A three-dimensional carrier diffusion microscopy imaging system based on two-photon excitation

Abstract

A three-dimensional diffusion microscopic imaging system of carriers based on two-photon excitation belongs to the field of microscopic imaging technology. According to the propagation order of light, it is composed of a wavelength-adjustable femtosecond optical device, a fully automatic optical parametric amplifier, a first achromatic lens, a pinhole, a second achromatic lens, a first aperture, a first fully polished reflector, a two-dimensional galvanometer, a scanning lens, a sleeve lens, a dichroic mirror, an objective lens, a piezoelectric displacement sample stage, a focused white light source, a second aperture, a beam splitter, a third achromatic lens, a fourth achromatic lens, a camera and a detector. MAPbI3 or MAPbBr3 crystal is placed on a high-precision piezoelectric displacement sample stage as a semiconductor sample. The present invention continuously changes the position of the excitation light beam on the sample surface by rotating the galvanometer, and the detector can be used to detect the fluorescence emitted from the same position on the sample surface, thereby obtaining the three-dimensional diffusion process information of the internal intercepted carriers of the sample under the action of the excitation light beam, and then revealing its diffusion law.

Description

Technical Field

[0001] The invention belongs to the technical field of microscopic imaging, and in particular relates to a three-dimensional carrier diffusion microscopic imaging system based on two-photon excitation. Background Art

[0002] In the thermal equilibrium state, the carrier concentration inside the semiconductor is relatively fixed. However, when certain external effects are applied, such as voltage or light, the carrier concentration inside the semiconductor will change. Taking light as an example, when the energy of the incident photon exceeds the bandgap width of the semiconductor, the photon will excite the electrons in the valence band to the conduction band, thereby generating electron-hole pairs. In this case, the carrier concentration inside the semiconductor will no longer be in equilibrium.

[0003] Perovskite crystal nanowires and nanoplates show great potential for applications in nanophotonics and optoelectronics due to their ultra-compact structures and unique optical and physical properties. However, conventional electronic and transient techniques are limited by sample size in describing the carrier behavior in these nanostructures. The behavior of these carriers, including diffusion coefficient, charge mobility, and diffusion length, is crucial for evaluating the quality of perovskite crystal nanowires (NWs) and nanoplates (NPs). The combination of these parameters determines the diffusion length of the carriers. Therefore, quantitatively describing the carrier behavior is particularly important in evaluating the quality of perovskite nanostructures.

[0004] However, due to the limitation of sample size, conventional techniques such as time-of-flight, Hall effect, dark current-voltage, fluorescence quenching, and transient spectroscopy are not applicable to measure the diffusion efficiency and charge mobility of individual perovskite NWs and NPs. The carrier lifetime can be determined by time-resolved photoluminescence.

[0005] Researchers usually use a time-resolved transient fluorescence scanning imaging microscope based on a time-correlated single-photon counting system to directly visualize the diffusion process of carriers in isolated perovskite nanowires or nanoplates. In most cases, the excitation of carriers is achieved by single-photon transitions, and their diffusion process is measured jointly by transient fluorescence and microscopic imaging systems. This method mainly observes the diffusion behavior of carriers in the two-dimensional plane direction. When a single photon excites the sample, the light-emitting area in the vertical direction of the sample is very wide, and the upper and lower areas of the beam waist emit light, resulting in poor spatial resolution of the beam in the vertical direction of the sample. Therefore, the traditional test method only converges the excitation beam to the semiconductor crystal film or surface to detect the diffusion process of carriers on the two-dimensional plane of the sample surface.

[0006] In contrast, two-photon microscopy uses the two-photon absorption characteristics of nonlinear optics to excite samples with long-wave, low-energy photons. When the laser beam converges perpendicular to the sample surface into the sample body, only the sample at the waist or focal position of the laser beam can be excited by the femtosecond laser to emit light. This method can not only achieve a deeper penetration depth than single photons, but also obtain more accurate spatial positioning in the longitudinal direction. Therefore, combining two-photon excitation technology with traditional transient fluorescence confocal microscopy systems can obtain the carrier diffusion behavior in the two-dimensional direction of the plane while accurately positioning the longitudinal position, and realize tomographic scanning of the carrier diffusion behavior in semiconductor materials at different depths. In addition, this technology can also detect deep into the interior of semiconductor materials, and its penetration depth is much higher than that of traditional single-photon transient fluorescence confocal microscopy systems. Summary of the invention

[0007] The purpose of the present invention is to provide a time-correlated single-photon counting system using a femtosecond laser with adjustable wavelength and repetition rate as a light source and a three-dimensional carrier diffusion microscopic imaging system based on two-photon excitation constructed using a two-dimensional galvanometer and microscopic imaging technology. The system has a repetition rate of up to 100 kHz, which is more suitable for detecting the diffusion behavior of carriers in semiconductor materials in three-dimensional space.

[0008] The carrier three-dimensional diffusion microscopic imaging system based on two-photon excitation of the present invention is composed of a wavelength-tunable femtosecond optical device (light Coversion), a fully automatic optical parametric amplifier (OPA), a first achromatic lens, a pinhole, a second achromatic lens, a first aperture, a first fully polished reflector, a two-dimensional galvanometer, a scanning lens, a sleeve lens, a dichroic mirror, an objective lens, a piezoelectric displacement sample stage, a focused white light source, a second aperture, a beam splitter, a third achromatic lens, a fourth achromatic lens, a camera and a detector in the order of light propagation. 3 or MAPbBr 3 The crystal is placed on a high-precision piezoelectric displacement sample stage as a semiconductor sample.

[0009] Furthermore, the reflection plane of the dichroic mirror is arranged at an angle of 45 degrees to the incident light, the optical symmetry centers of the first achromatic lens, the pinhole, the second achromatic lens, and the first aperture, i.e., the lens centers, are arranged collinearly, the optical symmetry centers of the two-dimensional galvanometer, the scanning lens, the sleeve lens, the objective lens, and the focused white light source are arranged collinearly, the optical symmetry centers of the second aperture and the fourth achromatic lens are arranged collinearly, and the piezoelectric displacement sample stage can be precisely moved in three dimensions with nanometer-level accuracy; the pinhole is arranged at the focus of the first achromatic lens; the distance between the two-dimensional galvanometer and the scanning lens is equal to the focal length of the scanning lens, the distance between the scanning lens and the sleeve lens is the sum of the focal lengths of the two lenses, and the distance between the sleeve lens and the objective lens is equal to the focal length of the sleeve lens.

[0010] The femtosecond laser emits photons, and the sample absorbs two long-wavelength photons at the same time to stimulate fluorescence. Because two-photon fluorescence excitation requires an intensity threshold, below which no fluorescence will be generated. Areas outside the focus cannot reach this threshold, so two-photon excitation can more accurately detect carrier diffusion. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 Schematic diagram of the structure of the three-dimensional two-photon transient confocal microscopy imaging system. DETAILED DESCRIPTION

[0012] Example 1: Figure 1 As shown, the various parts are: a wavelength-tunable femtosecond optical device 1, a fully automatic optical parametric amplifier 2, a first achromatic lens 3, a pinhole 4, a second achromatic lens 5, a first aperture 6, a first fully polished reflector 7, a two-dimensional galvanometer 8, a scanning lens 9, a sleeve lens 10, a dichroic mirror 11, an objective lens 12, a piezoelectric displacement sample stage 13, a focused white light source 14, a second aperture 15, a beam splitter 16, a third achromatic lens 17, a fourth achromatic lens 18, a camera 19, and a detector 20.

[0013] Figure 1In the figure, the first achromatic lens 3 is AC127-050-A-ML of Sorebo, with a focal length of 5 cm; the pinhole 4 is P50CBP of Sorebo; the second achromatic lens 5 is AC127-075-A-ML of Sorebo, with a focal length of 7.5 cm; the first aperture 6 is SM2D25 of Sorebo; the two-dimensional galvanometer 8 is DCB210-AG of Sorebo; the scanning lens 9 is SL50-3P of Sorebo, with a focal length of 5 cm; the tube lens 10 is TTL of Sorebo 200MP, focal length is 20cm; dichroic mirror 11 is DMSP425L of Sorebo; objective lens 12 is 50X or 100X objective lens; beam splitter 16 is BSW27 of Sorebo; third achromatic lens 17 is AC127-050-A-ML of Sorebo, focal length is 5cm; fourth achromatic lens 18 is AC127-050-A-ML of Sorebo, focal length is 5cm; camera 19 is TD-4K of Sanqiang TEDA; detector 20 is PD-100-CTC-FC of MICRO PHOTON DEVICES (MPD)).

[0014] First, the wavelength-tunable femtosecond optical device 1 generates a femtosecond laser with a wavelength (λ) = 315~2500nm as a pump light source (the wavelength of the pump light source is determined according to the band gap of the semiconductor sample to be detected, and the band gap is determined by the absorption wavelength of the semiconductor, that is, the band gap (eV) = 1240 / λ (nm), and the wavelength range of the pump light source must ensure that the semiconductor sample can absorb it). After adjustment by the fully automatic optical parametric amplifier 2, the infrared light with adjustable wavelength is output (the wavelength range after adjustment is 550~2000nm). The infrared light with adjustable wavelength is focused by the first achromatic lens 3 with a focal length of 5cm and forms a uniform collimated beam through the center of the pinhole 4; then, the uniform collimated beam is adjusted in the far field through the second achromatic lens 5 with a focal length of 7.5cm, so as to obtain a beam whose diameter remains unchanged as the distance changes; the beam is used as an excitation beam and passes through the first aperture 6 to make the beam uniform and conform to the Gaussian distribution, and then the beam is introduced into the two-dimensional galvanometer 8 through the first fully polished reflector 7. The two-dimensional galvanometer 8 is used to adjust the excitation of the excitation beam on the sample. The excitation beam passes through a scanning lens 9 with a focal length of 5 cm, a sleeve lens 10 with a focal length of 20 cm (the scanning lens 9 and the sleeve lens 10 ensure that when the galvanometer 8 moves parallel to the light transmission direction, the focus of the excitation beam is always on a focal plane (the focal plane is the sample surface), a dichroic mirror 11 and an objective lens 12 (50X), to obtain an excitation beam with a maximum resolution of λ / 2; the excitation beam is finally focused on the semiconductor sample of the piezoelectric displacement sample stage 13 (the focal position of the objective lens 12 is fixed, but whether the focus of the objective lens 12 is focused on the sample to be measured needs to be controlled by the movement of the piezoelectric displacement sample stage 13), thereby generating fluorescence (the sample needs to have a high light absorption rate, and can generate carriers and emit photon signals after being excited, that is, fluorescence; the sample can be MAPbBr 3 Crystal or MAPbI 3 Crystals can be prepared according to known literature).

[0015] The white light from the focused white light source 14 is irradiated onto the sample. The image on the sample passes through the objective lens 12 and the dichroic mirror 11, and then passes through the second aperture 15 (also to make the light beam uniform and conform to the Gaussian distribution and control the spot size) and the beam splitter 16 in sequence, and then is split into two beams of light. One of the beams passes through the third achromatic lens 17 with a focal length of 5 cm and is focused on the camera 19, so that the camera presents an image of the sample surface. The fluorescence generated by the excitation light beam after irradiating the sample passes through the objective lens 12 and the dichroic mirror 11, and then passes through the second aperture 15 and the beam splitter 16 in sequence, and then is split into two beams of light. One of the beams passes through the third achromatic lens 17 with a focal length of 5 cm and is focused on the camera 19. The spot of the fluorescence on the sample is observed by the camera 19, and the other beam of light passes through the fourth achromatic lens 18 and enters the detector 20. The position of the piezoelectric displacement sample stage 13 is adjusted so that the minimum size of the fluorescent spot on the sample surface is observed in the camera 19, even if the sample surface on the piezoelectric displacement sample stage 13 is strictly located on the focal plane of the objective lens 12, and then the second aperture 15 is adjusted so that the minimum size of the fluorescent spot fills the entire field of view of the camera 19, and then the white light source 14 is turned off; after passing through the fourth achromatic lens 18, it enters the detector 20 to collect the fluorescent signal, and the position of the excitation beam on the sample surface is continuously changed by rotating the two-dimensional galvanometer 8. The detector 20 can be used to detect the fluorescence emitted from the same position on the sample surface, thereby obtaining the three-dimensional diffusion process information of the carriers inside the sample under the action of the excitation beam. After collecting the fluorescence intensity data, the fluorescence dynamics can be fitted according to the following formula to obtain the diffusion parameters of the carriers (diffusion coefficient, charge mobility and recombination constant).

[0016] The photoexcited charge carrier density is determined by:

[0017]

[0018] In the formula, E The wavelength λ The energy of a single excitation pulse at ε is the absorption coefficient; d is the optical depth; h is Planck's constant, c is the speed of light, ν is the excitation volume, α is the ratio of charge produced per absorbed photon, R pump is the reflectivity of the sample when the excitation beam is incident at normal angle.

[0019] The boundary conditions for diffusion are:

[0020]

[0021] + and - represent the forward and backward first-order differentials. Gaussian functions are used to describe the initial (t =0) Distribution:

[0022]

[0023] In the formula is the carrier density at the center of the distribution; r is the distribution radius; is the total initial carrier density.

[0024] Diffusion parameters (diffusion coefficient, charge mobility and recombination constant) can be obtained from the fluorescence dynamics fitting through the diffusion model. The key to the imaging system of the present invention is to continuously change the position of the excitation beam on the sample surface by rotating the two-dimensional galvanometer 8, but the position of the fluorescence collection point on the sample surface remains unchanged, and then construct (the detector collects the fluorescence dynamics at different positions, the fluorescence lifetime and intensity at different positions will be different, and the fluorescence dynamics at these positions are fitted to construct a diffusion model) a fluorescence intensity image that can reflect the spatial distribution of charge carriers in the perovskite sample. The fluorescence dynamics at the measured position can be described by a two-dimensional diffusion model as a function reflecting the change of carrier density over time:

[0025]

[0026] in is the position at time t ( x , y ) carrier density (0≤ x ≤L x , 0≤ y ≤L;L x and L y is the side length of the crystal); D is the diffusion coefficient. is the charge recombination function, k 2 is the radiative electron-hole recombination rate constant, k 1 is the rate constant of non-radiative carrier capture caused by defects. Using the above equation, the carrier density at any location on the sample at a delay time t can be simulated. The fluorescence intensity IPL ( x , y , t ) and The diffusion model is used to fit the fluorescence dynamics, and the diffusion coefficient D and the recombination rate constant are obtained, thereby achieving the purpose of detecting the diffusion behavior of carriers in semiconductor materials in three-dimensional space.

[0027] Compared with the traditional confocal fluorescence microscopy technology, the present invention uses femtosecond laser for two-photon excitation, which achieves deep penetration excitation inside the semiconductor sample, thereby inducing the fluorescence luminescence phenomenon of the sample. Through the precise control of the high-precision piezoelectric displacement platform, the precise positioning of the excitation point inside the semiconductor sample is ensured. In this way, the diffusion behavior of carriers in the three-dimensional space inside the semiconductor sample can be imaged, thereby revealing its diffusion law.

Claims

1. A three-dimensional carrier diffusion microscopy imaging system based on two-photon excitation, characterized in that: According to the order of light propagation, the invention comprises a wavelength-tunable femtosecond optical device (1), a fully automatic optical parametric amplifier (2), a first achromatic lens (3), a pinhole (4), a second achromatic lens (5), a first aperture (6), a first fully polished reflector (7), a two-dimensional galvanometer (8), a scanning lens (9), a sleeve lens (10), a dichroic mirror (11), an objective lens (12), a piezoelectric displacement sample stage (13), a focused white light source (14), a second aperture (15), a beam splitter (16), a third achromatic lens (17), a fourth achromatic lens (18), a camera (19) and a detector (20). The semiconductor sample is placed on the high-precision piezoelectric displacement sample stage (13). The reflection plane of the dichroic mirror (11) is set at a 45-degree angle to the incident light. The first achromatic lens (3), the pinhole (4 The optical symmetry centers of the two-dimensional galvanometer (8), the scanning lens (9), the sleeve lens (10), the objective lens (12), and the focused white light source (14) are arranged in a collinear manner; the optical symmetry centers of the second diaphragm (15) and the fourth achromatic lens (18) are arranged in a collinear manner; the piezoelectric displacement sample stage (13) can be precisely moved in a three-dimensional direction with nanometer-level accuracy; the pinhole (4) is arranged at the focus of the first achromatic lens (3); the distance between the two-dimensional galvanometer (8) and the scanning lens (9) is equal to the focal length of the scanning lens (9); the distance between the scanning lens (9) and the sleeve lens (10) is the sum of the focal lengths of the two lenses; and the distance between the sleeve lens (10) and the objective lens (12) is equal to the focal length of the sleeve lens (10).

2. A three-dimensional carrier diffusion microscopy imaging system based on two-photon excitation as claimed in claim 1, characterized in that: The wavelength-tunable femtosecond optical device (1) generates a femtosecond laser with a wavelength of λ=315~2500nm as a pump light source, and after adjustment by a fully automatic optical parametric amplifier (2), outputs infrared light with a wavelength range of 550~2000nm; the wavelength-tunable infrared light is focused by a first achromatic lens (3) and passes through the center of a pinhole (4) to form a uniform collimated light beam; then, the uniform collimated light beam is adjusted in the far field by a second achromatic lens (5), thereby obtaining a light beam whose diameter remains unchanged as the distance changes; the light beam is used as an excitation light beam and passes through a first aperture (6) to make the light beam uniform and conform to a Gaussian distribution, and then the light beam is introduced into a two-dimensional galvanometer (8) through a first fully polished reflector (7). The two-dimensional galvanometer (8) is used to adjust the excitation position of the excitation light beam on the sample, and the excitation light beam passes through a scanning lens (9), a sleeve lens (10), a dichroic mirror (11) and an objective lens (12) to obtain a maximum resolution of λ / 2 The excitation light beam is finally focused on the semiconductor sample of the piezoelectric displacement sample stage (13), thereby generating fluorescence; white light from the focused white light source (14) is irradiated onto the sample, and the image on the sample passes through the objective lens (12) and the dichroic mirror (11), and then passes through the second aperture (15) and the beam splitter (16) in sequence, and then is split into two beams of light, one of which passes through the third achromatic lens (17) and then is focused on the camera (19), so that the camera presents an image of the sample surface; the fluorescence generated after the excitation light beam is irradiated onto the sample passes through the objective lens (12) and the dichroic mirror (11), and then passes through the second aperture (15) and the beam splitter (16) in sequence, and then is split into two beams of light, one of which passes through the third achromatic lens (17) and then is focused on the camera (19), so that the light spot of the fluorescence on the sample is observed by the camera (19), and the other light passes through the fourth achromatic lens (18) and enters the detector (20) to collect the fluorescence signal; The position of the piezoelectric displacement sample stage (13) is adjusted so that a fluorescent light spot of the smallest size on the sample surface is observed in the camera (19), and then the second aperture (15) is adjusted so that the fluorescent light spot of the smallest size fills the entire field of view of the camera (19), and then the white light source (14) is turned off; the position of the excitation light beam on the sample surface is continuously changed by rotating the two-dimensional galvanometer (8), and the fluorescence emitted from the same position on the sample surface is detected by the detector (20), thereby obtaining the three-dimensional diffusion process information of the intercepted current inside the sample under the action of the excitation light beam.

3. A three-dimensional carrier diffusion microscopy imaging system based on two-photon excitation as claimed in claim 1 or 2, characterized in that: The semiconductor sample is MAPbI3 or MAPbBr3 crystal.

4. A three-dimensional carrier diffusion microscopy imaging system based on two-photon excitation as claimed in claim 1 or 2, characterized in that: The focal length of the first achromatic lens (3) is 5 cm, the focal length of the second achromatic lens (5) is 7.5 cm, the focal length of the scanning lens (9) is 5 cm, the magnification of the objective lens (12) is 50X or 100X, the focal length of the third achromatic lens (17) is 5 cm, and the focal length of the fourth achromatic lens (18) is 5 cm.

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