A method for polarization detection using three-dimensional topological insulator films

Abstract

The present invention relates to a method for polarization detection using a three-dimensional topological insulator film. 0.8 Sb 0.2 )2Te3 thin film is used as a polarization detector. By rotating the detector, photocurrents are obtained at different laser incident angles. The ratio of the measured photocurrent to the reference photocurrent is then compared over multiple tests to uniquely determine the polarization state of the detected laser. The principle is that by rotating the quarter-wave plate, photocurrents at different polarization states are obtained. Then, a fitting formula is used to obtain reference photocurrent values ​​for circularly polarized and linearly polarized light. By comparing the ratio of the photocurrent measured at opposite incident angles to the reference photocurrent, the unique polarization state is determined. The backgate voltage can be used to adjust the performance of the detector to take into account the detection of both circular and linear polarization states, thereby determining the polarization state of the detected laser and calculating the Stokes parameters.

Description

Technical Field

[0001] The present invention relates to the field of polarization detection technology, in particular to a method for performing polarization detection using a three-dimensional topological insulator film. Background Art

[0002] Topological insulators are different from ordinary metals or insulators. Their unique physics makes them have potential application prospects in the fields of spin electronics and quantum computing, and they are now attracting much attention in these fields. (Bi 0.8 Sb 0.2 )2Te3 is a three-dimensional topological insulator material, unique in its topologically protected gapless surface states, which exhibit time-reversal symmetry and contain Dirac electrons with locked spin momentum. These surface states significantly suppress scattering from non-magnetic impurities, resulting in extremely high surface electron mobility. This makes three-dimensional topological insulators promising for applications in quantum computing and novel spintronic devices.

[0003] Typically, we use circularly polarized photoinduced current (CPGE) as an effective means of studying the spin-polarized photocurrent signal in three-dimensional topological insulators. The detection of the circularly polarized photocurrent can reflect the circular polarization state of the incident laser. Linearly polarized photoinduced current (LPGE) can also be used to detect the linearly polarized photocurrent signal in three-dimensional topological insulators. Similarly, the detection of the linearly polarized photocurrent can reflect the linear polarization state of the incident laser. Ultimately, the Stokes parameters, which contain information about the complete polarization state of the light being measured, are derived.

[0004] However, the current problem is that when using a three-dimensional topological insulator to detect the polarization state of the incident laser, the simultaneous presence of CPGE, LPGE, and photon drag effect current complicates the composition of the spin-polarized photocurrent. It is necessary to fit and analyze the photocurrent generated by the incident laser by rotating a quarter-wave plate to extract the circularly polarized and linearly polarized photocurrent signals to derive the laser's polarization state. This shows that using only the above two common techniques to perform polarization detection using a three-dimensional topological insulator still requires the assistance of a large number of optical components. Summary of the Invention

[0005] In light of this, the present invention aims to provide a method for polarization detection using a three-dimensional topological insulator thin film. Specifically, the method involves rotating the detector to obtain photocurrents at different laser incident angles. The ratio of the measured photocurrent to a reference photocurrent over multiple measurements is then compared to uniquely determine the polarization state of the detected laser. This method is convenient, low-cost, and provides accurate detection results.

[0006] To achieve the above object, the present invention adopts the following technical solution: a method for polarization detection using a three-dimensional topological insulator film, which is used to detect the topological insulator (Bi 0.8 Sb 0.2 )2Te3 is grown on a SrTiO3 substrate; the specific steps are as follows:

[0007] Step S1: In the topological insulator (Bi 0.8 Sb 0.2 )2Te3 sample was grown with a 10nm titanium electrode by magnetron sputtering, and a 100nm gold electrode was deposited by electron beam evaporation. The electrode was a square electrode with a side length of 0.5mm and an electrode spacing of about 2.5mm. The substrate surface was coated with silver paste to form a back gate electrode;

[0008] Step S2: Use a 1064nm laser as the excitation light source, let the laser pass through a chopper, a polarizer, and a quarter-wave plate, and irradiate vertically at the midpoint of the line connecting the two electrodes on the sample; the spot diameter is smaller than the distance between the two electrodes; the polarization direction of the polarizer is parallel to the fast axis direction of the quarter-wave plate.

[0009] Step S3: The sample is placed in a variable temperature Dewar flask, and a laser is irradiated at the center of the sample. The angle θ between the incident direction of the laser and the sample normal is changed by rotating the sample; a quarter-wave plate is rotated from 0 to 360 degrees at two opposite incident angles ±θ, with a step size of 5 degrees. The photocurrent at each quarter-wave plate angle is amplified by a current amplifier and a lock-in amplifier, and the amplified photocurrent is collected by a data acquisition card;

[0010] Step S4: The photocurrent at one of the incident angles is calculated using the formula (Bi 0.8 Sb 0.2 )Point group symmetry of 2Te3C 3v The polarization-related circularly polarized photocurrent and linearly polarized photocurrent can be extracted by fitting the symmetrical polarized photocurrent formula. The photocurrent fitting formula is as follows:

[0011]

[0012] in, is the angle of rotation of the quarter-wave plate, J total is the measured total photocurrent, C is the circularly polarized photocurrent signal caused by circularly polarized light, L1 and L2 are the linearly polarized photocurrent signals caused by linearly polarized light, and D is the background photocurrent caused by the thermoelectric effect and photovoltaic effect;

[0013] Step S5: applying a back gate voltage using an electrometer, changing the back gate voltage to adjust the performance of the three-dimensional topological insulator polarization detection device, measuring the total photocurrent at ±θ incident angles under different back gate voltages, and extracting the circularly polarized photocurrent signal C, linearly polarized photocurrent signals L1 and L2, and polarization-independent photocurrent signal D under different back gate voltages using formula (1) in step S4;

[0014] Step S6: Calculate |C| / (|C|+|L1|+|L2|) at different back-gate voltages from the circularly polarized photocurrent signal C and the linearly polarized photocurrent signals L1 and L2 extracted by fitting the experimental data in step S5 using formula (1). Plot |C| / (|C|+|L1|+|L2|) as the ordinate and the back-gate voltage as the abscissa, and find the back-gate voltage corresponding to the maximum value of |C| / (|C|+|L1|+|L2|), which is recorded as V G1 (±θ incident angle corresponds to V G1 The same, so only need to determine V at +θ angle of incidence G1 ). Set the back gate voltage to V G1 The total photocurrent measured at an incident angle of +θ is recorded as Calculate the back gate voltage as V G1 The circularly polarized photocurrent component at different quarter-wave plate rotation angles at +θ incident angle is Where C1 is the back gate voltage V G1 The circularly polarized photocurrent signal C is obtained by fitting the formula (1) at the incident angle of +θ. G1 The D obtained by fitting formula (1) at an incident angle of +θ is recorded as Set the back gate voltage to V G1 The total photocurrent measured at an incident angle of -θ is recorded as Calculate the back gate voltage as V G1 The circularly polarized photocurrent component at different quarter-wave plate rotation angles at -θ incident angle is Where C2 is the back gate voltage V G1 The circularly polarized photocurrent signal C is obtained by fitting the formula (1) at the incident angle of -θ. The back gate voltage is V G1 The D obtained by fitting formula (1) at an incident angle of -θ is recorded as

[0015] Find the back gate voltage when |C| / (|C|+|L1|+|L2|) is zero or close to zero, and record it as V G2 (±θ incident angle corresponds to V G2 The same, so only need to determine V at +θ angle of incidence G2 ). Set the back gate voltage to V G2 The total photocurrent measured at an incident angle of +θ is recorded as Calculate the back gate voltage as V G2 The linearly polarized photocurrent component at different quarter-wave plate rotation angles at +θ incident angle is Among them L 13 The back gate voltage is V G2 The linear polarization photocurrent signal L1 is obtained by fitting the formula (1) at the incident angle of +θ. G2 The D obtained by fitting formula (1) at an incident angle of +θ is recorded as Set the back gate voltage to V G2 The total photocurrent measured at an incident angle of -θ is recorded as Calculate the back gate voltage as V G2 The linearly polarized photocurrent component at different quarter-wave plate rotation angles at -θ incident angle is Among them L 14 The back gate voltage is V G2 The linear polarization photocurrent signal L1 is obtained by fitting the formula (1) at the incident angle of -θ. G2 The D obtained by fitting formula (1) at an incident angle of -θ is recorded as

[0016] Step S7: Adjust the back gate voltage to V G1 , which is the circular polarization detection mode. The laser to be measured is incident on the sample at the incident angle ±θ in step S3, and the total photocurrent at this time is measured to be and -θ at an incident angle The total photocurrent measured in step S6 Circularly polarized photocurrent component Polarization-independent photocurrent sum The ratio of As the reference photocurrent, calculate the ratio of the measured photocurrent and the reference photocurrent at the +θ incident angle and And the ratio of the measured photocurrent to the reference photocurrent at a -θ incident angle and by These ratios are used as polar diameters, with different polarization phase angles As the polar angle, plot the polarization angles at different states. Polar plot of the ratio. Find the ratio in the polar plot at +θ. and The polarization phase angle corresponding to the common intersection point, and the ratio is found in the polar coordinate image under -θ and The polarization phase angle corresponding to the common intersection point is finally found, and the only commonly determined polarization phase angle α under the ±θ incident angle is the circular polarization state of the laser to be measured;

[0017] Step S8: Adjust the back gate voltage to V G2 , which is the linear polarization detection mode. The laser to be measured is incident on the sample at the incident angle ±θ in step S3, and the total photocurrent at this time is measured to be and -θ at an incident angle Calculate the total photocurrent measured in step S6 The linearly polarized photocurrent component obtained Polarization-independent photocurrent sum The ratio of As the reference photocurrent, calculate the ratio of the measured photocurrent and the reference photocurrent at the +θ incident angle and And the ratio of the measured photocurrent to the reference photocurrent at a -θ incident angle and by These ratios are used as polar diameters, with different polarization phase angles As the polar angle, draw the polar coordinate graph of the ratio under different polarization states, and compare the polar coordinate image under +θ to find the ratio and The polarization phase angle corresponding to the common ratio is compared with the polar coordinate image under -θ to find the ratio. and The polarization phase angle corresponding to the common ratio is finally found, and the only commonly determined polarization phase angle ψ under the ±θ incident angle is found, which is the linear polarization state of the laser to be measured.

[0018] Step S9: Measure the laser power A using a power meter 2 The elliptical polarization angle α measured in step S7 and the azimuth angle ψ measured in step S8 are respectively substituted into the following Stokes parameter formula to calculate all Stokes parameters.

[0019] S0=A 2 (2)

[0020] S1=A 2 cos2ψcos2α (3)

[0021] S2=A 2 sin2ψcos2α (4)

[0022] S3=A 2 sin2α (5)

[0023] Where S0, S1, S2 and S3 are the four components of Stokes parameters. 2 Substituting into formula (2) we can get the Stokes parameter S0, and 2 Substituting α, ψ into formula (3) and formula (4) can obtain Stokes parameters S1 and S2. 2 Substituting α and α into formula (5) yields the Stokes parameter S3. Calculating the values ​​of all Stokes parameters yields the full polarization state information of the light to be measured.

[0024] In a preferred embodiment: the three-dimensional topological insulator material (Bi 0.8 Sb 0.2 )2Te3 was grown using molecular beam epitaxy (MBE) equipment, and the film thickness was 7nm.

[0025] In a preferred embodiment, the temperature of the variable temperature Dewar flask in step S3 is fixed at 77K.

[0026] In a preferred embodiment, the incident angle ±θ of the laser in step S3 is set to any angle between 60 degrees and 20 degrees.

[0027] In a preferred embodiment: the back gate voltage V G1 is -30V.

[0028] In a preferred embodiment, the back gate voltage V G2 It is 260V.

[0029] Compared with the prior art, the present invention has the following beneficial effects: accurate measurement results, simplicity and efficiency, high feasibility, and is conducive to future promotion and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Schematic diagram of the experimental optical path of a preferred embodiment of the present invention.

[0031] Figure 2 This is a curve showing the variation of the y-axis photocurrent with the quarter-wave plate rotation angle, obtained from steps S3 and S4, when the laser incident angle θ is 30° in a preferred embodiment of the present invention. The small circles represent experimental data, the solid line represents the fitting curve using formula (1), and the dashed lines represent curves for extracting the photocurrents of other polarizations.

[0032] Figure 3The different total photocurrents are measured at different back gate voltages in the preferred embodiment of the present invention, and the circularly polarized photocurrent signal is extracted by fitting the experimental data using formula (1). The proportion of the circularly polarized photocurrent signal in all polarized photocurrent signals at different back gate voltages, |C| / (|C|+|L1|+|L2|), is calculated.

[0033] Figure 4 The topological insulator (Bi 0.8 Sb 0.2 )2Te3 is used as a polarization detector, and the back gate voltage V G1 The polar plot of the circular polarization state of the incident light is detected by the ratio of the photocurrents at opposite incident angles. DETAILED DESCRIPTION

[0034] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0035] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present application belongs.

[0036] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application; as used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form, and it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or their combinations.

[0037] A method for polarization detection using three-dimensional topological insulator films, reference Figures 1 to 4 , the topological insulator (Bi 0.8 Sb 0.2 )2Te3 grown on SrTiO3 substrate; three-dimensional topological insulator material (Bi 0.8 Sb 0.2 )2Te3 is grown using MBE equipment; the method proposes a method for extracting topological insulators (Bi 0.8 Sb 0.2 )2Te3 reference photocurrent for polarization detection, the specific steps are as follows:

[0038] Step S1: In the topological insulator (Bi 0.8 Sb 0.2)2Te3 sample was grown with a 10nm titanium electrode by magnetron sputtering, and a 100nm gold electrode was deposited by electron beam evaporation. The electrode was a square electrode with a side length of 0.5mm and an electrode spacing of about 2.5mm. The substrate surface was coated with silver paste to make a back gate electrode.

[0039] Step S2: Use a 1064nm laser as the excitation light source. Pass the laser light through a chopper, polarizer, and quarter-wave plate, and illuminate the sample perpendicularly at the midpoint of the line connecting the two electrodes. The spot diameter should be smaller than the distance between the two electrodes. The polarization direction of the polarizer should be parallel to the fast axis of the quarter-wave plate.

[0040] Step S3: Place the sample in a variable temperature Dewar flask, irradiate the center of the sample with a laser, and change the angle θ between the laser incident direction and the sample normal direction by rotating the sample; rotate the quarter wave plate from 0 degrees to 360 degrees at two opposite positive and negative angles ±θ, with a step size of 5 degrees, amplify the photocurrent at each quarter wave plate angle through a current amplifier and a phase-locked amplifier, and collect the amplified photocurrent through a data acquisition card.

[0041] Step S4: The photocurrent at one of the incident angles is calculated using the formula (Bi 0.8 Sb 0.2 )Point group symmetry of 2Te3C 3v The polarization-related circularly polarized photocurrent and linearly polarized photocurrent can be extracted by fitting the symmetrical polarized photocurrent formula. The photocurrent fitting formula is as follows:

[0042]

[0043] in, is the polarization phase angle that changes periodically after the quarter-wave plate is rotated, J total is the measured total photocurrent, C is the circularly polarized photocurrent signal caused by circularly polarized light, L1 and L2 are the linearly polarized photocurrent signals caused by linearly polarized light, and D is the background photocurrent caused by the thermoelectric effect and photovoltaic effect.

[0044] Step S5: applying a back gate voltage using an electrometer, changing the back gate voltage to adjust the performance of the three-dimensional topological insulator polarization detection device, measuring the total photocurrent at ±θ incident angles under different back gate voltages, and extracting the circularly polarized photocurrent signal C, linearly polarized photocurrent signals L1 and L2, and polarization-independent photocurrent signal D under different back gate voltages using formula (1) in step S4;

[0045] Step S6: Calculate |C| / (|C|+|L1|+|L2|) at different back-gate voltages from the circularly polarized photocurrent signal C and the linearly polarized photocurrent signals L1 and L2 extracted by fitting the experimental data in step S5 using formula (1). Plot |C| / (|C|+|L1|+|L2|) as the ordinate and the back-gate voltage as the abscissa, and find the back-gate voltage corresponding to the maximum value of |C| / (|C|+|L1|+|L2|), which is recorded as V G1 (±θ incident angle corresponds to V G1 The same, so only need to determine V at +θ angle of incidence G1 ). Set the back gate voltage to V G1 The total photocurrent measured at an incident angle of +θ is recorded as Calculate the back gate voltage as V G1 The circularly polarized photocurrent component at different quarter-wave plate rotation angles at +θ incident angle is Where C1 is the back gate voltage V G1 The circularly polarized photocurrent signal C is obtained by fitting the formula (1) at the incident angle of +θ. G1 The D obtained by fitting formula (1) at an incident angle of +θ is recorded as Set the back gate voltage to V G1 The total photocurrent measured at an incident angle of -θ is recorded as Calculate the back gate voltage as V G1 The circularly polarized photocurrent component at different quarter-wave plate rotation angles at -θ incident angle is Where C2 is the back gate voltage V G1 The circularly polarized photocurrent signal C is obtained by fitting the formula (1) at the incident angle of -θ. The back gate voltage is V G1 The D obtained by fitting formula (1) at an incident angle of -θ is recorded as

[0046] Find the back gate voltage when |C| / (|C|+|L1|+|L2|) is zero or close to zero, and record it as V G2

[0047] (±θ incident angle corresponds to V G2 The same, so only need to determine V at +θ angle of incidence G2 ). Set the back gate voltage to V G2 The total photocurrent measured at an incident angle of +θ is recorded as Calculate the back gate voltage as V G2 The linearly polarized photocurrent component at different quarter-wave plate rotation angles at +θ incident angle is Among them L 13 The back gate voltage is V G2The linear polarization photocurrent signal L1 is obtained by fitting the formula (1) at the incident angle of +θ. G2 The D obtained by fitting formula (1) at an incident angle of +θ is recorded as Set the back gate voltage to V G2 The total photocurrent measured at an incident angle of -θ is recorded as Calculate the back gate voltage as V G2 The linearly polarized photocurrent component at different quarter-wave plate rotation angles at -θ incident angle is Among them L 14 The back gate voltage is V G2 The linear polarization photocurrent signal L1 is obtained by fitting the formula (1) at the incident angle of -θ. G2 The D obtained by fitting formula (1) at an incident angle of -θ is recorded as

[0048] Step S7: Adjust the back gate voltage to V G1 , which is the circular polarization detection mode. The laser to be measured is incident on the sample at the incident angle ±θ in step S3, and the total photocurrent at this time is measured to be and -θ at an incident angle The total photocurrent measured in step S6 Circularly polarized photocurrent component Polarization-independent photocurrent sum The ratio of As the reference photocurrent, calculate the ratio of the measured photocurrent and the reference photocurrent at the +θ incident angle and And the ratio of the measured photocurrent to the reference photocurrent at a -θ incident angle and by These ratios are used as polar diameters, with different polarization phase angles As the polar angle, plot the polarization angles at different states. Polar plot of the ratio. Find the ratio in the polar plot at +θ. and The polarization phase angle corresponding to the common intersection point, and the ratio is found in the polar coordinate image under -θ and The polarization phase angle corresponding to the common intersection point is finally found, and the only commonly determined polarization phase angle α under the ±θ incident angle is the circular polarization state of the laser to be measured;

[0049] Step S8: Adjust the back gate voltage to V G2, which is the linear polarization detection mode. The laser to be measured is incident on the sample at the incident angle ±θ in step S3, and the total photocurrent at this time is measured to be and -θ at an incident angle Calculate the total photocurrent measured in step S6 The linearly polarized photocurrent component obtained Polarization-independent photocurrent sum The ratio of As the reference photocurrent, calculate the ratio of the measured photocurrent and the reference photocurrent at the +θ incident angle and And the ratio of the measured photocurrent to the reference photocurrent at a -θ incident angle and by These ratios are used as polar diameters, with different polarization phase angles As the polar angle, draw the polar coordinate graph of the ratio under different polarization states, and compare the polar coordinate image under +θ to find the ratio and The polarization phase angle corresponding to the common ratio is compared with the polar coordinate image under -θ to find the ratio. and The polarization phase angle corresponding to the common ratio is finally found, and the only commonly determined polarization phase angle ψ under the ±θ incident angle is found, which is the linear polarization state of the laser to be measured.

[0050] Step S9: Measure the laser power A using a power meter 2 The elliptical polarization angle α measured in step S7 and the azimuth angle ψ measured in step S8 are respectively substituted into the following Stokes parameter formula to calculate all Stokes parameters.

[0051] S0=A 2 (2)

[0052] S1=A 2 cos2ψcos2α (3)

[0053] S2=A 2 sin2ψcos2α (4)

[0054] S3=A 2 sin2α (5)

[0055] Where S0, S1, S2 and S3 are the four components of Stokes parameters. 2 Substituting into formula (2) we can get the Stokes parameter S0, and 2Substituting α, ψ into formula (3) and formula (4) can obtain Stokes parameters S1 and S2. 2 Substituting α and α into formula (5) yields the Stokes parameter S3. Calculating the values ​​of all Stokes parameters yields the full polarization state information of the light to be measured.

[0056] In this embodiment, the three-dimensional topological insulator material (Bi 0.8 Sb 0.2 )2Te3 was grown using molecular beam epitaxy equipment, and the thickness of the film was 7nm.

[0057] In this embodiment, the temperature of the variable temperature Dewar flask in step S3 is fixed at 77K, the incident angle θ of the laser is set to 30°, and -θ is set to -30°.

[0058] In this embodiment, the circularly polarized photocurrent signal and the linearly polarized photocurrent signal under different back gate voltages measured in step S5 are set at a laser incident angle of 30° and the applied back gate voltage range is -200V to 260V. The back gate voltage V G1 =-30V, the back gate voltage V G2 It is 260V.

[0059] In this embodiment, if Figure 1 The samples shown were grown on SrTiO3 substrates using molecular beam epitaxy (Bi 0.8 Sb 0.2 )2Te3 thin film, approximately 7nm thick. The 1064nm laser used had an optical power of 110mW and a spot size of approximately 1mm. The laser light passed through a chopper, a polarizer, and a quarter-wave plate before striking the sample at the midpoint of the line connecting the two electrodes. The chopper frequency was 229Hz.

[0060] Figure 2 The embodiment of the present invention adopts a thickness of 7nm (Bi 0.8 Sb 0.2 )2Te3 thin film excited by 1064nm laser at 77K shows the variation curve of photocurrent with quarter-wave plate rotation angle, the formula fitting curve, and the circularly polarized photocurrent (C), linearly polarized photocurrent (L1 and L2), and polarization-independent current (D) obtained by fitting. The incident angle is 30 degrees. The optical power of the 1064nm laser irradiating the sample surface is 110mW. The hollow circles are experimentally measured data, and the solid line is the fitting curve obtained by fitting using formula (1). Through fitting, we can obtain the circularly polarized photocurrent C, linearly polarized photocurrents L1 and L2, and polarization-independent current D.

[0061] Since the chemical potential of the upper and lower surface states of the three-dimensional topological insulator can be adjusted by the back gate, we can measure different total photocurrents at different back gate voltages, and use formula (1) to fit the experimental data to extract the circularly polarized photocurrent signal, and calculate the proportion of the circularly polarized photocurrent signal in all polarized photocurrent signals at different back gate voltages |C| / (|C|+|L1|+|L2|) as follows Figure 3 As shown. It can be seen that in the range of back gate voltage from -200V to 260V, this proportion reaches the maximum value when the back gate voltage is set to -30V. The corresponding voltage is recorded as V G1 , reaches its minimum value when the back gate voltage is set to 260V, and the corresponding voltage is recorded as V G2 . It can be seen that when the back gate voltage is set to V G1 When the back gate voltage is set to V G2 When , the circularly polarized photoelectric response of the thin film device is the lowest and so low that it can be ignored, which means that only linearly polarized photoelectric response exists at this time.

[0062] Since the polarization state of laser light generally includes both circular polarization and linear polarization, if the circular polarization and linear polarization states of laser light can be detected separately, the entire polarization state of laser light can be measured. Therefore, for the device used in this method, the back gate voltage is adjusted to V G1 In order to obtain the maximum circularly polarized photoelectric response, the detection device can be adjusted to the circular polarization detection mode, that is, the circularly polarized photoelectric signal accounts for the largest proportion of all polarized photocurrent signals, and it is also the most accurate mode for circular polarization state detection. Since the circularly polarized photocurrent and linearly polarized photocurrent at opposite incident angles will cause the ratio of the detected photocurrent to the photocurrent reference value to change in polarization states other than the laser polarization state, the ratio of the total photocurrent measured under different polarization states to the theoretical value of the circularly polarized photocurrent or linearly polarized photocurrent in the polarization state will also change. By comparing the ratio of the detection value and the theoretical value under positive and negative opposite incident angles, the polarization state of this laser can be determined by the only identical ratio in the two cases. The total photocurrent measured at this time is and -θ at an incident angle The total photocurrent measured in step S6 The circularly polarized photocurrent extracted by fitting the experimental data with formula (1) is Polarization-independent photocurrent sum The ratio of As the reference photocurrent, calculate the ratio of the measured photocurrent and the reference photocurrent at the +θ incident angle and And the ratio of the measured photocurrent to the reference photocurrent at a -θ incident angle and by These ratios are used as polar diameters, with different polarization phase angles As the polar angle, the polar coordinate diagram of the ratio under different polarization states is plotted as follows Figure 4 As shown, compare the polar coordinate image under +θ to find the ratio and The polarization phase angle corresponding to the common ratio is compared with the polar coordinate image under -θ to find the ratio. and The polarization phase angle corresponding to the common ratio is finally found, and the only commonly determined polarization phase angle α under the ±θ incident angle is found to determine the circular polarization state of the laser to be measured.

[0063] Similarly, the back gate voltage can be adjusted to V G2 When the detection device is adjusted to the linear polarization detection mode, the circular polarization photoelectric signal accounts for the smallest proportion of all polarization photocurrent signals and can be ignored, which is the most accurate mode for linear polarization state detection. The laser to be measured is incident on the sample at the incident angles ±θ in step S3, and the total photocurrent at this time is measured to be and -θ at an incident angle The linearly polarized photocurrent extracted by fitting the experimental data in step S6 by formula (1) is Polarization-independent photocurrent sum That is As the reference photocurrent, calculate the ratio of the measured photocurrent and the reference photocurrent at the +θ incident angle and And the ratio of the measured photocurrent to the reference photocurrent at a -θ incident angle and by These ratios are used as polar diameters, with different polarization phase angles As the polar angle, draw the polar coordinate graph of the ratio under different polarization states, and compare the polar coordinate image under +θ to find the ratio and The polarization phase angle corresponding to the common ratio is compared with the polar coordinate image under -θ to find the ratio. and The polarization phase angle corresponding to the common ratio is finally found, and the only commonly determined polarization phase angle ψ under the ±θ incident angle is found to determine the linear polarization state of the laser to be measured.

[0064] Therefore, after obtaining the polarization phase angle corresponding to the circular polarization state determined by the circular polarization detection mode, that is, the elliptical polarization angle α, and the polarization phase angle corresponding to the linear polarization state determined by the linear polarization detection mode, that is, the azimuth angle ψ, the amplitude A corresponding to the laser light power of 110mW is calculated. 2 Substituting into the formula (2) in step S9, the Stokes parameter S0 can be obtained. 2 Substituting α, ψ into formula (3) and formula (4) in step S9 can obtain Stokes parameters S1 and S2. 2 Substituting α and α into formula (5) in step S9 yields the Stokes parameter S3. Calculating the values ​​of all Stokes parameters yields the full polarization state information of the light to be measured. As can be seen from the above embodiment, this embodiment is relatively convenient, concise, and efficient, and provides relatively accurate measurements.

[0065] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention should fall within the scope of the present invention.

Claims

1. A method for polarization detection using a three-dimensional topological insulator film, characterized in that: Topological insulators (Bi 0.8 Sb 0.2 )2Te3 is grown on a SrTiO3 substrate; the specific steps are as follows: Step S1: In the topological insulator (Bi 0.8 Sb 0.2 )2Te3 sample was grown with a 10nm titanium electrode by magnetron sputtering, and a 100nm gold electrode was deposited by electron beam evaporation. The electrode was a square electrode with a side length of 0.5mm and an electrode spacing of about 2.5mm. The substrate surface was coated with silver paste to form a back gate electrode; Step S2: Using a 1064 nm laser as the excitation light source, the laser passes through a chopper, a polarizer, and a quarter-wave plate, and is vertically irradiated at the midpoint of the line connecting the two electrodes on the sample; the spot diameter is smaller than the distance between the two electrodes; the polarization direction of the polarizer is parallel to the fast axis direction of the quarter-wave plate; Step S3: The sample is placed in a variable temperature Dewar flask, and a laser is irradiated at the center of the sample. The angle θ between the incident direction of the laser and the sample normal is changed by rotating the sample; a quarter-wave plate is rotated from 0 to 360 degrees at two opposite incident angles ±θ, with a step size of 5 degrees. The photocurrent at each quarter-wave plate angle is amplified by a current amplifier and a lock-in amplifier, and the amplified photocurrent is collected by a data acquisition card; Step S4: The photocurrent at one of the incident angles is calculated using the formula (Bi 0.8 Sb 0.2 )Point group symmetry of 2Te3C 3v The polarization-related circularly polarized photocurrent and linearly polarized photocurrent can be extracted by fitting the symmetrical polarized photocurrent formula. The photocurrent fitting formula is as follows: in, is the angle of rotation of the quarter-wave plate, J total is the measured total photocurrent, C is the circularly polarized photocurrent signal caused by circularly polarized light, L1 and L2 are the linearly polarized photocurrent signals caused by linearly polarized light, and D is the background photocurrent caused by the thermoelectric effect and photovoltaic effect; Step S5: applying a back gate voltage using an electrometer, changing the back gate voltage to adjust the performance of the three-dimensional topological insulator polarization detection device, measuring the total photocurrent at ±θ incident angles under different back gate voltages, and extracting the circularly polarized photocurrent signal C, linearly polarized photocurrent signals L1 and L2, and polarization-independent photocurrent signal D under different back gate voltages using formula (1) in step S4; Step S6: Calculate |C| / (|C|+|L1|+|L2|) at different back-gate voltages from the circularly polarized photocurrent signal C and the linearly polarized photocurrent signals L1 and L2 extracted by fitting the experimental data in step S5 by formula (1); plot |C| / (|C|+|L1|+|L2|) as the ordinate and the back-gate voltage as the abscissa, and find the back-gate voltage corresponding to the maximum value of |C| / (|C|+|L1|+|L2|), which is recorded as V G1 (±θ incident angle corresponds to V G1 The same, so only need to determine V at +θ angle of incidence G1 );Set the back gate voltage to V G1 The total photocurrent measured at an incident angle of +θ is recorded as Calculate the back gate voltage as V G1 The circularly polarized photocurrent component at different quarter-wave plate rotation angles at +θ incident angle is Where C1 is the back gate voltage V G1 The circularly polarized photocurrent signal C is obtained by fitting the formula (1) at the incident angle of +θ; the back gate voltage is V G1 The D obtained by fitting formula (1) at an incident angle of +θ is recorded as Set the back gate voltage to V G1 The total photocurrent measured at an incident angle of -θ is recorded as Calculate the back gate voltage as V G1 The circularly polarized photocurrent component at different quarter-wave plate rotation angles at -θ incident angle is Where C2 is the back gate voltage V G1 The circularly polarized photocurrent signal C is obtained by fitting the formula (1) at the incident angle of -θ; the back gate voltage is V G1 The D obtained by fitting formula (1) at an incident angle of -θ is recorded as Find the back gate voltage when |C| / (|C|+|L1|+|L2|) is zero or close to zero, and record it as V G2 (±θ incident angle corresponds to V G2 The same, so only need to determine V at +θ angle of incidence G2 );Set the back gate voltage to V G2 The total photocurrent measured at an incident angle of +θ is recorded as Calculate the back gate voltage as V G2 The linearly polarized photocurrent component at different quarter-wave plate rotation angles at +θ incident angle is Among them L 13 The back gate voltage is V G2 The linearly polarized photocurrent signal L obtained by fitting formula (1) at an incident angle of +θ is 1； Set the back gate voltage to V G2 The D obtained by fitting formula (1) at an incident angle of +θ is recorded as Set the back gate voltage to V G2 The total photocurrent measured at an incident angle of -θ is recorded as Calculate the back gate voltage as V G2 The linearly polarized photocurrent component at different quarter-wave plate rotation angles at -θ incident angle is Among them L 14 The back gate voltage is V G2 The linear polarization photocurrent signal L1 is obtained by fitting the formula (1) at the incident angle of -θ; the back gate voltage is V G2 The D obtained by fitting formula (1) at an incident angle of -θ is recorded as Step S7: Adjust the back gate voltage to V G1 , which is the circular polarization detection mode; the laser to be measured is incident on the sample at the incident angle ±θ in step S3, and the total photocurrent at this time is measured to be respectively and -θ at an incident angle The total photocurrent measured in step S6 Circularly polarized photocurrent component Polarization-independent photocurrent sum The ratio of As the reference photocurrent, calculate the ratio of the measured photocurrent and the reference photocurrent at the +θ incident angle and And the ratio of the measured photocurrent to the reference photocurrent at a -θ incident angle and by These ratios are used as polar diameters, with different polarization phase angles As the polar angle, plot the polarization angles at different states. Polar plot of the ratio. Find the ratio in the polar plot at +θ. and The polarization phase angle corresponding to the common intersection point, and the ratio is found in the polar coordinate image under -θ and The polarization phase angle corresponding to the common intersection point is finally found, and the only commonly determined polarization phase angle α under the ±θ incident angle is the circular polarization state of the laser to be measured; Step S8: Adjust the back gate voltage to V G2 , which is the linear polarization detection mode; the laser to be measured is incident on the sample at the incident angle ±θ in step S3, and the total photocurrent at this time is measured to be respectively and -θ at an incident angle Calculate the total photocurrent measured in step S6 The linearly polarized photocurrent component obtained Polarization-independent photocurrent sum The ratio of As the reference photocurrent, calculate the ratio of the measured photocurrent and the reference photocurrent at the +θ incident angle and And the ratio of the measured photocurrent to the reference photocurrent at a -θ incident angle and by These ratios are used as polar diameters, with different polarization phase angles As the polar angle, draw the polar coordinate graph of the ratio under different polarization states, and compare the polar coordinate image under +θ to find the ratio and The polarization phase angle corresponding to the common ratio is compared with the polar coordinate image under -θ to find the ratio. and The polarization phase angle corresponding to the common ratio is finally found, and the only commonly determined polarization phase angle ψ under the ±θ incident angle is found, which is the linear polarization state of the laser to be measured; Step S9: Measure the laser power A using a power meter 2 The elliptical polarization angle α measured in step S7 and the azimuth angle ψ measured in step S8 are respectively substituted into the following Stokes parameter formula to calculate all Stokes parameters; S0=A 2 (2) S1=A 2 cos2ψcos2α(3) S2=A 2 sin2ψcos2α(4) S3=A 2 sin2α(5) Where S0, S1, S2 and S3 are the four components of Stokes parameters; 2 Substituting into formula (2) we can get the Stokes parameter S0, and 2 Substituting α, ψ into formula (3) and formula (4) can obtain Stokes parameters S1 and S2. 2 Substituting α and α into formula (5) can obtain the Stokes parameter S3; the values ​​of all Stokes parameters are calculated, that is, the complete polarization state information of the light to be measured is obtained.

2. A method for polarization detection using a three-dimensional topological insulator thin film according to claim 1, characterized in that: The three-dimensional topological insulator material (Bi 0.8 Sb 0.2 )2Te3 was grown using molecular beam epitaxy equipment, and the thickness of the film was 7nm.

3. The method for polarization detection using a three-dimensional topological insulator thin film according to claim 1, wherein: The temperature of the variable temperature Dewar flask in step S3 is fixed at 77K.

4. The method for polarization detection using a three-dimensional topological insulator thin film according to claim 1, wherein: In step S3 , the incident angle θ of the laser light is set to any angle between 60 degrees and 20 degrees.

5. The method for polarization detection using a three-dimensional topological insulator thin film according to claim 1, wherein: The back gate voltage V in the circular polarization detection mode in step S7 is G1 is -30V.

6. The method for polarization detection using a three-dimensional topological insulator thin film according to claim 1, wherein: The back gate voltage V in the linear polarization detection mode in step S8 is G2 It is 260V.

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