A measurement system and method for carrier evolution dynamics

Abstract

This invention discloses a measurement system and method for carrier evolution dynamics, belonging to the field of precision optical measurement and materials processing technology. It splits a femtosecond laser into two beams as probe and pump pulses. The probe beam uses a linear delay stage to probe the sample at different delay times. The pump beam, after frequency doubling, is further split using a combination of a half-wave plate and a polarization beam splitter to generate dual pulses. The time interval between the two pulses is adjusted by changing the tilt angle of the glass delay plate. The collected data is converted into electron density. By introducing an electron-exciton coupling term into the femtosecond laser processing model, the electron-exciton collision coefficient in the carrier excitation region is fitted, clarifying the carrier evolution dynamics and providing a quantitative physical basis for parameter optimization of time-domain shaped femtosecond laser processing technology. This invention has advantages such as non-contact operation, high precision, ease of operation, and stable performance.

Description

Technical Field

[0001] This invention belongs to the field of precision optical measurement and materials processing technology, specifically referring to a measurement system and method for the dynamic process of charge carrier evolution. Background Technology

[0002] With the rapid development of micro-nano manufacturing and ultrafast laser processing technologies, time-domain shaped femtosecond lasers have become a powerful tool for achieving efficient and high-precision manufacturing. In the interaction between femtosecond lasers and ultrafast exciton-responsive materials, the spatiotemporal evolution of free electron density determines the final processing result. However, due to the high peak power and nonlinear effects of femtosecond lasers, the material is in a non-equilibrium state, involving multiphoton ionization, collisional ionization, and complex carrier dynamics processes.

[0003] Especially in dual-pulse processing, the self-trapped excitons (STEs) induced by the first pulse play a crucial role in the energy absorption and material property transformation of subsequent pulses. This technique, which utilizes the specific allocation of two beams over a specific time scale to modulate the material response, has demonstrated outstanding performance in multiple scientific and engineering fields. For example, in dual-pulse laser-induced breakdown spectroscopy, the re-excitation and spatial confinement of the initial plasma by the secondary laser can increase the intensity of atomic emission signals by orders of magnitude, optimizing the detection limits and stability of trace elements. It is now widely used in environmental pollutant monitoring, online analysis in industrial metallurgy, and deep-space material composition detection. In the electronics and semiconductor fields, dual-pulse testing is a fundamental experimental method for analyzing the dynamic characteristics of power switching devices, obtaining key parameters such as switching losses and current spikes, and is applicable throughout the entire device circuit design process. Furthermore, in combustion diagnostics and fluid dynamics research, secondary laser combined with fluorescence calibration technology, with its nanosecond-level time resolution and spatial synchronization capabilities, has become a core means of capturing transient evolution of turbulence, analyzing the dynamic paths of chemical reactions, and microscale flow characteristics. And when studying the dynamic processes of carrier evolution, the introduction of dual-pulse technology has profound physical necessity. Dual-pulse technology allows for precise intervention in the energy deposition process. By accurately controlling the time interval between two pulses, the evolution cycle of charge carriers can be cleverly utilized. This time-control provides additional degrees of freedom to the processing, enabling researchers to artificially intervene in the dynamics of trapped excitons and thus study the dynamics of charge carrier evolution.

[0004] Currently, researchers mainly utilize techniques such as time-resolved transient absorption spectroscopy to probe the dynamics of trapped excitons, obtaining key parameters such as free electron decay time, trapping time, and exciton lifetime. However, existing theoretical models mostly focus on the generation and decay of free electron density, often neglecting the crucial collisional interactions between free electrons and excitons. This ambiguity in the collision mechanism restricts the accuracy of the theoretical models for shaping femtosecond laser processing, making it difficult to accurately predict the morphology of fabricated microstructures. Although traditional pump-probe techniques have been widely used for carrier dynamics observation, achieving high-precision measurement of the electron-exciton collision coefficient under time-domain shaping pulses and coupling it into the carrier excitation region evolution model to clarify the carrier evolution dynamics remains a major challenge in the field of ultrafast processing. There is a lack of experimental methods and devices that can directly, quantitatively, and with high spatiotemporal resolution measure the electron-exciton collision coefficient to fill the gaps in theoretical models. Therefore, measuring the electron-exciton collision coefficient is of significant practical importance for improving the theory of shaping femtosecond laser processing and optimizing manufacturing process parameters. Summary of the Invention

[0005] To address the technical problems existing in the prior art, the present invention provides a measurement system and method for measuring the dynamic process of carrier evolution, the technical solution of which is as follows: On the one hand, a measurement system for the dynamic process of carrier evolution is provided, the system comprising: Laser modulation module: used to provide a stable raw laser source and to divide the femtosecond raw laser into a pump laser beam and a probe laser beam with a preset energy ratio; The time-domain shaping module is used to modulate the pump laser into a double-pulse sequence with a preset delay time Δt. One pump pulse is focused inside the transparent medium sample to induce the generation of self-trapped excitons. The other pump pulse arrives after a set delay time through a glass delay plate, triggering the interaction between free electrons and pre-stored excitons. When the probe laser pulse passes through the carrier excitation region generated by the pump double pulse, the free electrons and self-trapped excitons in the carrier excitation region will change the refractive index of the medium. Spatiotemporal synchronization control module: used to adjust the time delay τ between the dual-pulse sequence and the probe laser, and to control the scanning position of the probe laser on the sample to be tested; Interaction and Detection Module: Used to focus the dual-pulse sequence onto the sample to generate a carrier excitation region, and the detection laser passes through the carrier excitation region; The data acquisition module is used to capture the changes in physical properties after the probe laser passes through the carrier excitation region, generate spatiotemporal distribution data, and has an improved rate equation model built in. The model includes a collision term between self-trapped excitons and free electrons. The data acquired under different spatiotemporal coordinates are fitted to obtain the electron-exciton collision coefficient, clarify the dynamic process of carrier evolution, and thus provide a quantitative physical basis for the parameter optimization of the time-domain shaping femtosecond laser processing technology.

[0006] Optionally, the pump laser beam and the probe laser beam after beam splitting by the laser modulation module have different energies. A frequency-doubler generator is set in the optical path of the pump laser to increase its energy, ensuring that the pump laser is sufficient to excite nonlinear ionization while the probe laser does not interfere with the evolution process, thus achieving physical filtering of the pump background. Specifically, this includes: A femtosecond laser (1) is used to output a femtosecond raw laser. Optical isolator (2), used to prevent back reflection from damaging or destabilizing the femtosecond laser, is placed directly in front of the femtosecond laser; A half-wave plate (3) is used to change the polarization direction of linearly polarized light, thereby enabling precise energy adjustment; The first polarization beam splitter (4), used in combination with a half-wave plate, is used to split the original laser pulse into a pump laser beam and a probe laser beam according to a preset energy ratio. The frequency doubler generator (5) is used to halve the wavelength of the pump laser to obtain higher energy.

[0007] Optionally, the time-domain shaping module includes: A half-wave plate (7) is used to change the polarization direction of linearly polarized light, thereby enabling precise energy adjustment; The second polarization beam splitter (8), used in combination with a half-wave plate, is used to redistribute the pump energy to construct a double-pulse sequence with adjustable delay Δt. The total reflection mirror (9) is used to redirect the pump laser after the second beam splitting by 90°, thereby achieving a compact optical path folding layout; The rotating delay adjustment group (10,11) includes two symmetrical glass delay plates (10,11). By utilizing the difference in the speed of light propagation in the glass medium and air, the physical optical path of the beam in the glass delay plate is changed by adjusting the tilt angle of the glass delay plate to control the delay amount. During this process, the system maintains the initial geometric optical path of the first pump pulse and the second pump pulse consistent, thereby creating different time differences to adjust the time interval Δt between the two pulses in the double pulse sequence. The delay amount is referenced to the optical path of the reference plate (12) in the perpendicular incident state as the reference zero point, thereby achieving precise adjustment of the double pulse interval. The reference plate (12) and the rotation delay adjustment group (10,11) are both made of BK7 glass of the same material, providing a static time reference zero point for the dual pulses and ensuring the accuracy of time difference control.

[0008] Optionally, the spatiotemporal synchronization control module includes: The linear delay stage (16) adjusts the delay time of the probe laser relative to the pump laser by changing the optical path length of the probe laser in a large-scale space. Dichroic mirror (13) is used to combine one pump laser and one probe laser beam; The third polarization beam splitter (14) is used to finally combine the two pump lasers and the probe lasers; The movable sample stage (18) is used to carry the sample to be tested and to place it precisely on the focal plane of the objective lens. It is the place where electron-exciton collisions and carrier evolution occur. Through spatial step scanning and synchronous acquisition by the detector, a two-dimensional spatiotemporal evolution image of carrier distribution is obtained.

[0009] Optionally, the interaction and detection module irradiates the interior of the sample with a pump double-pulse laser, exciting the sample through a nonlinear multiphoton absorption and avalanche ionization process to obtain an excited sample containing free electrons and self-trapped excitons with spatiotemporal distribution. The detection laser passes through the carrier-excited region induced by the pump double pulse, and the intensity of the detection laser after passing through the sample is collected by a detector, including: Total reflection mirror (15) is used to change the optical path direction of the probe laser, thereby achieving a compact optical path folding layout; Objective lens (17) is used to focus the three laser pulses after time-space synchronization onto the same tiny spatial region of the sample stage, ensuring that the three beams of light physically interact in the same micro-region inside the sample. A filter (19) is placed in front of the detector (20) and is specifically used to filter out strong pump laser and stray light, allowing only the detection laser wavelength carrying evolution information to pass through; The detector (20) converts the change in the intensity of the probe laser after penetrating the sample into an electrical signal, capturing the weak transmittance fluctuations caused by carrier evolution.

[0010] Optionally, the data acquisition module includes: An electro-optic modulator (6) is used to make the intensity of the pump laser change periodically at a given frequency, which is the same as the reference signal frequency of the lock-in amplifier (21). The lock-in amplifier (21) only detects the probe signal that is exactly the same as the reference frequency. The lock-in amplifier (21) is used to extract the detection signal that is completely consistent with the reference frequency from the electrical signal collected by the detector (20) according to the reference frequency signal, filter out the self-luminescence of the carrier excitation region, the DC component of the detection laser itself and the low-frequency mechanical vibration noise, and convert it into a digital signal for transmission to the computer. A computer (22) is used to perform multi-parameter fitting on the data contained in the digital signal to obtain the electron-exciton collision coefficient of the stimulated sample.

[0011] Optionally, the improved rate equation model is as follows:

[0012]

[0013] Where, n e —Free electron density; I —Laser intensity; —Ionization cross section; —Avalanche ionization coefficient; —Photoionization section; —Electron-exciton collision coefficient; —The relaxation time of electrons; —The natural lifespan of excitons; —The rate of change of free electron density over time; —Multiphoton ionization term; —Avalanche ionization term; —Exciton photoelectric ionization term; —Exciton collision ionization term; —Loss item; —The time-dependent rate of change of the self-trapped exciton density; —Photoionization loss term; —Generated item; —Natural decay term; The improved model consists of two coupled equations, which describe the free electron density. and exciton density The evolution of exciton density is described in the following equations: the electron density equation includes multiphoton ionization, avalanche ionization, exciton photoionization, and a newly added collisional ionization term; the exciton density equation describes the consumption of excitons due to ionization, their generation due to electron capture, and their own decay; and an electron-exciton collision term is introduced to describe the inelastic scattering process between photoexcited carriers and metastable excitons generated by the previous pulse. It is the electron-exciton collision coefficient; By fitting the improved model to data collected under different spatiotemporal coordinates, the electron exciton collision coefficient, which determines the degree of mutual influence between two pulses, was obtained. .

[0014] On the other hand, a method for measuring the dynamic process of carrier evolution is provided, implemented by the aforementioned measurement system for the dynamic process of carrier evolution, the method comprising: The femtosecond raw laser is emitted from right to left, first passing through a half-wave plate and the first polarization beam splitter, which divides the beam into a pump branch and a probe branch. The pump laser passes through an electro-optic modulator, and the pump laser is modulated at high frequency. Combined with the lock-in amplifier at the end, it can achieve the extraction of weak signals with an extremely high signal-to-noise ratio. The pump laser then passes through a half-wave plate and a second polarization beam splitter, splitting into two paths to become a dual-pump pulse sequence, consisting of a pre-pump pulse and a main pump pulse that arrive sequentially. The pre-pump pulse generates initial free electrons through multiphoton ionization and forms self-trapped excitons in a very short time. Before the arrival of the main pulse, the excitons evolve to a preset concentration. The main pump pulse arrives at the same region after a specific delay time. The main pump pulse triggers the collisional ionization process between free electrons and pre-stored excitons, resulting in a nonlinear increment in the finally detected electron density. Based on the exciton relaxation characteristics of the sample under test, the glass delay plate is rotated and the arrival time of the main pulse is adjusted with femtosecond precision until the enhanced transmission signal caused by electron-exciton collision is observed at the detector end. At this time, the detection laser enters the detector through the filter and the changes in the detection signal under different glass delay plate rotation angles are recorded. Subsequently, the movable sample stage moves the sample, and the detection laser scans the carrier excitation region point by point, recording the changes in the detection signal at different displacements. The electrical signal collected by the detector is sent back to the lock-in amplifier. The lock-in amplifier extracts the detection signal that is completely consistent with the reference frequency from the electrical signal collected by the detector, and converts the detection signal into a digital signal and transmits it to the computer. Ultimately, the computer merges data from countless points into a spatiotemporal shadow map that reflects the dynamic process of carrier evolution. The shadow map records the transmittance at different radial positions and at different times. The improved rate equation model is then fitted to determine the electron-exciton collision coefficient.

[0015] The beneficial effects of the technical solution provided by this invention include at least the following: 1. This invention achieves ultra-high precision adjustment by adjusting the angle between the glass delay plate and the optical axis, utilizing the group velocity delay effect generated by the refractive index of the medium to replace the traditional mechanical displacement, and can achieve femtosecond-level time delay within a small angle range.

[0016] 2. This invention utilizes a detector (point detector) to scan the probe laser passing through the sample point by point, and combines lock-in amplification technology to extract the normalized transmittance change, effectively solving the problems of insufficient dynamic range of CCD shadow imaging and susceptibility to interference from self-emission in the carrier-excited region in the prior art, and providing high-fidelity raw data for subsequent multi-parameter fitting.

[0017] 3. This invention, through an improved rate equation model, can fit the electron exciton collision coefficient that determines the degree of mutual influence between two pulses, significantly improving the model's accuracy in predicting the processing morphology. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a system block diagram of a measurement system for the dynamic process of carrier evolution provided in an embodiment of the present invention; Figure 2 This is a detailed schematic diagram of a measurement system for the dynamic process of carrier evolution provided in an embodiment of the present invention; Figure 3 This is a flowchart of a method for measuring the dynamic process of carrier evolution provided in an embodiment of the present invention; In this array, 1 is a femtosecond laser, 2 is an optical isolator, 3 is a half-wave plate, 4 is the first polarization beam splitter, 5 is a second harmonic generator, 6 is an electro-optic modulator, 7 is a half-wave plate, 8 is the second polarization beam splitter, 9 is a total reflection mirror, 10 and 11 are rotating delay adjustment groups, 12 is a reference plate, 13 is a dichroic mirror, 14 is the third polarization beam splitter, 15 is a total reflection mirror, 16 is a linear delay stage, 17 is an objective lens, 18 is a movable sample stage, 19 is a filter, 20 is a detector, 21 is a lock-in amplifier, and 22 is a computer. Detailed Implementation

[0020] To make the technical problems, technical solutions and advantages of the present invention clearer, a detailed description will be given below in conjunction with the accompanying drawings and specific embodiments.

[0021] like Figure 1 and 2 As shown, this embodiment of the invention provides a measurement system for the dynamic process of carrier evolution, the system comprising: Laser modulation module: used to provide a stable raw laser source and to divide the femtosecond raw laser into a pump laser beam and a probe laser beam with a preset energy ratio; The time-domain shaping module is used to modulate the pump laser into a double-pulse sequence with a preset delay time Δt. One pump pulse is focused inside the transparent medium sample to induce the generation of self-trapped excitons. The other pump pulse arrives after a set delay time through a glass delay plate, triggering the interaction between free electrons and pre-stored excitons. When the probe laser pulse passes through the carrier excitation region generated by the pump double pulse, the free electrons and self-trapped excitons in the carrier excitation region will change the refractive index of the medium. Spatiotemporal synchronization control module: used to adjust the time delay τ between the dual-pulse sequence and the probe laser, and to control the scanning position of the probe laser on the sample to be tested; Interaction and Detection Module: Used to focus the dual-pulse sequence onto the sample to generate a carrier excitation region, and the detection laser passes through the carrier excitation region; The data acquisition module is used to capture the changes in physical properties after the probe laser passes through the carrier excitation region, generate spatiotemporal distribution data, and has an improved rate equation model built in. The model includes a collision term between self-trapped excitons and free electrons. The data acquired under different spatiotemporal coordinates are fitted to obtain the electron-exciton collision coefficient, clarify the dynamic process of carrier evolution, and thus provide a quantitative physical basis for the parameter optimization of the time-domain shaping femtosecond laser processing technology.

[0022] Optionally, the pump laser beam and the probe laser beam after beam splitting by the laser modulation module have different energies. A frequency-doubler generator is set in the optical path of the pump laser to increase its energy, ensuring that the pump laser is sufficient to excite nonlinear ionization while the probe laser does not interfere with the evolution process, thus achieving physical filtering of the pump background. Specifically, this includes: A femtosecond laser (1) is used to output a femtosecond raw laser. Optical isolator (2), used to prevent back reflection from damaging or destabilizing the femtosecond laser, is placed directly in front of the femtosecond laser; A half-wave plate (3) is used to change the polarization direction of linearly polarized light, thereby enabling precise energy adjustment; The first polarization beam splitter (4), used in combination with a half-wave plate, is used to split the original laser pulse into a pump laser beam and a probe laser beam according to a preset energy ratio. The frequency doubler generator (5) is used to halve the wavelength of the pump laser to obtain higher energy.

[0023] Optionally, the time-domain shaping module includes: A half-wave plate (7) is used to change the polarization direction of linearly polarized light, thereby enabling precise energy adjustment; The second polarization beam splitter (8), used in combination with a half-wave plate, is used to redistribute the pump energy to construct a double-pulse sequence with adjustable delay Δt. The total reflection mirror (9) is used to redirect the pump laser after the second beam splitting by 90°, thereby achieving a compact optical path folding layout; The rotating delay adjustment group (10,11) includes two symmetrical glass delay plates (10,11). By utilizing the difference in the speed of light propagation in the glass medium and air, the physical optical path of the beam in the glass delay plate is changed by adjusting the tilt angle of the glass delay plate to control the delay amount. During this process, the system maintains the initial geometric optical path of the first pump pulse and the second pump pulse consistent, thereby creating different time differences to adjust the time interval Δt between the two pulses in the double pulse sequence. The delay amount is referenced to the optical path of the reference plate (12) in the perpendicular incident state as the reference zero point, thereby achieving precise adjustment of the double pulse interval. The reference plate (12) and the rotation delay adjustment group (10,11) are both made of BK7 glass of the same material, providing a static time reference zero point for the dual pulses and ensuring the accuracy of time difference control.

[0024] Optionally, the spatiotemporal synchronization control module includes: The linear delay stage (16) adjusts the delay time of the probe laser relative to the pump laser by changing the optical path length of the probe laser in a large-scale space. Dichroic mirror (13) is used to combine one pump laser and one probe laser beam; The third polarization beam splitter (14) is used to finally combine the two pump lasers and the probe lasers; The movable sample stage (18) is used to carry the sample to be tested and to place it precisely on the focal plane of the objective lens. It is the place where electron-exciton collisions and carrier evolution occur. Through spatial step scanning and synchronous acquisition by the detector, a two-dimensional spatiotemporal evolution image of carrier distribution is obtained.

[0025] Optionally, the interaction and detection module irradiates the interior of the sample with a pump double-pulse laser, exciting the sample through a nonlinear multiphoton absorption and avalanche ionization process to obtain an excited sample containing free electrons and self-trapped excitons with spatiotemporal distribution. The detection laser passes through the carrier-excited region induced by the pump double pulse, and the intensity of the detection laser after passing through the sample is collected by a detector, including: Total reflection mirror (15) is used to change the optical path direction of the probe laser, thereby achieving a compact optical path folding layout; Objective lens (17) is used to focus the three laser pulses after time-space synchronization onto the same tiny spatial region of the sample stage, ensuring that the three beams of light physically interact in the same micro-region inside the sample. A filter (19) is placed in front of the detector (20) and is specifically used to filter out strong pump laser and stray light, allowing only the detection laser wavelength carrying evolution information to pass through; The detector (20) converts the change in intensity of the probe laser after penetrating the sample into an electrical signal, capturing the weak transmittance fluctuations caused by carrier evolution (the detector can be a point detector).

[0026] Optionally, the data acquisition module includes: An electro-optic modulator (6) is used to make the intensity of the pump laser change periodically at a given frequency, which is the same as the reference signal frequency of the lock-in amplifier (21). The lock-in amplifier (21) only detects the probe signal that is exactly the same as the reference frequency. The lock-in amplifier (21) is used to extract the detection signal that is completely consistent with the reference frequency from the electrical signal collected by the detector (20) according to the reference frequency signal, filter out the self-luminescence of the carrier excitation region, the DC component of the detection laser itself and the low-frequency mechanical vibration noise, and convert it into a digital signal for transmission to the computer. A computer (22) is used to perform multi-parameter fitting on the data contained in the digital signal to obtain the electron-exciton collision coefficient of the stimulated sample.

[0027] Existing classical models are typically based on a single-rate equation, mainly considering the following three terms:

[0028] Where, n e —Free electron density; I —Laser intensity; —k-photon ionization cross section; —The rate of change of free electron density; —Multiphoton ionization term; —Avalanche ionization term; — Loss item.

[0029] Disadvantages: While the classical model considers the generation and decay of self-trapped excitons, it neglects the crucial collisional interactions between free electrons and these excitons, thus failing to accurately describe the two-pulse processing process. In femtosecond two-pulse processing, the electron decay time is not a constant (as typically assumed in classical models), but varies with the pulse interval. Because it fails to account for the subsequent evolutionary contributions of excitons, the classical model deviates significantly from experimental results when predicting ablation pit morphology, failing to provide accurate guidance for shape-based pulse processing. Therefore, this invention provides an improved rate equation model.

[0030] Optionally, the improved rate equation model is as follows:

[0031]

[0032] Where, n e —Free electron density; I —Laser intensity; —Ionization cross section; —Avalanche ionization coefficient; —Photoionization section; —Electron-exciton collision coefficient; —The relaxation time of electrons; —The natural lifespan of excitons; —The rate of change of free electron density over time; —Multiphoton ionization term; —Avalanche ionization term; —Exciton photoelectric ionization term; —Exciton collision ionization term; —Loss item; —The time-dependent rate of change of the self-trapped exciton density; —Photoionization loss term; —Generated item; —Natural decay term; The improved model consists of two coupled equations, which describe the free electron density. and exciton density The evolution of excitons is described in the following equations: the electron density equation includes multiphoton ionization, avalanche ionization, exciton photoionization, and a newly added collisional ionization term; the exciton density equation describes the consumption of excitons due to ionization, their generation due to electron capture, and their own decay; an electron-exciton collision term is introduced to describe the inelastic scattering process between photoexcited carriers and metastable excitons generated by the previous pulse. It is the electron-exciton collision coefficient; By fitting the improved model to data collected under different spatiotemporal coordinates, the electron exciton collision coefficient, which determines the degree of mutual influence between two pulses, was obtained. .

[0033] like Figure 3 As shown, this embodiment of the invention also provides a method for measuring the dynamic process of carrier evolution, implemented by the aforementioned measurement system for the dynamic process of carrier evolution, the method comprising: The femtosecond raw laser is emitted from right to left, first passing through a half-wave plate and the first polarization beam splitter, which divides the beam into a pump branch and a probe branch. The pump laser passes through an electro-optic modulator, and the pump laser is modulated at high frequency. Combined with the lock-in amplifier at the end, it can achieve the extraction of weak signals with an extremely high signal-to-noise ratio. The pump laser then passes through a half-wave plate and a second polarization beam splitter, splitting into two paths to become a dual-pump pulse sequence, consisting of a pre-pump pulse and a main pump pulse that arrive sequentially. The pre-pump pulse generates initial free electrons through multiphoton ionization and forms self-trapped excitons in a very short time. Before the arrival of the main pulse, the excitons evolve to a preset concentration. The main pump pulse arrives at the same region after a specific delay time. The main pump pulse triggers the collisional ionization process between free electrons and pre-stored excitons, resulting in a nonlinear increment in the finally detected electron density. Based on the exciton relaxation characteristics of the sample under test, the glass delay plate is rotated and the arrival time of the main pulse is adjusted with femtosecond precision until the enhanced transmission signal caused by electron-exciton collision is observed at the detector end. At this time, the detection laser enters the detector through the filter and the changes in the detection signal under different glass delay plate rotation angles are recorded. Subsequently, the movable sample stage moves the sample, and the detection laser scans the carrier excitation region point by point, recording the changes in the detection signal at different displacements. The electrical signal collected by the detector is sent back to the lock-in amplifier. The lock-in amplifier extracts the detection signal that is completely consistent with the reference frequency from the electrical signal collected by the detector, and converts the detection signal into a digital signal and transmits it to the computer. Ultimately, the computer merges data from countless points into a spatiotemporal shadow map that reflects the dynamic process of carrier evolution. The shadow map records the transmittance at different radial positions and at different times. The improved rate equation model is then fitted to determine the electron-exciton collision coefficient.

[0034] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A measurement system for the dynamic process of carrier evolution, characterized in that, The system includes: Laser modulation module: used to provide a stable raw laser source and to divide the femtosecond raw laser into a pump laser beam and a probe laser beam with a preset energy ratio; The time-domain shaping module is used to modulate the pump laser into a double-pulse sequence with a preset delay time Δt. One pump pulse is focused inside the transparent medium sample to induce the generation of self-trapped excitons. The other pump pulse arrives after a set delay time through a glass delay plate, triggering the interaction between free electrons and pre-stored excitons. When the probe laser pulse passes through the carrier excitation region generated by the pump double pulse, the free electrons and self-trapped excitons in the carrier excitation region will change the refractive index of the medium. Spatiotemporal synchronization control module: used to adjust the time delay τ between the dual-pulse sequence and the probe laser, and to control the scanning position of the probe laser on the sample to be tested; Interaction and Detection Module: Used to focus the dual-pulse sequence onto the sample to generate a carrier excitation region, and the detection laser passes through the carrier excitation region; The data acquisition module is used to capture the changes in physical properties after the probe laser passes through the carrier excitation region, generate spatiotemporal distribution data, and has an improved rate equation model built in. The model includes a collision term between self-trapped excitons and free electrons. The data acquired under different spatiotemporal coordinates are fitted to obtain the electron-exciton collision coefficient, clarify the dynamic process of carrier evolution, and thus provide a quantitative physical basis for the parameter optimization of the time-domain shaping femtosecond laser processing technology.

2. The system according to claim 1, characterized in that, The pump laser beam and the probe laser beam after beam splitting by the laser modulation module have different energies. A frequency-doubler generator is set in the optical path of the pump laser to increase its energy, ensuring that the pump laser is sufficient to excite nonlinear ionization while the probe laser does not interfere with the evolution process. This achieves physical filtering of the pump background, specifically including: A femtosecond laser (1) is used to output a femtosecond raw laser. Optical isolator (2), used to prevent back reflection from damaging or destabilizing the femtosecond laser, is placed directly in front of the femtosecond laser; A half-wave plate (3) is used to change the polarization direction of linearly polarized light, thereby enabling precise energy adjustment; The first polarization beam splitter (4), used in combination with a half-wave plate, is used to split the original laser pulse into a pump laser beam and a probe laser beam according to a preset energy ratio. The frequency doubler generator (5) is used to halve the wavelength of the pump laser to obtain higher energy.

3. The system according to claim 1, characterized in that, The time-domain shaping module includes: A half-wave plate (7) is used to change the polarization direction of linearly polarized light, thereby enabling precise energy adjustment; The second polarization beam splitter (8), used in combination with a half-wave plate, is used to redistribute the pump energy to construct a double-pulse sequence with adjustable delay Δt. The total reflection mirror (9) is used to redirect the pump laser after the second beam splitting by 90°, thereby achieving a compact optical path folding layout; The rotating delay adjustment group (10,11) includes two symmetrical glass delay plates (10,11). By utilizing the difference in the speed of light propagation in the glass medium and air, the physical optical path of the beam in the glass delay plate is changed by adjusting the tilt angle of the glass delay plate to control the delay amount. During this process, the system maintains the initial geometric optical path of the first pump pulse and the second pump pulse consistent, thereby creating different time differences to adjust the time interval Δt between the two pulses in the double pulse sequence. The delay amount is referenced to the optical path of the reference plate (12) in the perpendicular incident state as the reference zero point, thereby achieving precise adjustment of the double pulse interval. The reference plate (12) and the rotation delay adjustment group (10,11) are both made of BK7 glass of the same material, providing a static time reference zero point for the dual pulses and ensuring the accuracy of time difference control.

4. The system according to claim 1, characterized in that, The spatiotemporal synchronization control module includes: The linear delay stage (16) adjusts the delay time of the probe laser relative to the pump laser by changing the optical path length of the probe laser in a large-scale space. Dichroic mirror (13) is used to combine one pump laser and one probe laser beam; The third polarization beam splitter (14) is used to finally combine the two pump lasers and the probe lasers; The movable sample stage (18) is used to carry the sample to be tested and to place it precisely on the focal plane of the objective lens. It is the place where electron-exciton collisions and carrier evolution occur. Through spatial step scanning and synchronous acquisition by the detector, a two-dimensional spatiotemporal evolution image of carrier distribution is obtained.

5. The system according to claim 1, characterized in that, The interaction and detection module irradiates the interior of the sample with a pump dual-pulse laser, exciting the sample through nonlinear multiphoton absorption and avalanche ionization processes to obtain an excited sample containing spatiotemporally distributed free electrons and self-trapped excitons. The detection laser passes through the carrier-excited region induced by the pump dual-pulse, and the intensity of the detection laser after passing through the sample is collected by a detector, including: Total reflection mirror (15) is used to change the optical path direction of the probe laser, thereby achieving a compact optical path folding layout; Objective lens (17) is used to focus the three laser pulses after time-space synchronization onto the same tiny spatial region of the sample stage, ensuring that the three beams of light physically interact in the same micro-region inside the sample. A filter (19) is placed in front of the detector (20) and is specifically used to filter out strong pump laser and stray light, allowing only the detection laser wavelength carrying evolution information to pass through; The detector (20) converts the change in the intensity of the probe laser after penetrating the sample into an electrical signal, capturing the weak transmittance fluctuations caused by carrier evolution.

6. The system according to claim 1, characterized in that, The data acquisition module includes: An electro-optic modulator (6) is used to make the intensity of the pump laser change periodically at a given frequency, which is the same as the reference signal frequency of the lock-in amplifier (21). The lock-in amplifier (21) only detects the probe signal that is exactly the same as the reference frequency. The lock-in amplifier (21) is used to extract the detection signal that is completely consistent with the reference frequency from the electrical signal collected by the detector (20) according to the reference frequency signal, filter out the self-luminescence of the carrier excitation region, the DC component of the detection laser itself and the low-frequency mechanical vibration noise, and convert it into a digital signal for transmission to the computer. A computer (22) is used to perform multi-parameter fitting on the data contained in the digital signal to obtain the electron-exciton collision coefficient of the stimulated sample.

7. The system according to claim 1, characterized in that, The improved rate equation model is as follows: ； ； Where, n e —Free electron density; I —Laser intensity; —Ionization cross section; —Avalanche ionization coefficient; —Photoionization section; —Electron-exciton collision coefficient; —The relaxation time of electrons; —The natural lifespan of excitons; —The rate of change of free electron density over time; —Multiphoton ionization term; —Avalanche ionization term; —Exciton photoelectric ionization term; —Exciton collision ionization term; —Loss item; —The time-dependent rate of change of the self-trapped exciton density; —Photoionization loss term; —Generated item; —Natural decay term; The improved model consists of two coupled equations, which describe the free electron density. and exciton density The evolution of excitons is described in the following equations: the electron density equation includes multiphoton ionization, avalanche ionization, exciton photoionization, and a newly added collisional ionization term; the exciton density equation describes the consumption of excitons due to ionization, their generation due to electron capture, and their own decay; an electron-exciton collision term is introduced to describe the inelastic scattering process between photoexcited carriers and metastable excitons generated by the previous pulse. It is the electron-exciton collision coefficient; By fitting the improved model to data collected under different spatiotemporal coordinates, the electron exciton collision coefficient, which determines the degree of mutual influence between two pulses, was obtained. .

8. A method for measuring the dynamic process of carrier evolution, characterized in that, The method is implemented by a measurement system for the carrier evolution dynamics according to any one of claims 1-7, and includes: The femtosecond raw laser is emitted from right to left, first passing through a half-wave plate and the first polarization beam splitter, which divides the beam into a pump branch and a probe branch. The pump laser passes through an electro-optic modulator, and the pump laser is modulated at high frequency. Combined with the lock-in amplifier at the end, it can achieve the extraction of weak signals with an extremely high signal-to-noise ratio. The pump laser then passes through a half-wave plate and a second polarization beam splitter, splitting into two paths to become a dual-pump pulse sequence, consisting of a pre-pump pulse and a main pump pulse that arrive sequentially. The pre-pump pulse generates initial free electrons through multiphoton ionization and forms self-trapped excitons in a very short time. Before the arrival of the main pulse, the excitons evolve to a preset concentration. The main pump pulse arrives at the same region after a specific delay time. The main pump pulse triggers the collisional ionization process between free electrons and pre-stored excitons, resulting in a nonlinear increment in the finally detected electron density. Based on the exciton relaxation characteristics of the sample under test, the glass delay plate is rotated and the arrival time of the main pulse is adjusted with femtosecond precision until the enhanced transmission signal caused by electron-exciton collision is observed at the detector end. At this time, the detection laser enters the detector through the filter and the changes in the detection signal under different glass delay plate rotation angles are recorded. Subsequently, the movable sample stage moves the sample, and the detection laser scans the carrier excitation region point by point, recording the changes in the detection signal at different displacements. The electrical signal collected by the detector is sent back to the lock-in amplifier. The lock-in amplifier extracts the detection signal that is completely consistent with the reference frequency from the electrical signal collected by the detector, and converts the detection signal into a digital signal and transmits it to the computer. Ultimately, the computer merges data from countless points into a spatiotemporal shadow map that reflects the dynamic process of carrier evolution. The shadow map records the transmittance at different radial positions and at different times. The improved rate equation model is then fitted to determine the electron-exciton collision coefficient.

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