Single-pulse transient nonlinear refractive index measurement device and measurement method

Abstract

The present invention discloses a device and method for measuring single-pulse transient nonlinear refractive index. The device comprises a detection system, a pumping system, and a data recording device. The detection system includes a detection laser source and a 4f imaging system. The 4f imaging system comprises a converging lens and an imaging lens, with a sample positioned between the converging lens and the imaging lens. The pumping system intersects the detection system at the sample at an angle. The detection system also includes a phased array stepped reflector device, through which the detection light emitted by the detection laser source is reflected to the converging lens. The measurement device of the present invention can significantly improve temporal resolution. The present invention also discloses a measurement method.

Description

Technical Field

[0001] The present invention relates to the field of optical parameter measurement, in particular to a device and method for measuring the single-pulse transient nonlinear refractive index of an electro-optical material. Background Art

[0002] The birth of transient optical imaging technology can be traced back to the 1870s, when American photographer E. Muybridge pioneered the technique by capturing horses in motion. More than a century later, scientists have now achieved imaging of femtosecond laser-induced lattice vibration waves with a single exposure. This makes single-exposure ultrafast imaging technology an irreplaceable advantage in measuring non-repeatable phenomena such as giant optical waves, irreversible structural dynamics in chemical reactions, and shock wave generation in inertial confinement fusion.

[0003] When it comes to ultrafast imaging of irreversible dynamic phenomena, traditional pump-probe technology is clearly insufficient. This requires a moving platform to achieve a time delay between two pulses of light, and multiple pulse repetitions are required to obtain images of the pump light spot at different time delays. However, this does not truly measure the temporal changes within a single pulse. Furthermore, traditional ultrafast electronic imaging techniques also suffer from the need for multiple measurements and time jitter. Therefore, single-shot imaging technology is needed to address these issues. Summary of the Invention

[0004] This invention addresses the problem of measuring transient nonlinear refraction using a single pulse. It uses a "stepped mirror array with a phase object" to achieve ultrafast single-pulse nonlinear refraction measurement. Using a stepped mirror array with a phase object, the invention splits a single pulse of probe light into multiple probe light pulse sequences with time delay differences. Using 4f phase coherent imaging technology, the nonlinear refractive dynamics of the material over a period of time after the pump light is applied can be captured, thereby enabling measurement of the transient nonlinear refractive index of a single pulse.

[0005] According to the present invention, a single-pulse transient nonlinear refractive index measuring device is provided, which includes a detection system, a pumping system and a data recording device. The detection system includes a detection laser source and a 4f imaging system. The 4f imaging system is composed of a converging lens and an imaging lens. The sample is located between the converging lens and the imaging lens. The pumping system intersects the detection system at the sample at an angle. The detection system also includes a phase array stepped reflector device. The detection light emitted by the detection laser source is reflected to the converging lens by the phase array stepped reflector device.

[0006] Preferably, the detection laser source is a picosecond laser source or a femtosecond laser source.

[0007] Preferably, the detection system further comprises a beam splitter, which is arranged between the detection laser source and the phase array stepped reflector device.

[0008] Preferably, the measuring device further comprises a sample holder for fixing the sample, and the sample holder is arranged at the rear focal plane of the converging lens and the front focal plane of the imaging lens.

[0009] Preferably, the pumping system includes a pumping laser source and a first convex lens, and the sample holder is located at the focal plane of the first convex lens.

[0010] Preferably, the pumping system further comprises a pump light absorption device, and the pump light absorption device is located on the other side of the sample holder relative to the pump laser source and the first convex lens.

[0011] Preferably, the data recording device is located at the image plane of the 4f imaging system.

[0012] Preferably, the 4f imaging system includes a converging lens and an imaging lens, the focal lengths of the converging lens and the imaging lens may be different, and the front focal plane of the converging lens is the object plane of the 4f imaging system.

[0013] Preferably, the object plane of the 4f imaging system is the surface of a phase array stepped reflector device.

[0014] Preferably, the phased array step reflector device comprises a substrate and a plurality of reflectors, wherein the plurality of reflectors are arranged in m rows and n columns, the surfaces of the plurality of reflectors have phase objects, and the reflective surfaces of the plurality of reflectors are parallel to the bottom surface of the substrate. The reflector in the first row and first column is denoted as L1C1, and the reflector in the mth row and nth column is denoted as L m C n , any of the reflectors in the phase array step reflector device is denoted as L i C j , the height from the bottom surface of the substrate to any of the reflectors is recorded as H(L i C j ), then the heights of the multiple reflectors satisfy H(L i C j )>H(L i+1 C j ), H(L i C j )>H(L i C j+1 ), H(L i C n )>H(L i+1 C1), where m, n, i, j are integers and 1≤i≤m, 1≤j≤n.

[0015] Preferably, the step height between two reflectors in the same row and adjacent columns is denoted as h, and the step heights between the multiple reflectors satisfy H(L i C j )-H(L i C j+1 )=h,H(L i C j )-H(L i+1 C j )=nh,H(L i C n )-H(L i+1 C1)=h, where the step height h is equal to the delay time resolution multiplied by the speed of light.

[0016] Preferably, the width of the step window of the phase array step mirror device is equal to N multiplied by the pixel size of the data recording device divided by the magnification of the imaging system, where N is the number of pixels of the data recording device occupied by each plane phase mirror.

[0017] The present invention provides a method for measuring a single-pulse transient nonlinear refractive index. The device for measuring the single-pulse transient nonlinear refractive index is used to measure a sample. The method comprises: step a), without placing a sample, shielding pump light, emitting probe light, and having a data recording device record background spot information of the probe light; step b), placing a sample, shielding pump light, emitting probe light, and having a data recording device record spot information of the sample's linear absorption of the probe light; step c), placing a sample, emitting a single-pulse probe light and pump light, and having a data recording device record spot information of the sample's nonlinear refraction of the single-pulse probe light; and step d), processing the spot information obtained in steps a), b), and c) to obtain the transient nonlinear refractive index of the sample.

[0018] Preferably, in step c), the light spot formed by the pump light on the sample is larger than and covers the light spot formed by the probe light on the sample.

[0019] Preferably, the step d) includes: step d1), dividing the sum of all pixel values ​​contained in the linear absorption detection light spot in step b) by the sum of all pixel values ​​contained in the detection light background spot in step a), to obtain the linear absorption coefficient of the sample; step d2), dividing the sum of all pixel values ​​contained in each step reflector image in the nonlinear refraction detection light spot in step c) by the sum of all pixel values ​​contained in each step reflector image corresponding to the linear absorption detection light spot in step b), to obtain the normalized transmittance containing only nonlinear absorption at different delay times; step d3), further dividing each pixel value of the nonlinear refraction detection light spot in step c) by the corresponding pixel value in the linear absorption detection light spot in step b), and the spot information after the division corresponds to the phase array The transmittance of the phase object at the center of each phase reflector of the column stepped reflector device is subtracted from the transmittance at the edge of each phase reflector to obtain the normalized transmittance difference containing nonlinear absorption at different delay times; step d4), the normalized transmittance difference containing nonlinear absorption obtained in step d3) is divided by the normalized transmittance containing only nonlinear absorption obtained in step d2) to obtain the normalized transmittance difference without nonlinear absorption; step d5), the normalized transmittance difference without nonlinear absorption obtained in step d4) is arranged in the order of delay time of the single-pulse detection light sequence to obtain a dynamic curve of the transmittance change caused by the nonlinear refraction of the sample as the delay time changes; step d6), the transient nonlinear refractive index of the sample is fitted using a pump-probe fitting program with a phase object.

[0020] The present invention uses a measurement device with a phase array stepped reflector device to measure the nonlinear refractive change of a sample excited by a pump pulse using a single pulse probe light. Compared with other measurement technologies, it has the following advantages:

[0021] 1. The optical path is very simple, requiring only a stepped mirror array with a phase object, a lens, and a CCD to obtain the nonlinear refractive index of the material.

[0022] 2. No expensive CCD is required to achieve ps or even fs-level data acquisition speed;

[0023] 3. The measurement is very convenient. There is no need to move the sample. Only one laser pulse is needed to obtain the nonlinear refractive index change of the material after the pump light.

[0024] 4. This method does not require a high-precision moving platform in the traditional optical path to achieve time delay. Only a time-delay step mirror is needed to achieve time delay.

[0025] 5. Compared with the current gated framing camera (more than ten picoseconds), its time resolution is greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Attachment Figure 1 A schematic diagram of a measuring device according to an embodiment of the present invention;

[0027] Figures 2a-2d Schematic diagram of the structure of the phase array stepped reflector device of the present invention.

[0028] in:

[0029] 1 is the detection beam, 2 is the beam splitter, 3 is the phase array step reflector device, 4 is the converging lens, 5 is the pump light, 6 is the first convex lens, 7 is the sample holder, 8 is the pump light absorption device, 9 is the imaging lens, and 10 is the data recording device. DETAILED DESCRIPTION

[0030] The aforementioned and other technical contents, features, and effects of the present invention will be clearly presented in the following detailed description of a preferred embodiment with reference to the accompanying drawings. The directional terms mentioned in the following embodiments, such as up, down, left, right, front, or back, are only used to refer to the directions in the accompanying drawings. Therefore, the directional terms used are used to illustrate and are not intended to limit the present invention. The present invention is further described in detail below with reference to the accompanying drawings:

[0031] For samples exhibiting Kerr refraction or carrier refraction, after being exposed to pump light, the particle population at different locations in the sample is related to the excitation light intensity and varies over time. If the probe light beam and the pump light beam arrive at the sample at different times, the corresponding probe light will record the magnitude of the sample's nonlinear refractive index at that delayed moment. At different delay moments, the sample material reacts differently to the probe light pulse, causing the changes in the material's nonlinear refractive index at different moments to be recorded by the probe light sequence. The change in the spatial distribution of the probe light spot intensity after the probe light passes through the sample represents the change caused by the sample's nonlinear refractive index. By adjusting the time delay between the pump light pulse and the probe light and processing the recorded spot image, the magnitude and sign of the nonlinear refractive index within the sample at different moments can be obtained.

[0032] refer to Figure 1, is a schematic diagram of a device for measuring a single-pulse transient nonlinear refractive index according to an embodiment of the present invention. The measuring device includes a detection laser source (not shown) that emits a detection beam 1, a beam splitter 2, a phase array step reflector device 3, a converging lens 4, a pump light source (not shown) that emits a pump light 5, a first convex lens 6, a sample holder 7, a pump light absorption device 8, an imaging lens 9, and a data recording device 10. The sample holder 7 is on the back focal plane of the converging lens 4, and the sample holder 7 is on the front focal plane of the imaging lens 9. The data recording device 10 is on the back focal plane of the imaging lens 9 corresponding to the sample holder 7. The beam splitter 2 is located between the phase array step reflector device 3 and the detection light source. The first convex lens 6 can converge the pump light 5 onto the sample on the sample holder 7. During measurement, the detection light beam 1 passes through the beam splitter 2 and is irradiated onto the phase array step reflector device 3. The detection light beam 1 is reflected by the phase array step reflector device 3 and then reflected again by the beam splitter 2 to the converging lens 4. The phase array step reflector device 3 produces a series of time delays for the detection light. The detection light with different time delays passes through the sample acted upon by the pump light 5 in turn. The sample has different nonlinear refraction effects on the detection light with different time delays, and the detection light spot is recorded by the data recording device 10.

[0033] In order to realize the measurement of single pulse transient nonlinear refractive index, the present invention also provides a phase array step reflector device. Figure 2a-2b is a schematic structural diagram of the phase array step reflector device 3, as shown in Figure 2a The phase array step reflector device 3 comprises a substrate and m rows by n columns of reflectors, at least part of the surface of the reflectors is coated with a phase object (such as Figure 2b As shown), the phase object is preferably coated on the central area of ​​the reflector surface, and the reflective surface of the reflector is parallel to the bottom surface of the substrate (as shown Figure 2c , as shown in 2d), the reflector located in the first row and first column is denoted as L1C1, and the reflector in the mth row and nth column is denoted as L m C n , any reflector in the phased array step reflector device is denoted as L i C j , the height from the bottom of the substrate to any reflector is recorded as H(L i C j ), then H(L i C j )>H(L i+1 C j ), H(L i C j )>H(L i C j+1 ), H(L i C n )>H(L i+1C1); further, if the height difference between two reflectors in adjacent columns of the same row, i.e., the step height, is denoted as h, then H(L i C j )-H(L i C j+1 )=h,H(L i C j )-H(L i+1 C j )=nh,H(L i C n )-H(L i+1 C1) = h, where m, n, i, and j are integers, and 1 ≤ i ≤ m, and 1 ≤ j ≤ n. The reflectors of the phased array step mirror device 3 form a stepped structure. The width of the steps of the step window is determined by the ratio of the image size to the size of the sample to be measured in the imaging system and the pixel size of the data recording device 10. The data recording device 10 can be a CCD, LCoS, or DMD. This is only an example and is not limited to this. The algorithm is: step window width = N × pixel size / system magnification, where N is the number of pixels occupied by each step; the step height h is equal to the delay time resolution multiplied by the speed of light.

[0034] In this embodiment, each reflective surface of the phased array step reflector device 3 is machined to 1mm x 1mm, with 20 stepped reflective units per row, for a total of 20 rows. The step height differences between each row and column are 120μm and 6μm, respectively. This yields 400 reflective units in a single pass. After a single pulse of probe light passes through the phased array step reflector device 3, 400 probe lights with different time delays are obtained. The step heights between each reflective mirror surface are determined based on measurement requirements, enabling femtosecond to picosecond temporal resolution. The reflected light, converged by the convex lens, overlaps and focuses at a single location. A sample exhibiting Kerr refraction or carrier refraction is placed at this location, and pump light is applied to the same point on the sample. The pump light spot size is larger than the probe light spot size, and the time delay between the pump light pulse and the probe light is adjusted. After the single pulse of probe light reflected by the phased array step reflector device 3 passes through the sample, it is reconstructed by the convex lens 9 and ultimately imaged on the data recording device 10. The change in the spatial distribution of the intensity of the probe spot after passing through the sample is the change caused by the nonlinear refraction of the sample. After image processing, the magnitude and sign of the nonlinear refraction within the sample at different moments can be obtained. In addition, by adding a beam splitter between the beam splitter 2 and the converging lens 4, two data recording devices 10 can be used for simultaneous detection: one to monitor the spatial distribution of the probe light, and the other to monitor the spatial distribution of the probe light after the pump light is applied, thereby improving the measurement signal-to-noise ratio.

[0035] In one embodiment, the sample is a semiconductor material zinc selenide (ZnSe), and the specific detection steps are as follows: 1) blocking the pump light, placing no sample on the sample holder, emitting probe light, and using a data recording device to receive the probe light spot after being reflected by the phase array step mirror device; the light spot received by the data recording device is the probe light background spot; 2) blocking the pump light, placing the sample to be tested on the sample holder, emitting probe light, and using a data recording device to receive the probe light spot after being reflected by the phase array step mirror device and passing through the sample; the light spot received by the data recording device is the probe light spot when linear absorption of the sample exists; 3) turning on the pump light, irradiating the sample to be tested with the pump light, emitting a single pulse of probe light, and using a data recording device to receive the light spot after being reflected by the phase array step mirror device and passing through the sample; the light spot received by the data recording device is the single pulse of probe light spot containing the nonlinear refractive information of the sample; 4) processing the three light spots obtained above to obtain the nonlinear refractive index of the sample to be tested. In the step 4), it includes: step d1), dividing the sum of all pixel values ​​contained in the linear absorption detection light spot in step b) by the sum of all pixel values ​​contained in the detection light background spot in step a), to obtain the linear absorption coefficient of the sample; step d2), dividing the sum of all pixel values ​​contained in each step reflector image in the nonlinear refraction detection light spot in step c) by the sum of all pixel values ​​contained in each step reflector image corresponding to the linear absorption detection light spot in step b), to obtain the normalized transmittance containing only nonlinear absorption at different delay times; step d3), then dividing each pixel value of the nonlinear refraction detection light spot in step c) by the corresponding pixel value in the linear absorption detection light spot in step b), and the spot information after the division corresponds to the phase array step. The normalized transmittance difference including nonlinear absorption at different delay times can be obtained by subtracting the transmittance at the edge of each phase reflector from the transmittance of the central phase object of each phase reflector of the ladder reflector device; step d4), dividing the normalized transmittance difference including nonlinear absorption obtained in step d3) by the normalized transmittance including only nonlinear absorption obtained in step d2), to obtain the normalized transmittance difference without nonlinear absorption; step d5), arranging the normalized transmittance difference without nonlinear absorption obtained in step d4) according to the order of delay time of the single-pulse detection light sequence, to obtain a dynamic curve of the transmittance change caused by the nonlinear refraction of the sample with the change of delay time; step d6), fitting the transient nonlinear refractive index of the sample using a pump-probe fitting program with a phase object.

[0036] The present invention uses a measuring device with a phase array stepped mirror device to achieve the measurement of nonlinear refractive index changes of a sample excited by a pump pulse using a single pulse probe light. Compared with other measurement technologies, it has the following advantages: 1. The optical path is very simple, and the nonlinear refractive index changes of the material can be obtained by only a stepped mirror array with a phase object, a lens and a CCD; 2. Data acquisition speeds of ps or even fs can be achieved without an expensive CCD; 3. The measurement is very convenient, and the nonlinear refractive index changes of the material after the action of the pump light can be obtained by only a single laser pulse without moving the sample; 4. In this method, the high-precision moving platform in the traditional optical path is not required to achieve time delay, and only a time-delayed stepped mirror is required to achieve time delay; 5. Compared with the current gated framing camera (tens of picoseconds), its time resolution is greatly improved.

[0037] The specific process of the experiment and theoretical calculation of Kerr refraction and carrier refraction measurement of ZnSe is as follows:

[0038] Since the probe light energy is very weak compared to the pump light, two-photon absorption of the pump light becomes the only way to generate free carriers, and its rate can be expressed as:

[0039]

[0040] I in the formula e is the pump light intensity at the sample, τ r is the free carrier lifetime. The change in the absorption coefficient caused by free carriers is proportional to the excess carrier concentration and can be expressed as:

[0041] △α f =σ α △N(t) (2)

[0042] Where σ α is the free carrier absorption cross section. Taking into account the combined effect of bound electrons and free carriers, using thin samples and slowly varying amplitude approximation, the propagation formulas of the probe light and pump light inside the ZnSe crystal are:

[0043]

[0044]

[0045]

[0046] I in the formula p represents the intensity of the detection light, Δφ prepresents the nonlinear phase shift of the probe light. When only the probe light acts on the sample, the nonlinear effect of the material is very weak. When the surrounding probe light passes through the sample, only linear absorption occurs, with no refractive index change. When the pump light acts on the sample, the stronger pump light excites the sample to produce nonlinearity. For the probe light sequence generated by the phase array step reflector device on the sample, the time delay between the probe light and the pump light is appropriately adjusted. At zero delay, the nonlinear refraction mechanism of ZnSe is optical Kerr refraction. Assuming negative refraction, there is a downward rapid recovery spike within the pulse width range. This is reflected in the spot image in several squares near zero delay. The light intensity distribution within the phase object is significantly weaker than the surrounding light intensity distribution. Other theoretical content will not be repeated here.

[0047] The foregoing descriptions are merely preferred embodiments of the present invention and should not be construed as limiting the scope of the present invention. All equivalent variations and modifications made in accordance with the claims and description of the present invention remain within the scope of the present invention. Furthermore, no embodiment or claim of the present invention is required to achieve all of the objectives, advantages, or features disclosed herein. Furthermore, the abstract and invention title are intended solely to assist in patent document retrieval and are not intended to limit the scope of the present invention.

Claims

1. A device for measuring single-pulse transient nonlinear refractive index, characterized in that: The measuring device includes a detection system, a pumping system and a data recording device. The detection system includes a detection laser source and a 4f imaging system. The 4f imaging system consists of a converging lens and an imaging lens. The sample is located between the converging lens and the imaging lens. The pumping system intersects the detection system at the sample at an angle. The detection system also includes a phase array step reflector device. The detection light emitted by the detection laser source is reflected to the converging lens by the phase array step reflector device. The phase array step reflector device includes a substrate and a plurality of reflectors. The plurality of reflectors are arranged in m rows and n columns. The plurality of reflectors have a surface area of ​​1000 nm. The surface has a phase object, and the reflecting surfaces of the multiple reflectors are parallel to the bottom surface of the substrate. The reflector located in the first row and first column is recorded as L1C1, the reflector in the mth row and nth column is recorded as LmCn, and any reflector in the phase array step reflector device is recorded as LiCj. The height from the bottom surface of the substrate to any reflector is recorded as H(LiCj). Then the heights of the multiple reflectors satisfy H(LiCj)>H(Li+1Cj), H(LiCj)>H(LiCj+1), H(LiCn)>H(Li+1C1), where m, n, i, j are integers, and 1≤i≤m, 1≤j≤n.

2. The device for measuring the single-pulse transient nonlinear refractive index according to claim 1, characterized in that: The detection laser source is a picosecond laser source or a femtosecond laser source.

3. The device for measuring single-pulse transient nonlinear refractive index according to claim 1, characterized in that: The detection system further includes a beam splitter, which is arranged between the detection laser source and the phase array stepped reflector device.

4. The device for measuring single-pulse transient nonlinear refractive index according to claim 1, characterized in that: The measuring device further includes a sample holder for fixing the sample, and the sample holder is arranged at the back focal plane of the converging lens and the front focal plane of the imaging lens.

5. The device for measuring the single-pulse transient nonlinear refractive index according to claim 4, characterized in that: The pumping system includes a pumping laser source and a first convex lens, and the sample holder is located at the focal plane of the first convex lens.

6. The device for measuring single-pulse transient nonlinear refractive index according to claim 5, characterized in that: The pumping system further includes a pumping light absorbing device, which is located on the other side of the sample holder relative to the pumping laser source and the first convex lens.

7. The device for measuring single-pulse transient nonlinear refractive index according to claim 1, characterized in that: The data recording device is located at the image plane of the 4f imaging system.

8. The device for measuring single-pulse transient nonlinear refractive index according to claim 1, characterized in that: The focal lengths of the converging lens and the imaging lens may be different, and the front focal plane of the converging lens is the object plane of the 4f imaging system.

9. The device for measuring single-pulse transient nonlinear refractive index according to claim 1, characterized in that: The object plane of the 4f imaging system is the surface of the phase array stepped reflector device.

10. The device for measuring single-pulse transient nonlinear refractive index according to claim 1, characterized in that: The step height between two reflectors in the same row and adjacent columns is denoted as h, and the step heights between the multiple reflectors satisfy H(L i C j )-H(L i C j+1 )=h,H(L i C j )-H(L i+1 C j )=nh,H(L i C n )-H(L i+1 C1)=h, where the step height h is equal to the delay time resolution multiplied by the speed of light.

11. The device for measuring single-pulse transient nonlinear refractive index according to claim 1, characterized in that: The width of the step window of the phase array step mirror device is equal to N multiplied by the pixel size of the data recording device divided by the magnification of the imaging system, where N is the number of pixels of the recording device occupied by each plane phase mirror.

12. A method for measuring a single-pulse transient nonlinear refractive index, comprising measuring a sample using the device for measuring a single-pulse transient nonlinear refractive index as claimed in any one of claims 1 to 11, wherein: The measuring method comprises: Step a), without placing the sample, shielding the pump light, emitting the detection light, and the data recording device recording the background spot information of the detection light, Step b), placing the sample, shielding the pump light, emitting the probe light, and the data recording device recording the linear absorption of the probe light spot by the sample, Step c), placing the sample, emitting a single pulse of probe light and pump light, and a data recording device recording the nonlinear refraction spot information of the single pulse of probe light of the sample, Step d) processing the light spot information obtained in steps a), b) and c) to obtain the transient nonlinear refractive index of the sample.

13. The method for measuring single-pulse transient nonlinear refractive index according to claim 12, characterized in that: In the step c), the light spot formed by the pump light on the sample is larger than and covers the light spot formed by the probe light on the sample.

14. The method for measuring single-pulse transient nonlinear refractive index according to claim 12, characterized in that: In the step d), it includes: Step d1) dividing the sum of all pixel values ​​contained in the linear absorption detection light spot in step b) by the sum of all pixel values ​​contained in the detection light background spot in step a) to obtain the linear absorption coefficient of the sample; Step d2) dividing the sum of all pixel values ​​contained in each step reflector image in the nonlinear refraction detection light spot in step c) by the sum of all pixel values ​​contained in each corresponding step reflector image in the linear absorption detection light spot in step b) to obtain normalized transmittance containing only nonlinear absorption at different delay times; In step d3), each pixel value of the nonlinear refraction detection light spot in step c) is divided by the corresponding pixel value of the linear absorption detection light spot in step b). The light spot information after the division corresponds to the transmittance of the central phase object of each phase reflector of the phase array step reflector device, and the transmittance of the edge of each phase reflector is subtracted to obtain the normalized transmittance difference including nonlinear absorption at different delay times; Step d4), dividing the normalized transmittance difference including nonlinear absorption obtained in step d3) by the normalized transmittance including only nonlinear absorption obtained in step d2) to obtain a normalized transmittance difference excluding nonlinear absorption; Step d5) arranging the normalized transmittance differences obtained in step d4) after removing nonlinear absorption in the order of the delay time of the single-pulse detection light sequence to obtain a kinetic curve of the transmittance change caused by the nonlinear refraction of the sample versus the delay time; Step d6), using a pump-probe fitting program with a phase object to fit the transient nonlinear refractive index of the sample.

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