Microscopic system and method for detecting transient carriers based on Fourier image processing

Abstract

The invention relates to a microscopic system and method for detecting transient carriers based on Fourier image processing. The transportation condition of the carriers is obtained after image signals are efficiently collected and processed. The system comprises an ultrafast laser light source module, a time control module, a beam combining module and a signal acquisition module, the ultrafast laser light source module is used for emitting pump light and probe light; the time control module is used for modulating the optical path of the probe light emitted by the ultrafast laser light source module; the beam combining module is used for combining the pump light emitted by the ultrafast laser light source module and the detection light modulated by the time control module; the signal acquisition module comprises a camera, the combined light, reflected by the sample, of the beam combining module is received through the camera, imaging is carried out, and the camera is an area-array camera.

Description

Technical Field

[0001] This application relates to the field of carrier parameter characterization of semiconductor materials, and in particular to a microscopic system and method for detecting transient carriers based on Fourier image processing. Background Technology

[0002] Transient grating technology is a tool for characterizing the ultrafast dynamics of charge carriers in semiconductor materials. It utilizes the interference of two pump beams to form a periodically distributed density grating in the sample. By measuring the diffraction of this density grating with the probe light, the attenuation information of the transient grating is recorded, thereby obtaining the transport properties and relaxation processes of various energy carriers (such as electrons, phonons, and spins) across multiple time and spatial scales. This technology is widely used in electronics, optoelectronics, and energy harvesting devices due to its advantages of flexible selection of probe length, sensitivity to in-situ transport, and ease of heterodyne detection.

[0003] However, in practice, this technique can only detect the change in intensity of a specific wave vector q over time for each periodic density grating formed on the sample surface. Since transport properties and dynamic information are often obtained from the characteristics of multiple wave vectors q, the experiment requires repeated operations to change the grating period, which greatly increases the time cost and operational difficulty.

[0004] A search of existing technical literature revealed a patent application (application number 202111450315.2) entitled "A Device and Method for Measuring Carrier Diffusion Coefficient Based on Micro-region Transient Spectroscopy." This patent includes a femtosecond laser source, a supercontinuum white light pump-probe unit, a microscopic focusing unit, and a spectral measurement unit. Femtosecond laser pulses output from the femtosecond laser source pass through the supercontinuum white light pump-probe unit to generate pump and probe light pulses with adjustable delay times. The two beams are combined and focused onto the sample by an objective lens, with the probe spot coinciding with the center of the pump spot. The divergence angle of the probe light is adjusted to increase its spot size on the sample. The area covered by the pump light is the photogenerated carrier excitation region, and the area covered by the probe light is the photogenerated carrier diffusion region. By moving the position of the fiber optic probe receiving surface relative to the probe spot in the spectral measurement unit, transient spectra of different regions of the probe spot are acquired. The carrier concentration at the corresponding position in the diffusion region and its change over time are detected to obtain parameters such as the photogenerated carrier diffusion coefficient and diffusion length. However, this patent uses point scanning to acquire signals, which has problems such as failing to obtain information about the two-dimensional spatial distribution characteristics of charge carriers and lacking information about different wave vectors obtained based on Fourier image processing. Summary of the Invention

[0005] Therefore, it is necessary to provide a microscopic system and method for detecting transient charge carriers based on Fourier image processing to address the above-mentioned technical problems, so as to obtain the transport status of charge carriers after efficiently acquiring and processing image signals.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: On the one hand, the present invention provides a microscopic system for detecting transient charge carriers based on Fourier image processing, the system comprising: an ultrafast laser source module, a time control module, a beam combining module, and a signal acquisition module; The ultrafast laser source module is used to emit pump light and probe light; The time control module is used to modulate the optical path of the probe light emitted by the ultrafast laser source module; The beam combining module is used to combine the pump light emitted by the ultrafast laser source module and the probe light modulated by the time control module. The signal acquisition module includes a camera, which receives the combined light reflected by the sample from the beam combining module and performs imaging. The camera is an area array camera.

[0007] Preferably, the ultrafast laser source module includes: a laser seed source, an optical parametric amplifier, and a compressor; The seed light emitted by the laser seed source passes through the optical parametric amplifier and then reaches the compressor. The optical parametric amplifier outputs pump light, and the compressor outputs probe light.

[0008] Preferably, the time control module includes: a delay line; The probe light emitted by the ultrafast laser source module reaches the beam combining module after passing through the delay line.

[0009] Preferably, the time control module further includes: a telephoto lens; The probe light emitted by the ultrafast laser source module passes sequentially through the delay line and the telephoto lens before reaching the beam combining module.

[0010] Preferably, the beam combining module includes: a beam combining plate; The pump light emitted by the ultrafast laser source module and the probe light modulated by the time control module are combined by the beam combiner and then reach the signal acquisition module.

[0011] Preferably, a plurality of attenuators are provided between the beam combining module and the ultrafast laser source module, and between the time control module and the ultrafast laser source module.

[0012] Preferably, the signal acquisition module further includes: a beam splitter, an imaging objective, a filter, and a focusing lens; The beam combined by the beam combining module reaches the sample after passing through the imaging objective lens. After being reflected by the sample, it passes through the beam splitter, filter and focusing lens in sequence before reaching the camera, which is placed on the focal plane of the focusing lens.

[0013] Preferably, the focusing lens is an achromatic focusing lens.

[0014] On the other hand, the present invention also provides a microscopic method for detecting transient carriers based on Fourier image processing, applied to the aforementioned microscopic system for detecting transient carriers based on Fourier image processing, the method comprising: Place the sample, turn on the ultrafast laser source module and camera, and set the parameters; The optical path of the probe light emitted by the ultrafast laser source module is modulated by the time control module to obtain the original data of the background item and the original data of the transient image signal at different times. The original data of the background term and the original data of transient image signals at different times are processed by a preset Fourier amplitude spectrum processing algorithm to obtain the carrier amplitude spectrum of the sample.

[0015] Preferably, the preset Fourier amplitude spectrum processing algorithm is calculated as follows:

[0016]

[0017] in, This indicates that the amplitude spectrum is obtained after performing a Fourier transform on the data. Represents a certain wave vector exist The amplitude spectrum with spatial distribution characteristics in the Fourier domain at time t. Indicates in At any given time, the position coordinates are... The carrier distribution under the given conditions This represents the raw data of transient image signals at different times, specifically, the data obtained by the camera from reflected light and image formation at different positions on the delay line of the time control module. The position coordinates in the image obtained by imaging at different times grayscale value, This represents the raw data for the background item, i.e., when the pump light is blocked and only the probe light provides illumination. The position coordinates in the image obtained by imaging at different times The grayscale value.

[0018] Compared with the prior art, the beneficial effects of the present invention are: This invention provides a microscopic system and method for detecting transient carriers based on Fourier image processing. By using surface detection, the reflected light after the sample is reflected is received and imaged. Then, by using Fourier image processing, multiple wave vector components q in the image are extracted all at once in the Fourier domain. This helps to simplify the experimental process of existing transient grating technology and reduce experimental time costs and operational difficulties. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of a microscopic system for detecting transient carriers based on Fourier image processing in one embodiment; Figure 2 This is a schematic diagram of the complete architecture of a microscopic system for detecting transient carriers based on Fourier image processing in one embodiment; Figure 3 This is a schematic diagram of a microscopic method for detecting transient carriers based on Fourier image processing in one embodiment; Figure 4 This is a schematic diagram of the numerical simulation results of a microscopic method for detecting transient carriers based on Fourier image processing in one embodiment; Figure 5 This is a schematic diagram of the experimental results of a microscopic method for detecting transient carriers based on Fourier image processing in one embodiment; 1. Laser seed source; 2. Parametric amplifier; 3. Compressor; 4. First mirror; 5. Second mirror; 6. Delay line; 7. Third mirror; 8. Long focal length lens; 9. Beam combiner; 10. Beam splitter; 11. Imaging objective; 12. Sample; 13. Fourth mirror; 14. Filter; 15. Focusing lens; 16. Camera. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0021] Example 1 like Figure 1 As shown, this embodiment proposes a microscopic system for detecting transient charge carriers based on Fourier image processing. The system includes: an ultrafast laser source module, a time control module, a beam combining module, and a signal acquisition module. The ultrafast laser source module is used to emit pump light and probe light; The time control module is used to modulate the optical path of the probe light emitted by the ultrafast laser source module. The core component is the delay line 6, which achieves femtosecond-level time resolution by controlling the optical path difference between the pump light and the probe light when they reach the sample. The modulated optical path can pass through twice. The beam combining module is used to combine the pump light emitted by the ultrafast laser source module and the probe light modulated by the time control module. The signal acquisition module includes a camera 16, which receives the combined light reflected by the beam combining module from the sample 12 and performs imaging. The camera 16 is an area array camera. The multi-pixel parallel acquisition and efficient data processing benefit from the spatial resolution advantage of the area array camera. In this embodiment, an area array camera is used as the signal acquisition device, which can acquire the time-varying signals of multiple pixels on the surface of the sample 12 at one time. Compared with the traditional point-by-point scanning technology, this parallel acquisition method significantly shortens the data acquisition time, while preserving the spatial distribution information of the sample 12, providing important data support for studying the non-uniformity of the sample 12.

[0022] Example 2 This embodiment further provides a supplementary description of the microscopic system for detecting transient charge carriers based on Fourier image processing proposed in the above embodiment.

[0023] The ultrafast laser source module includes: a laser seed source 1 (1030nm), an optical parametric amplifier 2, and a compressor 3. The required laser wavelength can be adjusted from 650–900nm and 1200–2500nm, and the repetition frequency can be adjusted from single pulse to 200kHz. The laser wavelength is selected according to the sample to be tested, and the repetition frequency is based on the camera 16 in the signal acquisition module being able to acquire more than 100 pulses in a single exposure. The seed light emitted by the laser seed source 1 passes through the optical parametric amplifier 2 and then reaches the compressor 3. The optical parametric amplifier 2 outputs pump light, and the compressor 3 outputs probe light. Specifically, the wavelengths of the pump light and the probe light can be freely selected according to the properties of the sample 12. The selection criteria are that the pump light excites photogenerated carriers, i.e., electron-hole pairs, in the sample 12, while the probe wavelength should be able to respond to the change in reflectivity of the excited photogenerated carriers.

[0024] The time control module includes a second reflector 5, a delay line 6, and a third reflector 7. The delay line 6 has a travel range of 200 mm and a minimum displacement of 100 mm / s, and is used to modulate the optical path of the probe light so that the probe light passes through twice. The probe light emitted by the ultrafast laser source module passes sequentially through the second reflector 5, the delay line 6, and the third reflector 7 before reaching the beam combining module.

[0025] The time control module also includes a telephoto lens 8, which defocuses the probe light on the surface of the sample 12 to expand the illumination area of ​​the probe light. The probe light emitted by the ultrafast laser source module passes sequentially through the second reflector 5, the delay line 6, the third reflector 7, and the telephoto lens 8 before reaching the beam combining module.

[0026] The beam combining module includes: a first reflector 4 and a beam combining plate 9, used to combine pump light and probe light; The pump light emitted by the ultrafast laser source module is reflected by the first reflector 4 and then reaches the beam combiner 9. The pump light reflected by the first reflector 4 and the probe light modulated by the time control module are combined by the beam combiner 9 and then reach the signal acquisition module.

[0027] Several attenuation plates are provided between the beam combining module and the ultrafast laser source module, and between the time control module and the ultrafast laser source module. The attenuation plates are used to adjust the single pulse energy density reaching the sample 12.

[0028] The signal acquisition module also includes: a beam splitter 10, an imaging objective lens 11, a fourth reflecting mirror 13, a filter 14, and a focusing lens 15; The beam combined by the beam combining module passes through the imaging objective lens 11 and reaches the sample 12. After being reflected by the sample 12, it passes through the beam splitter 10, the fourth reflecting mirror 13, the filter 14 and the focusing lens 15 in sequence before reaching the camera 16. The camera 16 is placed on the focal plane of the focusing lens 15, thus forming a standard infinity microscopic imaging system. Specifically, a beam splitter 10 and an imaging objective lens 11 are sequentially arranged along the first axis. The beam splitter 10 is placed between the beam combiner 9 and the imaging objective lens 11 in the detection optical path, at a 45° angle to the optical axis, and is used to receive the transient image signal reflected from the sample 12. The imaging objective lens 11 is positioned above the sample 12, and the magnification of the imaging objective lens 11 is selected to allow clear viewing of the surface details of the sample 12, with a magnification of 50x or higher. The sample 12 is on the focal plane of the imaging objective lens 11 and is perpendicular to the center of the incident light source. A second beam splitter 10 is sequentially arranged along the second axis. The system consists of four reflectors 13, a filter 14, a focusing lens 15, and a camera 16. The light reflected from the sample 12 passes sequentially through the beam splitter 10 and the fourth reflector 13 before reaching the filter 14. The fourth reflector 13 is placed between the beam splitter 10 and the filter 14 at a 45° angle to the optical axis. The filter 14 is used to filter the light reflected from the sample 12, with the parameter selected to filter out the pump light. The focusing lens 15 is used to focus the light filtered by the filter 14 through the probe light. The camera 16 is used to capture the light focused by the focusing lens 15 to obtain a transient image signal.

[0029] The focusing lens 15 is an achromatic focusing lens.

[0030] like Figure 2As shown, the system is a Fourier image processing-based microscopy system for detecting transient charge carriers, with the addition of a first mirror 4, a second mirror 5, a third mirror 7, and a fourth mirror 13. It eliminates the need for complex optical modulation devices, reducing hardware costs and simplifying system debugging and maintenance. It can be directly integrated into conventional optical microscopy platforms without requiring large-scale modifications to existing equipment. This compatibility enables the system to be rapidly applied in fields such as ultrafast spectroscopy and micro / nano material characterization, reducing equipment upgrade costs for users.

[0031] Example 3 like Figure 3 As shown, this embodiment proposes a microscopic method for detecting transient carriers based on Fourier image processing, applied to the aforementioned microscopic system for detecting transient carriers based on Fourier image processing. The method includes: Place the sample, turn on the ultrafast laser source module and camera, and set the parameters; Specifically, the sample 12 is placed on the focal plane of the imaging objective lens 11, the ultrafast laser source module is turned on, a suitable wavelength and repetition frequency are selected, the camera 16 is turned on, the required operating parameters are set, an appropriate amount of attenuator is added, and the energy density of the laser single pulse reaching the surface of the sample 12 is controlled.

[0032] The optical path of the probe light emitted by the ultrafast laser source module is modulated by the time control module to obtain the original data of the background item and the original data of the transient image signal at different times. Specifically, the position of the delay line 6 is fixed, the pump light and the probe light are applied to the sample 12, and a series of images are captured by the camera 16 and stored as raw data of the transient image signal of the pump-probe at zero point.

[0033] At the same delay line 6 position, the pump light is blocked, and only the probe light illuminates the sample 12. A series of images are then captured by the camera 16 and stored as the raw data for the background item.

[0034] The position of delay line 6 was changed multiple times, and a series of images were taken by camera 16 at different positions. These images, along with the raw transient image signal data at the zero point, were stored together as the raw transient image signal data of pump-probe at different times.

[0035] The original data of the background term and the original data of transient image signals at different times are processed by a preset Fourier amplitude spectrum processing algorithm to obtain the carrier amplitude spectrum of the sample.

[0036] Specifically, the raw data is input into a computer and processed using a preset Fourier amplitude spectrum processing algorithm to obtain the carrier amplitude spectrum of the sample. A single-exponential fitting is then performed on the decay curve of the amplitude spectrum to obtain the carrier lifetime and bipolar diffusion coefficient of the sample. In image processing, the Fourier transform is a mathematical tool that converts an image from the spatial domain (i.e., an image represented by pixels) to the frequency domain (i.e., a representation by frequency components). In the frequency domain, high-frequency components correspond to image edges, details, and noise, while low-frequency components represent smooth regions and the overall structure. After the Fourier transform, any image can be decomposed into a superposition of trigonometric functions with different frequencies, amplitudes, and phases. And a wave vector... Each corresponds to one of the trigonometric functions. It is a physical quantity in the Fourier domain that reflects the spatial distribution information of the image in real space, recording the transport of photogenerated carriers excited after a grating of period L pumps the material. Its effective range is related to the diffusion distribution morphology of the photogenerated carriers. In transient grating technology, a density grating can only measure one corresponding wave vector. The evolution of the sine wave is observed, and in this embodiment, multiple values ​​can be obtained at once simply by measuring the attenuation of any image. The sinusoidal attenuation trend is because in the Fourier domain any image can be viewed as multiple... The superposition of sine waves.

[0037] The preset Fourier amplitude spectrum processing algorithm is calculated as follows:

[0038]

[0039] in, This indicates that the amplitude spectrum is obtained after performing a Fourier transform on the data. Represents a certain wave vector exist The amplitude spectrum with spatial distribution characteristics in the Fourier domain at time t. Indicates in At any given time, the position coordinates are... The carrier distribution under the given conditions This represents the raw data of transient image signals at different times, specifically, the data obtained by the camera from reflected light and image formation at different positions on delay line 6 of the time control module. The position coordinates in the image obtained by imaging at different times grayscale value, This represents the raw data for the background item, i.e., when the pump light is blocked and only the probe light provides illumination. The position coordinates in the image obtained by imaging at different times grayscale value, Represents the bipolar diffusion coefficient of charge carriers. Represents a certain wave vector exist The signal amplitude at any given moment depends on the carrier relaxation process after the material is photoexcited, and includes carrier recombination and decay information. The background term representing system noise is time-independent. Specifically, for materials with isotropic carrier transport, the amplitude spectrum It exhibits rotational invariance in the frequency domain; in other words, pixels equidistant from the image center can be represented by the same spatial frequency. Therefore, in two-dimensional reciprocal space, we can radially average different components with the same spatial frequency and process them in one dimension, representing them as a wave vector. From the spatiotemporal evolution of photogenerated carriers, it can be seen that the concentration of carriers injected into the pump region is highest at the instant the material is excited by light, and then slowly diffuses outward and gradually recombines and disappears. This pattern causes a certain wave vector amplitude As time goes by The decay gradually decreases. If we perform a single exponential fit on this decay curve, the characteristic time... It can be represented as:

[0040] This represents the carrier lifetime.

[0041] like Figure 4 As shown, the diffusion coefficient was tested through numerical simulation experiments. =1 and carrier lifetime =1000, a series of images are generated. (a)-(c) record the random diffusion of photoexcited charge carriers at different times. Subsequently, Fourier transforms are performed on the images to extract their amplitude spectra. (d) shows the wave vector. The normalized amplitude spectrum from 0.13 to 0.35 evolves over time. The figure shows that the amplitude gradually decreases with increasing time, which coincides with the spatiotemporal evolution of photogenerated carriers. At the moment the material is photoexcited, the pump region has the highest concentration of injected carriers, which then slowly diffuse outwards and gradually recombine and disappear. Furthermore, as… As the value increases, the time required for the grating amplitude to disappear is shorter, thus indicating the characteristic time. have Value independence. After performing a single exponential fit on the decay curve, we will determine the different values. The characteristic time below As shown in (e), by performing a linear fit on the data points, the slope becomes the diffusion coefficient. =1.004±0.005, the reciprocal of the intercept is the carrier lifetime. =1000.7±261.7, the relative errors between the result and the preset value were 0.4% and 0.07%, respectively, verifying the feasibility of transient microscopy based on Fourier image processing to detect ultrafast carrier dynamics; like Figure 5 As shown, sample 12 in this embodiment is a single-crystal silicon thin film. The pump light wavelength is 515nm generated by a frequency doubling crystal, the probe light wavelength is 650nm, and the repetition frequency is 200kHz. (a) shows three different wave vectors. The evolution of the normalized amplitude spectrum over time satisfies a large wave vector. Carrier diffusion transport exhibits a relatively small amplitude decay characteristic time. For all detectable wave vectors... =1.05~4.82μm -1 After performing a single exponential fit on the decay curves, we will... The characteristic time below In (b), a linear fit is performed on the data points to obtain the diffusion coefficient. carrier lifetime The results are consistent with the reported nominal values, further validating the feasibility of transient microscopy for probing ultrafast carrier dynamics based on Fourier image processing. From (b), it can be intuitively seen that the 19 wave vector components in the image are processed in the Fourier domain. The fact that all samples were extracted individually at once confirms the effectiveness of surface detection methods.

Claims

1. A microscopy system for detecting transient carriers based on Fourier image processing, characterized in that, The application relates to a superfast laser light source module, a time control module, a beam combination module and a signal acquisition module. The superfast laser light source module is used for emitting pump light and probe light. The time control module is used for modulating the optical path of the probe light emitted by the superfast laser light source module. The beam combination module is used for combining the pump light emitted by the superfast laser light source module and the probe light modulated by the time control module. The signal acquisition module comprises a camera (16), the camera (16) receives the combined light of the beam combination module reflected by a sample (12) and performs imaging, and the camera (16) is a plane array camera. The superfast laser light source module comprises a laser seed source (1), an optical parametric amplifier (2) and a compressor (3).

2. The microscopic system for detecting transient carriers based on Fourier image processing according to claim 1, characterized in that, The seed light emitted by the laser seed source (1) reaches the compressor (3) after passing through the optical parametric amplifier (2), the optical parametric amplifier (2) outputs pump light, and the compressor (3) outputs probe light. The time control module comprises a delay line (6).

3. The microscopic system for detecting transient carriers based on Fourier image processing according to claim 1, wherein, The probe light emitted by the superfast laser light source module reaches the beam combination module after passing through the delay line (6). The time control module further comprises a long-focus lens (8).

4. The microscopic system for detecting transient carriers based on Fourier image processing according to claim 3, characterized in that The probe light emitted by the superfast laser light source module reaches the beam combination module after sequentially passing through the delay line (6) and the long-focus lens (8). The beam combination module comprises a beam combination sheet (9).

5. The microscopic system for detecting transient carriers based on Fourier image processing according to claim 1, wherein, The pump light emitted by the superfast laser light source module and the probe light modulated by the time control module reach the signal acquisition module after being combined through the beam combination sheet (9). A plurality of attenuation sheets are arranged between the beam combination module and the superfast laser light source module and between the time control module and the superfast laser light source module.

6. The microscopic system for detecting transient carriers based on Fourier image processing according to claim 3 or 5, characterized in that The signal acquisition module further comprises a light splitting flat sheet (10), an imaging objective lens (11), a filter sheet (14) and a focusing lens (15).

7. The microscopic system for detecting transient carriers based on Fourier image processing according to claim 1, wherein, The combined light of the beam combination module reaches the sample (12) after passing through the imaging objective lens (11), is reflected by the sample (12) and then sequentially passes through the light splitting flat sheet (10), the filter sheet (14) and the focusing lens (15) to reach the camera (16), and the camera (16) is placed on the focal plane of the focusing lens (15). The focusing lens (15) is an achromatic focusing lens.

8. The microscopic system for detecting transient carriers based on Fourier image processing according to claim 7, characterized in that The application further discloses a carrier amplitude spectrum measurement method.

9. Microscopy method for the detection of transient charge carriers based on Fourier image processing, applied to a microscopy system for the detection of transient charge carriers based on Fourier image processing according to any one of claims 1 to 8, characterized in that, The application further discloses a carrier amplitude spectrum measurement method. The preset Fourier amplitude spectrum processing algorithm calculation expression is as follows: ​ ​ 10. The microscopic method for detecting transient carriers based on Fourier image processing according to claim 9, characterized in that, ​ in, This indicates that the amplitude spectrum is obtained after performing a Fourier transform on the data. Represents a certain wave vector exist The amplitude spectrum with spatial distribution characteristics in the Fourier domain at time t. Indicates in At any given time, the position coordinates are... The carrier distribution under the given conditions This represents the raw data of the transient image signal at different times, that is, under the condition that the reflected light is received and imaged by the camera (16) at different positions of the delay line (6) of the time control module. The position coordinates in the image obtained by imaging at different times grayscale value, This represents the raw data for the background item, i.e., when the pump light is blocked and only the probe light provides illumination. The position coordinates in the image obtained by imaging at different times The grayscale value.

Images