from the original signal to extract a coherent acoustic phonon echo signal.

In the step (1), the two-dimensional semiconductor sample is prepared by a chemical vapor deposition or mechanical stripping method, and the substrate is made of a transparent material.

The photon energy of the pump laser in the step (2) is at least 1eV, and the photon energy of the detection laser is equal to the photon energy corresponding to the measured exciton formant.

The pumping detection pulse in the step (3) adopts a micro-area femtosecond laser pumping detection system, and the detection laser adopts an emission type light path; the time delay scanning of the pumping and detecting pulses is controlled by an optical delay line, and the vertical incidence and the space focusing of pumping and detecting laser spots are carried out by a microscope objective. The pump laser power is smaller than the damage threshold of the multilayer two-dimensional semiconductor, and the detection laser power is smaller than the pump laser power. And (5) adopting a lock direction amplification technology for balance detection. In the step of curve fitting, a double-exponential decay function is adopted for function fitting.

According to the invention, a beam of pumping femtosecond laser and a beam of detecting femtosecond laser are vertically focused to the same point of a sample through a micro-area micro-optical system; the photon energy of the pump laser is greater than the A exciton resonance energy of the multilayer two-dimensional semiconductor, and the photon energy of the detection laser is equal to the A exciton resonance energy of the multilayer two-dimensional semiconductor; the reflected component of the detected laser beam after being incident on the sample is led into one probe of the balance detector; the signal-to-noise ratio of the detected laser differential reflection signal is improved by adopting a balance detector and a phase-locked amplification means; measuring and simultaneously obtaining signals of photogenerated carriers and coherent acoustic phonon echoes evolving along with time by changing time delay of pumping and detection pulses; obtaining coherent acoustic phonon echo signals through curve fitting and separation

Has the advantages that: compared with the prior art, the invention has the following remarkable advantages: the method can solve the problems of all-optical induction and direct observation of complete waveform information of a coherent acoustic phonon packet with GHz frequency in a multilayer two-dimensional semiconductor, is beneficial to calculating and analyzing basic physical parameters such as sound velocity of the multilayer two-dimensional semiconductor, has the measured time resolution up to 150fs, and can directly and accurately observe the waveform and the time evolution ultrafast process of each level of echo of the coherent acoustic phonon.

Drawings

FIG. 1 is an optical absorption spectrum of a multilayer two-dimensional semiconductor thin film of the present invention;

FIG. 2 is a composite signal diagram of photogenerated carriers and coherent acoustic phonon echoes in a multilayer two-dimensional semiconductor;

fig. 3 shows the separated coherent acoustic phonon echo signals in the embodiment of the present invention.

Detailed Description

The technical scheme of the invention is further explained by combining the attached drawings.

The purpose of this example is to measure the coherent acoustic phonon echo of a multilayer molybdenum disulfide sample.

The laser is selected as a titanium gem femtosecond laser (the pulse width is about 100fs) with the repetition frequency of 80MHz, and a femtosecond laser pumping detection system with the output wavelength of 800nm is adopted; a microscope objective with a numerical aperture of 0.4 is adopted; the signal-to-noise ratio is improved by adopting phase-locked amplification detection; the optical chopper modulates the pump laser, and the chopping frequency is about 4 kHz; the phase lock amplifier selects SR 830.

A multi-layered two-dimensional molybdenum sample of about 200nm thickness was prepared on a quartz substrate using mechanical stripping and its micro-domain optical absorption spectrum was measured (as shown in fig. 1). According to the optical absorption spectrum, the sample is an indirect band gap semiconductor, the 1s exciton resonance peak is about 660nm, and therefore 400nm femtosecond laser is selected as the pump, and 660nm femtosecond laser is selected as the probe.

After being combined, pumping and detection lasers are focused on the upper surface of the multilayer two-dimensional molybdenum sample; meanwhile, the microscope objective collects the reflected detection laser and focuses the reflected detection laser on a probe of the balanced photoelectric detector; an optical beam splitter is adopted to split part of the detection laser before entering the micro objective lens, and the split detection laser passes through an adjustable optical attenuation sheet and is focused on another probe for balancing photoelectric detection.

Adjusting the power of pumping laser and the power of detection laser to 10uW and 5uW respectively, and enabling light spots of the pumping laser and the detection laser to coincide with each other in space and focus on the same position of a sample; blocking the pump laser, carrying out optical chopping on the detection laser, and adjusting an optical attenuation sheet to enable the SR830 voltage reading to tend to zero so as to realize balanced detection; introducing pump laser into a light path, and carrying out optical chopping on the pump laser; and time scanning is carried out through an optical delay line, and a detection laser differential reflection signal caused by injection of photon-generated carriers is observed and obtained. Further, the differential reflection signal of the detection laser is adjusted to be strongest by optimizing conditions of collimation, focusing and the like of the pumping and the detection laser.

On the basis, the other conditions are kept unchanged, the pump laser power is increased to 100uW, time scanning is carried out again, and periodic coherent acoustic phonon echo signals emerging on the detection laser differential reflection signals are observed, as shown in FIG. 2. Fitting the measured composite signal by using a double-exponential decay function; the fitting signal is subtracted from the original signal to obtain a coherent acoustic phonon echo signal, as shown in fig. 3.

Claims

1. A coherent acoustic phonon echo induction and detection method in a multilayer two-dimensional semiconductor is characterized by comprising the following steps: (1) growing or transferring a multi-layer two-dimensional semiconductor sample on a substrate; (2) measuring the optical absorption spectrum of a sample, and selecting pumping and detecting laser wavelength according to the exciton resonance peak position; (3) pumping and detecting pulses are vertically incident and focused on the surface of the multilayer two-dimensional semiconductor through a microscope objective, and time resolution signals of transient absorption of photon-generated carriers are optimized; (4) improving the power of pumping laser, inducing coherent acoustic phonon packets, carrying out time scanning and observing the emergence of coherent acoustic phonon echo signals; (5) a phase-locked amplifier is adopted for balanced detection, the signal-to-noise ratio of a detection laser differential reflection signal is improved, and a coherent acoustic phonon echo signal is measured; (6) and fitting the measured transient absorption time-resolved signal by adopting a double-exponential decay function, and subtracting the fitted signal from the original signal to extract a coherent acoustic phonon echo signal.

2. The coherent acoustic phonon echo inducing and detecting method in multilayer two-dimensional semiconductor according to claim 1, wherein the specific steps of the step (3) comprise: focusing the pump and detection laser on the upper surface of the multilayer two-dimensional semiconductor sample after combination; meanwhile, the microscope objective collects the reflected detection laser and focuses the reflected detection laser on a probe of the balanced photoelectric detector; an optical beam splitter is adopted to split part of the detection laser before entering the micro objective lens, and the split detection laser passes through an adjustable optical attenuation sheet and is focused on another probe for balancing photoelectric detection.

3. The method for coherent acoustic phonon echo induction and detection in multilayer two-dimensional semiconductor according to claim 1, wherein the concrete step of the step (4) comprises adjusting the power of the pump laser and the detection laser respectively and enabling the two spots to coincide with each other in space and focus on the same position of the sample; blocking the pump laser, carrying out optical chopping on the detection laser, and enabling the voltage reading of the phase-locked amplifier to tend to zero by adjusting an optical attenuation sheet; introducing pump laser into a light path, and carrying out optical chopping on the pump laser; and time scanning is carried out through the optical delay line, and a detection laser differential reflection signal caused by injection of photon-generated carriers is observed and obtained.

4. The method for coherent acoustic phonon echo induction and detection in multilayer two-dimensional semiconductor according to claim 1, wherein the two-dimensional semiconductor sample in step (1) is prepared by chemical vapor deposition or mechanical lift-off, and the substrate is made of transparent material.

5. The method according to claim 1, wherein the photon energy of the pump laser in step (2) is at least 1eV, and the photon energy of the detection laser is equal to the photon energy corresponding to the measured exciton formant.

6. The method for inducing and detecting coherent acoustic phonon echo in a multilayer two-dimensional semiconductor according to claim 1, wherein a micro-area femtosecond laser pumping detection system is adopted in the step (3), and an emission type light path is adopted for detection laser; the time delay scanning of the pumping and detecting pulses is controlled by an optical delay line, and the vertical incidence and the space focusing of pumping and detecting laser spots are carried out by a microscope objective.

7. The method of claim 1, wherein the pump laser power is less than a damage threshold of the multilayer two-dimensional semiconductor, and the probing laser power is less than the pump laser power.

8. The method of claim 1, wherein the function fitting is a bi-exponential decay function.