Nondestructive testing device and method for uniformity in saturable absorption mirror surface

Abstract

The invention discloses a reflective pumping detection nondestructive testing device and method capable of saturatively absorbing uniformity in a mirror surface. The device is based on a wavelength degeneracy pumping-detection principle, and comprises a femtosecond laser source, a beam splitter, an optical delay line, a double-frequency optical chopper, a two-dimensional precision translation stage, a coaxial focusing system, a spatial filtering diaphragm arranged in a reflection light path, a photoelectric detector, a lock-in amplifier and a signal processing unit. According to the method, different point positions of a sample are accurately positioned through a two-dimensional translation stage, a nonlinear differential reflectance dynamic curve of each point is measured under low power, the maximum differential reflectance and the carrier lifetime are extracted as characteristic parameters, the spatial relative standard deviation of the characteristic parameters is calculated, and lossless and quantitative in-plane uniformity evaluation is realized. According to the method, the interference of the pump light is effectively filtered through the space filtering diaphragm, the signal-to-noise ratio is remarkably improved in combination with the phase-locked amplification technology, and the problem that the nonlinear optical performance uniformity of the saturable absorber mirror cannot be quantitatively evaluated through a traditional method is solved.

Description

Technical Field

[0001] This invention relates to the field of optical inspection technology, and specifically to a non-collinear reflective pump-probe non-destructive testing device and method for evaluating the in-plane uniformity of the nonlinear optical performance of a reflective saturable absorber mirror (SAM). Background Technology

[0002] In the fields of optical and semiconductor material characterization, pump-probe technology, as a high-precision time-resolved measurement method, has been widely applied in carrier dynamics research, such as carrier lifetime measurement. Existing patent document CN116047253A proposes a method and apparatus for measuring carrier lifetime based on a pump laser. This method generates a broad-spectrum probe light using a passive fiber supercontinuum spectrometer, and combines this with a filter to select specific wavelengths, a balanced amplified photodetector, and a reference light calibration mechanism, significantly improving the applicability and accuracy of measuring carrier lifetime in various materials. These techniques primarily focus on the quantification of carrier lifetime or material dynamic characteristics, demonstrating the high-resolution advantages of pump-probe technology in both the time and energy domains.

[0003] However, despite significant advancements in pump-probe technology for carrier lifetime measurement, existing techniques have failed to effectively apply the pump-probe principle to the non-destructive testing of in-plane uniformity in saturable absorber mirrors (SAMs). Traditional SAM uniformity testing methods primarily rely on microscopic observation, white light interferometry, or point-by-point measurements, often lacking high spatial resolution and non-destructive evaluation capabilities, and unable to reflect in real-time local response differences of the mirror surface under pump light. In recent years, some studies have attempted to indirectly evaluate the uniformity of SAMs through material structure characterization. For example, Chen et al. prepared Bi2Te3-Gold SAMs using pulsed laser deposition (PLD) and indirectly evaluated the in-plane structural uniformity by measuring the standard deviation of Raman spectral peak positions and amplitude ratios (see Chen, HR., Tsai, CY., Cheng, HM. et al. Sci Rep 6, 38444(2016)). Although this method demonstrated structural consistency, Raman spectroscopy only indirectly reflects material properties and may not directly correspond to the saturable absorption optical performance of the SAM. Furthermore, as an absolute indicator, standard deviation is less intuitive and comparable than relative standard deviation when comparing parameters of different orders of magnitude or units. Summary of the Invention

[0004] Therefore, to address the aforementioned technical problems, this invention applies high-precision pump-probe technology to the non-destructive uniformity detection of saturable absorber mirrors (SAMs), providing a device and method for SAM uniformity detection based on the reflective pump-probe principle: The nonlinear differential reflectivity dynamics curves (ΔR / R(t)) of multiple points in the SAM are measured using reflective pump-probe technology, and characteristic parameters such as the maximum differential reflectivity (ΔR / R) are extracted. max By calculating the carrier lifetime τ and the relative standard deviation (RSD) of these parameters, a quantitative and non-destructive in-plane uniformity assessment is achieved. On the one hand, this directly quantifies the uniformity of optical performance because (ΔR / R)max is directly related to the device's saturable absorption characteristics (such as modulation depth and saturable flux), avoiding indirect inference. On the other hand, the RSD, expressed as a percentage, provides an intuitive and normalized uniformity index, facilitating cross-sample comparison and quality control, and offering an innovative solution for the performance evaluation and quality control of saturable absorber mirrors.

[0005] The technical solution of the present invention is as follows: A non-destructive testing device for evaluating the uniformity within a saturable absorber mirror, comprising: Femtosecond laser source, beam splitter, optical delay line, dual-frequency optical chopper, two-dimensional precision translation stage, coaxial focusing system, spatial filter aperture for blocking pump light stray light, photodetector, lock-in amplifier and signal processing unit; The pulsed laser generated by the femtosecond laser source is split into pump light and probe light by a beam splitter. The pump light and the probe light, delayed by an optical delay line, are simultaneously modulated by a dual-frequency optical chopper and coaxially focused by a coaxial focusing system onto the same area of ​​the sample surface on a two-dimensional precision translation stage. The light signal reflected from the sample surface is filtered by a spatial filter to remove the pump light. The probe light is received by a photodetector and converted into an electrical signal. Then, a lock-in amplifier extracts a weak nonlinear signal with the probe light modulation frequency as a reference. Finally, a signal processing unit acquires the nonlinear dynamic curves of each point. By calculating the relative standard deviation of the characteristic parameters, a non-destructive and quantitative evaluation of the uniformity within the saturable absorber under test is achieved. The lock-in amplifier is connected to the dual-frequency optical chopper and the photodetector. The signal processing unit is connected to the lock-in amplifier, the optical delay line, and the two-dimensional precision translation stage.

[0006] Furthermore, the center wavelength of the femtosecond laser source output is 400nm-3μm, and the pulse width is 100fs-300fs.

[0007] Furthermore, the power of the pump light separated by the beam splitter is less than 5mW, and the power of the probe light is less than 0.5mW.

[0008] Furthermore, the two-dimensional precision translation stage is an electrically controlled linear displacement stage with a positioning accuracy of 0.1-1μm.

[0009] Furthermore, the dual-frequency optical chopper has a modulation frequency range of 100Hz-3000Hz.

[0010] Furthermore, the coaxial focusing system uses convex lenses with a focal length of 5-15mm.

[0011] Furthermore, the spatial filter aperture is an adjustable aperture aperture, installed after the coaxial focusing system in the reflected light path and before the photodetector. After its aperture is adjusted, it can completely block the pump light spot reflected back from the sample, while allowing the probe light spot to pass through completely, thereby achieving spatial separation of the pump light and the probe light.

[0012] The corresponding testing methods and steps are as follows: This invention provides a non-destructive testing method for evaluating the uniformity within a saturable absorber mirror. The method is based on the reflective pump-probe principle and achieves quantitative evaluation through spatial scanning and lock-in amplification techniques. The method includes the following steps: S1. System Initialization and Optical Path Calibration: The saturable absorber sample to be tested is fixed on a two-dimensional precision translation stage. The optical path is adjusted so that the pump light and the probe light, after being split by the beam splitter, are precisely coaxially focused on the same point on the sample surface by the coaxial focusing system. Then, the aperture and position of the spatial filter aperture set in the reflection optical path are adjusted. By monitoring the output signal of the photodetector to minimize, it is ensured that the pump light spot reflected from the sample is completely blocked by the spatial filter aperture, while allowing the probe light spot to pass through completely. This effectively suppresses pump light stray interference and lays the foundation for high signal-to-noise ratio measurement. S2. Single-point dynamic measurement: The control signal processing unit drives a two-dimensional precision translation stage, which carries the sample and moves it along a preset path. This sequentially and precisely positions N (N≥5) different measurement points on the sample surface to the laser focal point. For each measurement point, the following sub-steps are executed cyclically: S2.1 Time Delay Scan: Controls the optical delay line to perform continuous or step scans, covering a preset time delay range from before the pump-probe pulse overlap to after carrier recombination is completed; S2.2 Lock-in signal extraction: During the time-delay scanning process, the lock-in amplifier uses the modulation frequency applied to the pump light by the dual-frequency optical chopper (104) as the reference frequency to synchronously acquire and demodulate the detection light intensity signal from the photodetector (108). This process can extract the weak nonlinear reflectivity change signal caused by pump light excitation from the strong noise background, and finally obtain the high signal-to-noise ratio nonlinear differential reflection dynamics curve ΔR / R(t) at the measurement point. S3. Feature Parameter Extraction: From each nonlinear differential reflection dynamics curve ΔR / R(t) obtained in step S2.2, the experimental data is fitted using a double-exponential or multi-exponential function to extract the characteristic parameter P characterizing the nonlinear optical response of the sample. The characteristic parameter P is preferably the peak value of the fitted curve of the dynamics curve, i.e., the maximum differential reflectivity (ΔR / R). max , and carrier recombination lifetime τ; S4. Quantitative analysis and output of in-plane uniformity: After completing the measurements at all N measurement points, the signal processing unit processes the acquired N characteristic parameter values ​​(P1, P2, ..., P...). N The core of statistical analysis is to calculate the relative standard deviation (RSD) of the data set. in It is the sample mean. It is the standard deviation. It is the value of the i-th data point. The total number of data points is used, and this RSD value is output and displayed as the final quantitative index for evaluating the in-plane uniformity of the nonlinear optical performance of the saturable absorber mirror.

[0013] Furthermore, the number of measurement points is no less than 5, and they are evenly distributed on the sample surface.

[0014] Compared with the prior art, the present invention has the following beneficial technical effects: 1. Innovative principles and high spatial resolution: This invention is the first to combine the measurement of nonlinear differential reflectivity dynamics curves with the performance uniformity of saturable absorber mirrors, directly detecting the local nonlinear optical response of the device in its working state (i.e., when excited by pump light). This fundamentally avoids the limitations of traditional methods such as microscopes or white light interferometers, which can only characterize static morphology rather than functional characteristics. At the same time, it characterizes the uniformity of the material structure and nonlinear absorption properties of two-dimensional material saturable absorber mirrors. Furthermore, by combining a coaxial focusing system with a high-precision translation stage, it can achieve micron-level spatial resolution and accurately locate the performance differences at various points within the plane.

[0015] 2. High precision, high sensitivity, and high signal-to-noise ratio: The device achieves efficient spatial separation of the pump and probe beams in the reflected optical path by introducing a spatial filter aperture, fundamentally suppressing interference from stray pump light. Combined with a phase-locked detector loop consisting of a dual-frequency optical chopper and a lock-in amplifier, it can extract extremely weak nonlinear reflection change signals (ΔR / R) from a strong noise background, significantly improving the signal-to-noise ratio and thus ensuring the accuracy of the characteristic parameters (ΔR / R). max The accuracy and repeatability of the τ measurement results.

[0016] 3. Truly non-destructive and quantitative evaluation: By strictly limiting the power of the pump and probe lights (e.g., pump light < 5mW, probe light < 0.5mW), the measurement process is ensured to prevent any thermal or optical damage to sensitive samples (such as two-dimensional materials), thus achieving true non-destructive testing. Finally, the relative standard deviation (RSD) of the characteristic parameters in space is used as a unified and objective quantitative indicator, completely changing the traditional method's limitation to only qualitative or semi-quantitative assessment of uniformity. This provides reliable data support for the quality control and performance grading of saturable absorber mirrors.

[0017] 4. Automation, high efficiency, and good versatility: The entire measurement process is automatically controlled by the signal processing unit, including two-dimensional scanning, delay line control, data acquisition, and processing analysis. It is easy to operate and has high detection efficiency. This device and method do not depend on a specific two-dimensional material system and are applicable to the uniformity detection of all saturable absorber mirrors based on the principle of reflective saturable absorption, including but not limited to those based on graphene, transition metal sulfides, black phosphorus, topological insulators, and other materials, thus having wide applicability. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the overall structure of the detection device of the present invention; In the figure: femtosecond laser source-101, beam splitter-102, optical delay line-103, dual-frequency optical chopper-104, two-dimensional precision translation stage-105, coaxial focusing system-106, spatial filter aperture-107, photodetector-108, lock-in amplifier-109, and signal processing unit-110; Figure 2 This is a schematic diagram of the sample positioning process using a spatial filter aperture and a two-dimensional translation stage. Figure 3 This is a schematic diagram of the kinetic curves measured at different positions on a saturable absorber mirror with a PtSe2 film thickness of 3 nm. Figure 4 These are the characteristic parameters (ΔR / R) of a 3nm PtSe2 saturable absorber mirror. max τ distribution diagram. Detailed Implementation

[0019] Example 1: Construction and Initialization of a Reflective Pump-Based Non-Destructive Testing Device The detection method is based on the reflective pump-probe principle. Multiple measurement points are precisely located using a two-dimensional translation stage, and the nonlinear differential reflectivity dynamics curve is extracted. The relative standard deviation (RSD) of the characteristic parameters is then calculated to quantify uniformity. The entire detection process is non-destructive and quantitative, and a high signal-to-noise ratio is ensured through spatial filter apertures and lock-in amplification technology.

[0020] As shown in the attached diagram. Figure 1 The detection device shown. The core components and parameters of the device include: Femtosecond laser source 101: A Ti:sapphire laser (center wavelength 800 nm, pulse width ~100 fs, repetition frequency 100 kHz) is selected, with stable output power to ensure the time resolution of the pump-probe process.

[0021] Beam splitter 102: A 90:10 beam splitter is used to split the incident laser into pump light (90%) and probe light (10%). The pump light power is set to ~1.5 mW and the probe light power is set to ~0.13 mW to avoid sample damage with low power.

[0022] Optical delay line 103: It employs an electrically controlled linear displacement stage (accuracy 1 μm) and is equipped with a hollow retroreflector to scan the time delay between the pump light and the probe light (range: -10 ps to +10 ps).

[0023] Dual-frequency optical chopper 104: performs optical chopping modulation of pump light and probe light at different frequencies. The modulation frequency of pump light is set to 730Hz, the modulation frequency of probe light is set to 440Hz, and a reference signal is provided for lock-in amplification.

[0024] 2D Precision Translation Stage 105: Employs an electrically controlled linear displacement stage (positioning accuracy 0.5μm) for carrying and moving samples. The stage has a travel of 150mm x 150mm and supports automated scanning.

[0025] Coaxial focusing system 106: Employs a convex lens with a focal length of 10 mm to coaxially focus the pump light and probe light onto the sample surface, with a spot diameter of ~33 μm.

[0026] Spatial filter stop 107: as attached Figure 2 As shown, this is an adjustable aperture stop (aperture range 0.1-5 mm), installed in the reflected light path to filter out stray pump light. By adjusting the aperture, only the probe light is ensured to pass through, thus improving the signal-to-noise ratio.

[0027] Photodetector 108: Employs a silicon photodiode with a response wavelength range of 400-1100 nm, used to receive detection light signals.

[0028] Lock-in amplifier 109: Using the modulation frequency of the probe light, 440 Hz, as a reference, it extracts nonlinear signals and suppresses noise.

[0029] Signal processing unit 110: Composed of an industrial computer and self-developed software, it controls the translation stage and delay line, and processes data.

[0030] Device initialization includes optical path calibration: adjusting each component to ensure the pump light and probe light are strictly coaxial, and ensuring that the spatial filter 107 accurately blocks the reflected components of the pump light. Specifically, as shown in the attached... Figure 2 As shown in the schematic diagram, the aperture of the aperture is adjusted until the pump light signal is minimized by monitoring the output of the photodetector.

[0031] As attached Figure 1 The detection device shown operates as follows: The pulsed laser (center wavelength 800 nm, pulse width ~100 fs) generated by the femtosecond laser source 101 is split into pump light (power ~1.5 mW) and probe light (power ~0.13 mW) by the beam splitter 102; the probe light is first precisely delayed by the optical delay line 103, and then passes together with the pump light through the dual-frequency optical chopper 104, and is modulated to different frequencies of 730 Hz and 440 Hz respectively; the two modulated beams are coaxially focused by the coaxial focusing system 106 onto the same area (spot diameter ~33 μm) of the sample surface placed on the two-dimensional precision translation stage 105; the coaxial light signal reflected from the sample surface passes through the spatial filter aperture 107, which can be precisely adjusted to completely block the reflected pump light while allowing the probe light to pass through, thereby effectively filtering out pump light stray interference. The probe light is received by the photodetector 108 and converted into an electrical signal. This signal is then demodulated by the lock-in amplifier 109 with the modulation frequency of the probe light (440 Hz) as a reference, thereby extracting a weak nonlinear reflectivity change signal (ΔR / R) from the noise background. Finally, the signal processing unit 110 synchronously controls the optical delay line 103 and the two-dimensional precision translation stage 105 to scan and position, acquires the signal from the lock-in amplifier 109, automatically obtains the nonlinear differential reflection dynamics curves at different points on the sample, and realizes a non-destructive and quantitative evaluation of the uniformity within the saturable absorber mirror under test by calculating the spatial relative standard deviation of the characteristic parameters.

[0032] Example 2: Detection of In-Face Uniformity of Saturable Absorber Mirrors in Two-Dimensional Materials This embodiment describes in detail the specific method for detecting the in-plane uniformity of a two-dimensional material saturable absorber (SAM) using the device invented in Embodiment 1 of this patent.

[0033] 1. Sample preparation and fixation The sample to be tested is a two-dimensional material PtSe2 saturable absorber mirror, and the preparation process is as follows: A 3 nm platinum atomic layer was deposited on a gold-plated quartz mirror substrate using a magnetron sputtering system. The sample was placed in a quartz tube furnace, and selenium powder was vaporized in the upstream region at 220°C under a 10% H2 / Ar atmosphere. The sample was then reacted with selenium vapor in the downstream region at 450°C for 2 hours to completely selenize the platinum layer into a PtSe2 thin film. After the sample cools, it is fixed on the two-dimensional translation stage 105, as shown in the attached figure. Figure 2 As shown. The translation stage ensures that the sample plane is perpendicular to the focusing optical path and that the sample center is aligned with the laser focus; 2. Automated measurement process The measurement process is automatically controlled by the signal processing unit 110, including spatial scanning and time-delay scanning: Spatial scanning: as attached Figure 2 As shown, the translation stage moves along a preset path to sequentially position six points (evenly distributed with a spacing of 20 mm) on the sample surface to the laser focus. During the measurement of each point, the translation stage ensures that the positioning error is <1 μm. Time Delay Scan: For each point, the optical delay line 103 is controlled to scan its introduced time delay (denoted as Δt) in steps of 0.1 ps, covering a range of -1 ns to +1 ns, to detect pump-probe pulse overlap and subsequent carrier recombination dynamics. Signal Extraction: At each delay point, the lock-in amplifier 109 extracts the probe light intensity signal at a reference frequency of 440 Hz, and the nonlinear differential reflection dynamics curve ΔR / R(t) is obtained by the signal processing unit 110. A typical curve is attached. Figure 3 As shown, this illustrates the change of ΔR / R over time delay; 3. Feature Parameter Extraction: Feature parameters P are extracted from each dynamic curve through double-exponential or multi-exponential curve fitting. The peak value of the fitted curve is the maximum differential reflectance (ΔR / R). max At the same time, the carrier lifetime τ is obtained; 4. Quantitative analysis of uniformity After the measurement is completed, the signal processing unit counts (ΔR / R) at 6 points. max And τ value, and perform statistical analysis: Calculate the relative standard deviation (RSD): The formula for calculating RSD is as follows: ,in, The standard deviation is... This is the average value; in this embodiment, (ΔR / R) represents 6 points. maxThe values ​​are: 0.0560537, 0.0561525, 0.0570097, 0.0584852, 0.0576489, 0.0571626; the average value is... = 0.0570854, standard deviation =0.000919061, therefore RSD = (0.000919061 / 0.0570854) × 100% = 1.61%; the τ values ​​for the six points are: 2.23346, 2.22504, 2.16476, 2.13707, 2.21787, 2.14439; the average value is... = 2.18709, standard deviation =0.0432705, therefore RSD = (0.0432705 / 2.18709) × 100% = 1.98%; Results output: The spatial relative standard deviations of the measured differential reflectivity and carrier lifetime are RSD. 微分反射率 =1.61%, RSD 载流子寿命 = 1.98%. Considering the high signal-to-noise ratio achieved by the device of this invention through its spatial filter aperture and lock-in amplification technology, and the micron-level positioning accuracy provided by the two-dimensional precision translation stage, it can be concluded that the above RSD value mainly reflects the true spatial distribution of the nonlinear optical performance of the sample itself, rather than the inherent noise or error of the measurement system. This data objectively confirms that the sample has a highly consistent nonlinear optical response in the plane. Based on this measured result, the relative standard deviation (RSD) of the characteristic parameters ≤ 5% can be used as an exemplary indicator for evaluating the excellent uniformity in the saturable absorber mirror. The uniformity evaluation results have been output in graphical form, as shown in the attached figure. Figure 4 As shown, (ΔR / R) is visually represented. max The distribution of values.

Claims

1. A nondestructive testing device for in-plane uniformity of a saturable absorber mirror, comprising: include: Femtosecond laser Source (101), beam splitter (102), optical delay line (103), dual-frequency optical chopper (104), two-dimensional precision translation stage (105), coaxial focusing system (106), spatial filter aperture for blocking pump light stray light (107), photodetector (108), lock-in amplifier (109) and signal processing unit (110). The pulsed laser generated by the femtosecond laser source (101) is split into pump light and probe light by the beam splitter (102); the pump light and the probe light delayed by the optical delay line (103) are simultaneously modulated by the dual-frequency optical chopper (104) and coaxially focused by the coaxial focusing system (106) onto the same area of ​​the sample surface on the two-dimensional translation stage (105); the light signal reflected from the sample surface is filtered by the spatial filter aperture (107) to remove the pump light, and the probe light is received by the photodetector (108) and converted into an electrical signal, which is then modulated by the lock-in amplifier (109) at a frequency of The rate is used to extract weak nonlinear signals, and finally the signal processing unit (110) analyzes and obtains the nonlinear dynamic curves of each point. By calculating the spatial relative standard deviation of the characteristic parameters, the non-destructive and quantitative evaluation of the uniformity within the saturable absorbable mirror under test is realized. The lock-in amplifier (109) is connected to the dual-frequency optical chopper (104) and the photodetector (108). The signal processing unit (110) is connected to the lock-in amplifier (109), the optical delay line (103) and the two-dimensional precision translation stage (105).

2. The apparatus of claim 1, wherein, The center output of the femtosecond laser source (101) Wavelength 400nm-3μm, pulse width 100fs-300fs.

3. The apparatus of claim 1, wherein, The pump light separated by the beam splitter (102) The power is less than 5mW, and the power of the probe light is less than 0.5mW.

4. The apparatus of claim 1, wherein, The two-dimensional precision translation stage (105) is electrically controlled. Linear displacement stage with positioning accuracy of 0.1-1μm.

5. The apparatus of claim 1, wherein, The dual-frequency optical chopper (104) modulates the frequency. Frequency range 100Hz-3000Hz.

6. The apparatus of claim 1, wherein, The coaxial focusing system (106) uses a focal length A convex lens with a diameter of 5-15mm.

7. The apparatus of claim 1, wherein, The spatial filter aperture (107) is an adjustable aperture. The aperture stop is installed after the coaxial focusing system (106) and before the photodetector (108) in the reflected light path. Its aperture can be adjusted to completely block the pump light spot reflected back from the sample, while allowing the probe light spot to pass through completely, thereby realizing the spatial separation of the pump light and the probe light.

8. A method for detecting uniformity of a saturable absorber mirror using the apparatus of any one of claims 1-7. Its characteristics are The process includes the following steps: S1. Sample mounting and optical path calibration: Fix the saturable absorber sample to be tested onto the two-dimensional precision translation... On the platform (105), the optical system is adjusted so that the pump light and the probe light are coaxially focused on the same area of ​​the sample surface through the coaxial focusing system (106); then, the spatial filter (107) is adjusted to block the pump light reflected back from the sample and allow the probe light to pass through; S2. Single-point dynamic measurement: control the two-dimensional precision translation stage (105) to move one measurement point on the sample to the laser focal point; at this point, scan the optical delay line (103), and extract the nonlinear differential reflection dynamic curve ΔR / R(t) of this point using the lock-in amplifier (109); S3. Feature parameter extraction: extract the feature parameter P representing the nonlinear optical response from the dynamic curve ΔR / R(t); S4. Space scanning and uniformity quantification: repeat steps S2 to S3 to measure the characteristic parameters P1, P2,..., PN of the pre-set N different measurement points on the sample in turn N ; according to the N characteristic parameters, calculate the statistical standard deviation or relative standard deviation, and take the value as a quantitative index for evaluating the in-plane uniformity of the saturable absorption mirror.

9. The method of claim 8, wherein, The feature parameter P is the dynamic curve maximum differential reflectance value (ΔR / R) max and carrier lifetime τ.

10. The method of claim 8, wherein, The number of measurement points is not less than 5, and the distribution is uniform. ​

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