Active terahertz polarization conversion and detection method based on gap regulation metasurface

Abstract

According to the active terahertz polarization conversion and detection method based on the gap regulation metasurface, any completely polarized light can be decomposed into two orthogonal polarization components, the two components have a certain phase difference and amplitude ratio, and target polarization response can be obtained by changing the phase difference or the amplitude ratio. Therefore, in order to obtain the required polarized light, the phase difference between the two orthogonal components of the incident polarized light can be changed by using any phase delayer, namely a wave plate, so that a target polarization state is obtained; according to the detection method, based on the reciprocity principle of an electromagnetic field, a device with a polarization conversion function can be reconfigured into a polarization detection system under time reversal symmetry, different polarization states are switched according to a time sequence through the conversion relation between a Jones matrix and a Muller matrix, and the detection accuracy is improved. And the light intensity with different polarization components is measured in sequence to realize full Stokes polarization detection of incident polarized light.

Description

Technical Field

[0001] The present invention relates to the technical field of active terahertz polarization metasurface design, and in particular to an active terahertz polarization conversion and detection method based on gap-tuned metasurface. Background Technique

[0002] Terahertz waves are electromagnetic waves with a frequency range of 0.1 THz to 10 THz and a wavelength range of 0.03 mm to 3 mm. Since terahertz waves are in the transition region between photonics and electronics, they have both photonic and electronic characteristics. Terahertz has low photon energy, and many materials can exhibit fingerprint spectrum characteristics in the terahertz band, making it have potential application prospects in fields such as material structure exploration, biomedicine, and food detection; at the same time, terahertz has a wider frequency band range and shorter wavelength than microwaves, making it have important application values in ultra-wideband wireless communication, high-resolution radar imaging, etc. These unique properties and applications are expected to bring great convenience to human production and life and are called one of the most important emerging disciplines in this century.

[0003] Polarization, as one of the important physical properties of electromagnetic waves, represents the vector characteristics of the electric field of electromagnetic waves and plays an important role in the interaction between light and matter. In the field of communication, linear polarization is generally used to transmit information, while circular polarization is used to transmit information in complex climate environments. Outside the field of wireless communication, polarization control is also widely used in fields such as polarization imaging, radar detection, and electromagnetic stealth. To meet different application scenarios, it is necessary to control the polarization state of electromagnetic waves. Being able to freely control the polarization state of electromagnetic waves is a current research frontier direction and also has important application prospects in many fields.

[0004] For the polarization control of terahertz waves, traditional polarization control devices are usually made of birefringent materials such as calcite and quartz. However, traditional materials are usually large in size and have serious dispersion, which do not meet the design requirements of device miniaturization and high precision. Metamaterials can have arbitrary equivalent permeability and permittivity by changing the shape, size, and arrangement of sub-wavelength structures. Metasurface is the two-dimensional counterpart of three-dimensional metamaterials. It not only has some electromagnetic characteristics similar to metamaterials but also has many excellent characteristics such as low loss, smaller size, higher efficiency, and easy preparation. Related research has attracted extensive interest from researchers in recent years and is widely used in the design of various imaging devices and can also be used to achieve functions that ordinary natural materials cannot achieve, such as perfect lenses and negative refraction.

[0005] Most traditional metasurfaces are purposefully distributed with basic units in a periodic or aperiodic manner to realize various wavefront control functional devices. Their functions mostly remain fixed after construction, and the processing of light propagation cannot be adjusted in real time, failing to meet the requirements of multifunctionality, multi-scenarios, and integration in many practical applications. To achieve dynamic polarization control of terahertz waves, a dynamic OMS (Optical metasurface, OMS) needs to be designed. Researchers usually adopt two methods: one is to use dynamically controllable constituent materials, such as liquid crystals, etc., whose optical properties can be adjusted by external driving, thereby realizing the adjustment of optical response and the OMS reconstruction function. However, materials such as liquid crystals have a long response time and low modulation efficiency. The other is to directly modify its geometric parameters through mechanical driving. For example, using MEMS actuators can achieve faster and more precise execution operations, and the current design and manufacturing technologies are also becoming increasingly mature.

[0006] Based on the reciprocity principle of electromagnetic fields, devices with polarization conversion functions can be reconstructed into polarization detection systems under time-reversal symmetry. Common terahertz polarization detection methods can be divided into the following two categories: one is based on optical elements such as wire grid polarizers and wave plates, and polarization detection is achieved through selective transmission or phase delay. However, due to the typical bandwidth of the wire grid polarizer being 3 THz, there are large losses in the high-frequency band. Moreover, the extinction ratio of the wire grid polarizer is limited, and its typical value is about 1000:1. Even if two wire grid polarizers with orthogonal transmission axes are placed in the optical path, the terahertz intensity passing through these two polarizers will not become zero, resulting in the generation of other interference signals. The other is to use electro-optic (EO) sampling technology to achieve polarization detection by intensity modulation of polarization-sensitive optoelectronic signals. When the terahertz electric field is incident on the electro-optic crystal, the refractive index ellipsoid of the crystal is changed through the E0 effect, and the change of the refractive index ellipsoid is detected by using the polarization change of the ultrafast laser pulse that is also incident on the E0 crystal in the terahertz time domain spectroscopy (TDS). However, in this type of method, the sensitivity of the electromagnetic wave signal depends on the crystal orientation and the polarization states of the terahertz pulse and the probe pulse in the TDS, with low flexibility.

[0007] With the development of the research on metasurfaces, many terahertz polarization detection methods based on metasurfaces have emerged by utilizing their flexible wavefront control, mainly including the following two categories. One category is spatial separation methods such as amplitude division method, focal plane division method, and aperture division method. Through the periodic arrangement of metasurface unit structures with different wavefront controls, light with different polarizations is separated in space and the intensities of each component are measured to achieve polarization detection. However, multiplexing inevitably leads to a further increase in the size and complexity of the system, sacrificing spatial resolution and system flexibility. The second category is the time-division method, that is, by switching different polarization states in a time series, the light intensities with different polarization components are measured in turn. Since the time-division method only requires a single detector and simple optical elements, and each pixel can record all polarization information in turn, it reduces the hardware complexity and cost, and can also maintain a high spatial resolution. By dynamically adjusting the polarization state combination, the device can adapt to different detection requirements, so it has high flexibility and scalability, and is more in line with the current development trend of miniaturization and high precision of technology. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide an active terahertz polarization conversion based on gap-tuned metasurface.

[0009] Another technical problem to be solved by the present invention is to provide a detection method for the above-mentioned active terahertz polarization conversion based on gap-tuned metasurface.

[0010] To solve the above technical problems, the technical solution of the present invention is:

[0011] An active terahertz polarization conversion based on gap-tuned metasurface, any completely polarized light can be decomposed into two orthogonal polarization components, and these two components have a certain phase difference and amplitude ratio. By changing the phase difference or amplitude ratio, the target polarization response can be obtained. Therefore, in order to obtain the required polarized light, any phase retarder, i.e., wave plate, can be used to change the phase difference between the two orthogonal components of the incident polarized light, so as to obtain the target polarization state.

[0012] Preferably, for the above-mentioned active terahertz polarization conversion based on gap-tuned metasurface, after a linearly polarized light beam is incident on the wave plate, it is divided into two orthogonal components, the ordinary light (o-light) and the extraordinary light (e-light), due to the birefringence effect of the crystal, and the refractive indices correspond to n o and n e . In the non-optic axis direction, n o ≠n e , that is, the propagation speeds of the two light beams in the crystal are different, so when passing through a crystal with a thickness of d, a certain phase difference δ will be generated:

[0013]

[0014] This phase difference is the phase delay of the light wave in the slow axis direction relative to the light wave in the fast axis direction. When two linearly polarized light beams with perpendicular vibration directions and a certain phase difference are superposed, polarized light in any polarization state can be obtained according to different phase differences and amplitude ratios.

[0015] As the two-dimensional counterpart of three-dimensional metamaterials, metasurfaces have electromagnetic properties similar to those of metamaterials. When used to achieve polarization conversion of terahertz waves, it can be understood that the metasurface interacts with electromagnetic waves to regulate the amplitude and phase of the incident light, thereby achieving the required polarization response. After normalization, the incident light and the outgoing light can be respectively represented by the Jones matrix as follows:

[0016]

[0017] Among them, a 1 and a 2 are respectively the amplitudes of the two orthogonal polarization components of the incident light, a = a 2 / a 1 is the amplitude ratio of the two components, and δ is the phase difference between the two orthogonal polarizations. Similarly, a' 1 and a' 2 are respectively the amplitudes of the two polarization components corresponding to the reflected light (transmitted light), a' = a' 2 / a' 1 is the amplitude ratio of the two components of the outgoing light (transmitted light), and δ' is the phase difference between the two components of the reflected light (transmitted light).

[0018] The above detection method for active terahertz polarization conversion based on gap-tuned metasurfaces, based on the reciprocity principle of electromagnetic fields, a device with polarization conversion function can be reconstructed into a polarization detection system under time-reversal symmetry. In the present invention, through the conversion relationship between the Jones matrix and the Muller matrix, different polarization states are switched according to the time sequence, and the light intensities with different polarization components are measured in turn to achieve full Stokes polarization detection of the incident polarized light.

[0019] Preferably, the above detection method for active terahertz polarization conversion based on gap-tuned metasurfaces is as follows:

[0020] First, for the incident light with an unknown polarization state, its polarization state is described by the stokes vector as follows:

[0021]

[0022] To measure S in , the incident light first passes through the metasurface. To facilitate the detection of circularly polarized light, it then passes through a polarizer and is finally detected by a photodetector. When the incident light is modulated, its stokes vector becomes:

[0023] S out= M(gap)·S in #(11)

[0024] Where M is the 4×4 Muller matrix of the measurement system, which is a function of the metasurface gap gap:

[0025] M(gap) = Mp·M meta (gap)#(12)

[0026] Where M P and M meta are the Muller matrices of the polarizer and the metasurface respectively. M(gap) can be obtained by converting the system Jones matrix.

[0027] First, we describe the FP-type metasurface with the Jones matrix. The Jones matrix of the entire measurement system is:

[0028] J = J P ·J meta #(13)

[0029] Where J P and J meta are the Jones matrices of the polarizer and the metasurface respectively. If the angle between the transmission axis of the polarizer and the X-axis is θ P , then the Jones matrix of the polarizer is:

[0030]

[0031] The designed metasurface is not chiral, and its cross polarization is ignored. Therefore, the Jones matrix of the metasurface is a diagonal matrix, that is

[0032]

[0033] Where r x and r y represent the complex amplitudes of the X- and Y-polarized light reflected by the metasurface respectively. Therefore,

[0034]

[0035] According to the conversion formula between the Jones matrix and the Muller matrix:

[0036]

[0037] Where * represents the complex conjugate, represents the tensor product,

[0038]

[0039] Also, since the photodetector can only detect the light intensity, therefore, Equation (11) can be rewritten as:

[0040] S out，0 = M 0 (gap)·S in #(19)

[0041] Among them, M 0 is the first row of M, that is:

[0042] M v (gap)= [m 00 (gap)m 01 (gap)m 02 (gap)m 03 (gap)]#(20)

[0043] Substituting Equation (16) and Equation (18) into Equation (17), the first row of the measurement system Muller matrix can be obtained as follows:

[0044]

[0045] Among them,

[0046] By discretely changing the gap of the metasurface through the motor, the following linear equations can be obtained:

[0047]

[0048] Among them, respectively represent the first row of the system Muller matrix corresponding to the i-th gap, represents the optical power of the reflected light at this gap, and n represents that M under n different gaps has been measured 0 (gap) and the reflected light power;

[0049] By solving the linear equations (22) with S in as the independent variable, the full Stokes parameters of the incident light can be obtained. It should be noted that since the above linear equations have four independent variables, in order to obtain the correct solution, the number of equations should be greater than or equal to 4 and the rank of the coefficient matrix is non-zero. Because of the inevitable systematic errors in the experiment, the above equations may become overdetermined equations, and at this time, an appropriate optimization method should be used to fit the optimal solution.

[0050] Beneficial effects:

[0051] The above-mentioned active terahertz polarization conversion and detection method based on gap-tuned metasurface combines technologies such as electro-control, mechanics, and metasurface design to provide an active terahertz polarization conversion and detection method based on gap-tuned metasurface, which can achieve active regulation of reflected terahertz polarization and detection of incident terahertz polarization. It mainly solves the following problems: (1) Most of the existing regulation methods for active terahertz polarization conversion metasurfaces use materials that can be dynamically controlled, and generally have problems such as long response time, low modulation efficiency, and poor stability. (2) The signal sensitivity of existing terahertz polarization detection methods depends on the polarization states of terahertz pulses and probe pulses in the TDS system, and spatial separation methods such as the amplitude division method sacrifice spatial resolution, etc., all of which have problems such as high hardware complexity, poor system flexibility, and high cost. Description of the Drawings

[0052] Figure 1 It is a structural diagram of an FP resonator.

[0053] Figure 2 It is a schematic diagram of the metasurface microstructure.

[0054] Figure 3 It is a schematic diagram of an active metasurface based on gap regulation.

[0055] Figure 4 It is a schematic diagram of the unit structure.

[0056] Figure 5 It is an amplitude and phase spectral diagram.

[0057] Figure 6 It is a diagram of the phase difference change in the resonant region.

[0058] Figure 7 It is a diagram of the variation of amplitude, phase, and the phase difference between two orthogonal components with gap.

[0059] Figure 8 It is an effect diagram of polarization conversion.

[0060] Figure 9 It is S out Curve comparison diagram. Detailed Embodiments

[0061] The following describes a basic temperature and pressure dual-mode sensing unit of the non-electric multi-modal tactile sensor of the present invention in conjunction with embodiments and drawings.

[0062] Embodiment 1

[0063] An active terahertz polarization conversion and detection method based on gap-tuned metasurface is as follows:

[0064] (1) Polarization conversion

[0065] Any completely polarized light can be decomposed into two orthogonal polarization components, which have a certain phase difference and amplitude ratio. By changing the phase difference or amplitude ratio, the target polarization response can be obtained. Therefore, in order to obtain the required polarized light, any phase retarder, i.e., a wave plate, can be used to change the phase difference between the two orthogonal components of the incident polarized light, thereby obtaining the target polarization state. After a linearly polarized light beam is incident on a wave plate, it is divided into two orthogonal components, the ordinary light (o-light) and the extraordinary light (e-light), due to the birefringence effect of the crystal, and the refractive indices correspond to n o and n e . In the non-optic axis direction, n o ≠n e , that is, the propagation speeds of the two light beams in the crystal are different. Then, when passing through a crystal with a thickness of d, a certain phase difference δ will be generated:

[0066]

[0067] This phase difference is the phase delay of the light wave in the slow axis direction relative to the light wave in the fast axis direction. In this way, two linearly polarized light beams with perpendicular vibration directions and a certain phase difference are superimposed. According to the different phase differences and amplitude ratios, polarized light with any polarization state can be obtained.

[0068] As the two-dimensional counterpart of three-dimensional metamaterials, metasurfaces have electromagnetic properties similar to those of metamaterials. When used to achieve polarization conversion of terahertz waves, it can be understood that metasurfaces interact with electromagnetic waves to regulate the amplitude and phase of the incident light, thereby achieving the required polarization response. After normalization, the incident light and the outgoing light can be represented by Jones matrices respectively as:

[0069]

[0070] where a 1 and a 2 are the amplitudes of the two orthogonal polarization components of the incident light respectively, a = a 2 / a 1 is the amplitude ratio of the two components, and ψ is the phase difference between the two orthogonal polarizations. Similarly, a′ 1 and a′ 2 are the amplitudes of the two polarization components corresponding to the reflected light (transmitted light) respectively, a′ = a′ 2 / a′ 1 is the amplitude ratio of the two components of the outgoing light (transmitted light), and δ′ is the phase difference between the two components of the reflected light (transmitted light).

[0071] To elaborate on the regulation principle of the phase difference between the two orthogonal components by metasurfaces, take the incident light as a linearly polarized light at 45° to the X-axis as an example. As Figure 2As shown, Fig. (a) is a square metal microstructure, and Fig. (b) is a rectangular metal microstructure. The linearly polarized light incident at 45° can be decomposed into two orthogonal components along the X and Y directions, and their amplitudes are the same, that is, the amplitude ratio is 1. Then, when the structure shown in Fig. (a) is considered, since the lengths in the X and Y directions are equal and completely symmetric, the two components have the same equivalent refractive index in the X-axis and Y-axis directions, that is, the same propagation phase. Therefore, the phase difference between the two components of the outgoing light remains unchanged, and polarization conversion cannot occur. When the structure shown in Fig. (b) is considered, since the lengths in the X and Y directions are not equal, the equivalent refractive indices of the two components are also different, resulting in different propagation phases, and the phase difference also changes accordingly, thus realizing the conversion of the polarization state of the outgoing light.

[0072] It should be noted that for a passive metasurface, when the unit structure is determined, the phase difference between the two orthogonal components of the electric field is also fixed, and thus the polarization conversion ability for electromagnetic waves is limited. To achieve a more comprehensive and flexible control of the polarization state, this method combines the metasurface with a planar mirror to form an FP resonator, and realizes the polarization control function from a zero-order waveplate to a full-wave plate by adjusting the air gap between the two.

[0073] The FP resonator, also known as a plane-parallel cavity, is a passive optical resonator jointly invented by Charles Fabry and Alfred Perot. As Figure 1 shown, an FP cavity usually consists of two parallel plates M 1 , M 2 with certain reflectivities r, r':

[0074] If the amplitude of the incident light is A, and the refractive indices of surfaces M 1 and M 2 are t, t' respectively, then the amplitude of the reflected light is:

[0075]

[0076] The amplitude of the transmitted light is:

[0077]

[0078] When the two surfaces are kept parallel, the optical path difference between adjacent reflected light and transmitted light is ΔL = 2ndcosθ, and the phase difference is The complex amplitudes of the reflected light and transmitted light are respectively:

[0079]

[0080] From the above equations, by summing the geometric series, the complex amplitudes of the total reflected light and total transmitted light can be obtained as:

[0081]

[0082] Therefore, by changing the cavity length d of the FP cavity, the phase difference between adjacent light beams can be changed, thereby realizing the tuning of the complex amplitudes of the reflected light and the transmitted light. Combining the regulation of the phase difference between two orthogonal components of the incident electromagnetic wave by the metasurface, the free regulation of the phase difference between the two orthogonal components from 0 to 2π can be realized, thus realizing the active polarization regulation from a zero-order wave plate to a full-wave plate.

[0083] (2) Polarization detection

[0084] Based on the reciprocity principle of the electromagnetic field, devices with polarization conversion functions can be reconstructed into polarization detection systems under time-reversal symmetry. In the present invention, according to the conversion relationship between the Jones matrix and the Muller matrix, different polarization states are switched in time series, and the light intensities with different polarization components are measured in sequence to realize the full Stokes polarization detection of the incident polarized light.

[0085] First, for the incident light with an unknown polarization state, its polarization state is described by the Stokes vector as follows:

[0086]

[0087] To measure S in , the incident light first passes through the metasurface. To facilitate the detection of circularly polarized light, it then passes through a polarizer and is finally detected by a photodetector. When the incident light is modulated, its Stokes vector becomes:

[0088] S out = M(gap)·S in #(11)

[0089] where M is the 4×4 Muller matrix of the measurement system, which is a function of the metasurface gap gap:

[0090] M(gap)= M P ·M mEta (gap)#(12)

[0091] where M P and M meta are the Muller matrices of the polarizer and the metasurface, respectively. M(gap) can be obtained by converting the system Jones matrix.

[0092] First, we describe the FP-type metasurface with the Jones matrix. The Jones matrix of the entire measurement system is:

[0093] J = J P ·J meta #(13)

[0094] where J P and J metaThe Jones matrices of the polarizer and the metasurface respectively. If the angle between the transmission axis of the polarizer and the X-axis is θ P , then the Jones matrix of the polarizer is:

[0095]

[0096] The designed metasurface is achiral, and its cross polarization is ignored. Therefore, the Jones matrix of the metasurface is a diagonal matrix, that is

[0097]

[0098] where r x and r y represent the complex amplitudes of the X- and Y-polarized light reflected by the metasurface respectively. Therefore,

[0099]

[0100] According to the conversion formula between the Jones matrix and the Mueller matrix:

[0101]

[0102] where * represents the complex conjugate, represents the tensor product,

[0103]

[0104] Also, since the photodetector can only detect the light intensity, therefore, Equation (11) can be rewritten as:

[0105] S out，0 = M 0 (gap)·S in #(19)

[0106] where M 0 is the first row of M, that is:

[0107] M 0 (gap)= [m 00 (gap)m 01 (gap)m 02 (gap)m 03 (gap)]#(20)

[0108] Substituting Equation (16) and Equation (18) into Equation (17), the first row of the Mueller matrix of the measurement system can be obtained as:

[0109]

[0110] where,

[0111] By discretely changing the gap of the metasurface through a motor, the following system of linear equations can be obtained:

[0112]

[0113] Wherein, respectively represent the first row of the system Muller matrix corresponding to the i-th gap, represents the optical power of the reflected light at this gap, and n represents that a total of n different gaps are measured for M 0 (gap) and the reflected light power.

[0114] By solving the system of linear equations (22) with S in as the independent variable, the full Stokes parameters of the incident light can be obtained. It should be noted that since the above system of linear equations has four independent variables, in order to obtain the correct solution, the number of equations should be greater than or equal to 4 and the rank of the coefficient matrix is non-zero. Because of the inevitable systematic errors in the experiment, the above system of equations may become an overdetermined system of equations. At this time, a suitable optimization method should be used to fit the optimal solution.

[0115] Example 2

[0116] For the active terahertz polarization conversion and its detection method based on gap regulation of the metasurface described in Example 1, the schematic diagram of the active metasurface based on gap regulation is as shown Figure 3 in the figure. Different materials are distinguished by different gray levels in the figure. The dark gray substrate selects quartz material with a dielectric constant of 3.75, and the light gray C-shaped split ring is a metal material, gold. The terahertz wave is incident vertically along the +z direction, and after multiple reflections in the FP cavity composed of the C-shaped split ring and the planar reflective gold mirror, it exits along the -z direction through the quartz substrate. Figure 4 The schematic diagram of the unit structure is shown, and the geometric parameters adopted in the present invention are marked. The period P x = P y = 170 μm, the substrate thickness The outer radius R of the C-shaped split ring is 75 μm, the inner radius r is 55 μtm, and the opening angle is The metal thickness t Au = 200 nm. The change range of the gap gap between the structure and the mirror is 0 - 300 μm.

[0117] To analyze the regulation effect of the unit structure, the present invention uses the CST Microwave Studio electromagnetic simulation software for parameter scanning, device design and simulation. To obtain the amplitude and phase distributions of the reflected light, modeling is performed according to the above geometric parameters in the frequency-domain and time-domain solvers. The Unit cell boundary condition is adopted in the x and y directions of the unit structure, and the perfectly matched Open boundary condition is set in the z direction. By performing parameter scanning with gap as the variable, the scanning range is determined to be 5 - 300 μm, and the scanning interval is 5 μm, the following spectral lines can be obtained, where Figure 5 (a) and (b) are respectively the S11 amplitude and phase spectral lines of y-polarized incidence and y-polarized reflection, Figure 5 (c) and (d) are respectively the S22 amplitude and phase spectral lines of x-polarized incidence and x-polarized reflection.

[0118] From Figure 6 it can be seen that there is an obvious resonance region. According to the phase change, 0.65 THz in the resonance frequency band is selected as the target frequency point, and the phase difference between the x-direction and y-direction polarizations with respect to the change of gap is observed. As Figure 6 shown, as the gap changes, the distance between the two phase curves also changes, that is, the phase difference between the two orthogonal components changes with the change of gap.

[0119] Processing the time-domain simulation data with Matlab can obtain the changes of the amplitude, phase and the phase difference between the two orthogonal components of the structure at 0.65 THz with respect to gap as Figure 7 (a), (b) and (c) shown. It can be seen from the figure that the phase difference between the two orthogonal components can cover 0 - 2π, and the reflectivity is greater than 95% when gap ≥ 10 μm. It can be seen that the designed metasurface can achieve the polarization conversion function from a zero-waveplate to a full-waveplate with high efficiency.

[0120] To intuitively reflect the polarization conversion effect of the designed metasurface, taking the incidence of 45° linearly polarized light as an example, the above data is plotted on the Poincaré sphere by the method of plotting points, and the effect is as Figure 8 shown. It can be seen that the simulation results are in good agreement with the theoretical situation.

[0121] To verify the polarization detection effect of the designed metasurface, taking the incidence of right-handed circularly polarized light as an example, the above simulation results S22 and S11 are used as the Jones matrix of the metasurface, and the reflected light passes through a 45° linear polarizer and then reaches the photodetector. The first row of the Muller matrix of the test system is obtained through Equation (21), and finally the optimal solution of the linear equation system (22) with n = 60 is fitted by the optimization method, and the solution is:

[0122]

[0123] The solid angle included angle with the perfect right-handed circularly polarized light on the Poincaré sphere is 0.049°, that is, the error is extremely small. To more intuitively show the accuracy of polarization detection, the solved S in is respectively multiplied by 60 different measurement system Muller matrices to obtain the inverse solution of S varying with the gap out curve, and compared with the actual S out curve to obtain the result as Figure 9 shown.

[0124] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art of the present technology, without departing from the principle of the present invention, several improvements and refinements can also be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. An active terahertz polarization conversion based on a gap-controlled metasurface, characterized in that: Any completely polarized light can be decomposed into two orthogonal polarization components. These two components have a certain phase difference and amplitude ratio. The target polarization response can be obtained by changing the phase difference or amplitude ratio. In order to obtain the required polarized light, an arbitrary phase retarder, i.e., a wave plate, is used to change the phase difference between the two orthogonal components of the incident polarized light, thereby obtaining the target polarization state.

2. The active terahertz polarization conversion based on gap-controlled metasurface according to claim 1, characterized in that: After a beam of linear polarized light enters the wave plate, it is divided into two orthogonal components, ordinary light and extraordinary light, due to the birefringence effect of the crystal. The refractive indices are n and n respectively. o and n e ; In the non-optical axis direction, n o ≠n e , that is, the two beams of light have different propagation speeds in the crystal, and when they pass through a crystal with a thickness of d, a certain phase difference δ will be generated: The phase difference is the phase delay of the light wave in the slow axis direction relative to the light wave in the fast axis direction. In this way, two linearly polarized lights with mutually perpendicular vibration directions and a certain phase difference are superimposed, and polarized light of any polarization state can be obtained according to the different phase difference and amplitude ratio. As the two-dimensional counterpart of three-dimensional metamaterials, metasurfaces have similar electromagnetic properties to metamaterials. When used to achieve polarization conversion of terahertz waves, it can be understood that the metasurface interacts with electromagnetic waves to regulate the amplitude and phase of the incident light, thereby achieving the required polarization response; the incident light and the outgoing light can be expressed by the Jones matrix after normalization as follows: Among them, a1 and a2 are the amplitudes of the two orthogonal polarization components of the incident light, a=a2 / a1 is the amplitude ratio of the two components, and δ is the phase difference of the two orthogonal polarizations; similarly, a′1 and a′2 are the amplitudes of the two polarization components corresponding to the reflected light, a′=a′2 / a′1 is the amplitude ratio of the two components of the outgoing light, and δ is the phase difference of the two components of the reflected light.

3. The above-mentioned detection method of active terahertz polarization conversion based on gap-controlled metasurface is characterized by: Based on the reciprocity principle of electromagnetic fields, devices with polarization conversion function can be reconstructed into polarization detection systems under time reversal symmetry. The present invention switches different polarization states in a time series through the conversion relationship between the Jones matrix and the Muller matrix, and sequentially measures the light intensity with different polarization components to achieve full Stokes polarization detection of the incident polarized light.

4. The detection method of active terahertz polarization conversion based on gap-controlled metasurface according to claim 3, characterized in that: Here are the steps: First, for the incident light of unknown polarization state, the Stokes vector is used to describe its polarization state as follows: To measure S in , the incident light first passes through the metasurface, and in order to facilitate the detection of circularly polarized light, it passes through a polarizer and is finally detected by the photodetector; when the incident light is modulated, its Stokes vector becomes: S out =M(gap)·S in #(11) Where M is the 4×4 Muller matrix of the measurement system, which is a function of the metasurface gap: M(gap)=M P ·M meta (gap)#(12) Among them, M P and M meta are the Muller matrices of the polarizer and metasurface, respectively; M(gap) can be obtained by transforming the system Jones matrix; First, we use the Jones matrix to describe the FP-type metasurface. The Jones matrix of the entire measurement system is: J=J P ·J meta #(13) Among them, J P and J meta are the Jones matrices of the polarizer and the metasurface respectively; if the angle between the transmission axis of the polarizer and the X-axis is θ P , then the Jones matrix of the polarizer is: The designed metasurface has no chirality and its cross polarization is ignored, so the metasurface Jones matrix is ​​a diagonal matrix, that is, Among them, r x and r y Respectively represent the complex amplitudes of X and Y polarized light reflected by the metasurface; therefore, According to the conversion formula of Jones matrix and Muller matrix: Where * represents complex conjugation, represents the tensor product, Since the photoelectric detector can only detect light intensity, equation (11) can be rewritten as: S out，0 =M0(gap)·S in #(19) Among them, M0 is the first row of M, that is: M0(gap)=[m 00 (gap)m 01 (gap)m 02 (gap)m 03 (gap)]#(20) Substituting equations (16) and (18) into equation (17), we can obtain the first row of the Muller matrix of the measurement system: in, By discretely changing the gap of the metasurface through the motor, the following linear equations can be obtained: in, They represent the first row of the system Muller matrix corresponding to the i-th gap, represents the optical power of the reflected light under the gap, and n represents that M0(gap) and the reflected light power under n different gaps are measured; By solving S in The linear equations (22) with π as independent variables can be used to obtain the full Stokes parameters of the incident light. It is worth noting that since the above linear equations have four independent variables, in order to obtain the correct solution, the number of equations should be greater than or equal to 4 and the rank of the coefficient matrix should be non-zero. Due to the inevitable systematic errors in the experiment, the above equations may become overdetermined. In this case, a suitable optimization method should be used to fit the optimal solution.

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