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, any phase retarder, namely a wave plate, is used for changing the phase difference between two orthogonal components of the incident polarized light, so that a target polarization state is obtained; the detection method is 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 based on a gap-controlled metasurface and a detection method thereof. Background Art

[0002] Terahertz waves are electromagnetic waves with frequencies ranging from 0.1 THz to 10 THz and wavelengths from 0.03 mm to 3 mm. Terahertz waves lie in the transition zone between photonics and electronics, possessing both photonic and electronic properties. Terahertz photons have low energy, and many materials exhibit fingerprint spectral characteristics in the terahertz band, making them promising applications in material structure exploration, biomedicine, and food testing. Furthermore, terahertz waves have a wider frequency band and shorter wavelength than microwaves, making them valuable in ultra-wideband wireless communications and high-resolution radar imaging. These unique properties and applications are expected to greatly facilitate human production and life, earning them the title of one of the most important emerging disciplines of this century.

[0003] Polarization, a key physical property of electromagnetic waves, represents the vector characteristics of the electric field of electromagnetic waves and plays a crucial role in the interaction between light and matter. In the field of communications, linear polarization is generally used to transmit information, while circular polarization is used to transmit information in complex climate environments. Beyond wireless communications, polarization manipulation is also widely used in fields such as polarization imaging, radar detection, and electromagnetic stealth. To meet diverse application scenarios, it is necessary to control the polarization state of electromagnetic waves. The ability to freely manipulate the polarization state of electromagnetic waves is currently a cutting-edge research direction and holds important application prospects in many fields.

[0004] For 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 severe dispersion, which does not meet the design requirements of miniaturization and high precision of devices. Metamaterials can have arbitrary equivalent magnetic permeability and dielectric constant by changing the shape, size and arrangement of subwavelength structures. Metasurfaces are the two-dimensional counterpart of three-dimensional metamaterials. They not only have some electromagnetic properties similar to metamaterials, but also have many excellent properties such as low loss, smaller size, higher efficiency and easy preparation. Related research has attracted widespread interest among researchers in recent years and is widely used in the design of various imaging devices. It can also be used to achieve functions that ordinary natural materials cannot achieve, such as perfect lenses and negative refraction.

[0005] Traditional metasurfaces are typically constructed by purposefully distributing basic units in a periodic or aperiodic pattern to implement various wavefront manipulation devices. However, these functions remain fixed after construction, and their processing of light propagation cannot be adjusted in real time. This makes them unable to meet the multifunctional, multi-scenario, and integrated requirements of many practical applications. To achieve dynamic polarization control of terahertz waves, a dynamic optical metasurface (OMS) (OMS) must be designed. Researchers typically employ two approaches: First, dynamically controllable materials, such as liquid crystals, whose optical properties can be adjusted via external actuation, thereby enabling optical response adjustment and OMS reconfiguration. However, materials like liquid crystals suffer from long response times, low modulation efficiency, and poor stability. Second, metasurface reconfiguration can be achieved by directly modifying its geometric parameters through mechanical actuation. For example, MEMS actuators (MEMS) can achieve faster and more precise actuation, and current design and manufacturing technologies are maturing.

[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, since the typical bandwidth of the wire grid polarizer is 3THz, the loss is large in the high frequency band. In addition, 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 mutually orthogonal transmission axes are placed in the optical path, the terahertz intensity passing through the two polarizers will not become zero, resulting in the generation of other interference signals. The second is to use electro-optical (E0) sampling technology to achieve polarization detection by intensity modulating the polarization-sensitive photoelectric signal. A terahertz electric field is incident on an electro-optical crystal, changing the crystal's refractive index ellipsoid through the E0 effect. This change in the refractive index ellipsoid is then detected using the polarization changes of an ultrafast laser pulse also incident on the E0 crystal in a terahertz time-domain spectroscopy (TDS) system. However, the sensitivity of the electromagnetic wave signal in this method depends on the crystal orientation and the polarization states of the terahertz pulse and probe pulse in TDS, resulting in limited flexibility.

[0007] With the development of metasurface research, leveraging their flexible wavefront control, numerous metasurface-based terahertz polarization detection methods have emerged. These methods primarily fall into two categories. The first involves spatial separation methods, including amplitude division, focal plane division, and aperture division. These methods utilize a periodic arrangement of metasurface unit structures with different wavefront controls to spatially separate light of different polarizations and measure the intensity of each component, thereby achieving polarization detection. However, multiplexing inevitably increases system size and complexity, sacrificing spatial resolution and system flexibility, resulting in high application costs. The second category involves time-sharing methods, which sequentially measure the intensity of light with different polarization components by switching between different polarization states in a time-sequential manner. Because time-sharing methods require only a single detector and simple optical components, and each pixel can sequentially record all polarization information, they reduce hardware complexity and cost while maintaining high spatial resolution. By dynamically adjusting the polarization state combination, the device can adapt to diverse detection requirements, offering greater flexibility and scalability, further aligning with current technological trends toward miniaturization and high precision.

[0008] However, most current polarization-conversion metasurfaces are passive metasurfaces, which can only achieve one-to-one or many-to-one polarization state conversion from one polarization state to another, that is, the polarization conversion that can be achieved is limited; and the polarization detector needs to rely on complex optical systems to measure the spectrum, which needs further research and improvement. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide an active terahertz polarization conversion based on a gap-controlled metasurface.

[0010] Another technical problem to be solved by the present invention is to provide a detection method for active terahertz polarization conversion based on the above-mentioned gap-controlled metasurface.

[0011] In order to solve the above technical problems, the technical solution of the present invention is:

[0012] An active terahertz polarization conversion method based on gap-controlled metasurface is proposed. A linearly polarized light is incident on a wave plate and then split into two orthogonal components, ordinary light (o light) and extraordinary light (e light), by 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 the 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. In this way, two beams of linearly polarized light with perpendicular vibration directions and a certain phase difference are superimposed. Depending on the phase difference and amplitude ratio, polarized light of any polarization state can be obtained.

[0015] In the aforementioned active terahertz polarization conversion method based on a gap-controlled metasurface, any fully polarized light can be decomposed into two orthogonal polarization components. 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, to obtain the desired polarized light, an arbitrary phase retarder, or 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.

[0016] Preferably, in the above-mentioned active terahertz polarization conversion method based on gap-controlled metasurface, the metasurface is the two-dimensional counterpart of the three-dimensional metamaterial and has electromagnetic properties similar to those of the metamaterial. When used to achieve polarization conversion of terahertz waves, the metasurface interacts with the electromagnetic wave and regulates the amplitude and phase of the incident light, thereby achieving the required polarization response. The incident light and the output light are normalized and expressed by the Jones matrix as follows:

[0017]

[0018] 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 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), 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).

[0019] The above-mentioned detection method of active terahertz polarization conversion based on gap-controlled metasurface is based on the reciprocity principle of electromagnetic fields. Devices with polarization conversion function can be reconstructed into polarization detection systems under time reversal symmetry. Through the conversion relationship between the Jones matrix and the Muller matrix, different polarization states are switched in a time series, and the light intensity with different polarization components is measured in turn to realize full Stokes polarization detection of the incident polarized light.

[0020] Preferably, the above-mentioned detection method of active terahertz polarization conversion based on gap-controlled metasurface comprises the following specific steps:

[0021] ① By measuring the complex amplitude S22 (corresponding to r x , that is, x polarization incident, x polarization reflected) and S11 (corresponding to r y , i.e. y polarization incident, y polarization reflected), different M0(gap) can be obtained: Formula (21)

[0022]

[0023] in,

[0024] At the same time, since formula (21) is the coefficient matrix of the linear equation group (22), the corresponding reflected light intensity under different gaps is measured, that is, We get a system of equations (22) with four unknowns,

[0025]

[0026] 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, n represents that M0(gap) and the reflected light power under n different gaps are measured, and the four values of the Stokes parameters are obtained by solving the equation group.

[0027] The above-mentioned detection method of active terahertz polarization conversion based on gap-controlled metasurface has the following specific steps:

[0028] (1) For incident light of unknown polarization state, the Stokes vector is used to describe its polarization state as follows:

[0029]

[0030] 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:

[0031] S out =M(gap)·S in #(11)

[0032] Where M is the 4×4 Muller matrix of the measurement system, which is a function of the metasurface gap:

[0033] M(gap)=M P ·M meta (gap)#(12)

[0034] Among them, M P and M meta are the Muller matrices of the polarizer and metasurface, respectively, and M(gap) is obtained by transforming the system Jones matrix;

[0035] (2) The Jones matrix is used to describe the FP metasurface. The Jones matrix of the entire measurement system is:

[0036] J=JP ·J meta #(13)

[0037] Among them, J P and J meta are the Jones matrices of the polarizer and the metasurface respectively; if the angle between the polarizer transmission axis and the X axis is θ P , then the Jones matrix of the polarizer is:

[0038]

[0039] The designed metasurface has no chirality and its cross polarization is ignored, so the metasurface Jones matrix is a diagonal matrix, that is,

[0040]

[0041] Among them, r x and r y Respectively represent the complex amplitudes of X and Y polarized light reflected by the metasurface. Therefore,

[0042]

[0043] According to the conversion formula of Jones matrix and Muller matrix:

[0044]

[0045] Where * represents complex conjugate, represents the tensor product,

[0046]

[0047] Since the photoelectric detector can only detect light intensity, equation (11) can be rewritten as:

[0048] S out，0 =M0(gap)·S in #(19)

[0049] Among them, M0 is the first row of M, that is:

[0050] M0(gap)=[m 00 (gap)m 01 (gap)m 02 (gap)m 03 (gap)]#(20)

[0051] Substituting equations (16) and (18) into equation (17), we can obtain the first row of the Muller matrix of the measurement system:

[0052]

[0053] in,

[0054] By discretely changing the gap of the metasurface through the motor, the following linear equations can be obtained:

[0055]

[0056] 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;

[0057] By solving S in The full Stokes parameters of the incident light can be obtained by solving the linear equation system (22) with as the independent variable.

[0058] Preferably, the above-mentioned detection method for active terahertz polarization conversion based on gap-controlled metasurface has the following polarization conversion functions:

[0059] (1) Measure the reflection spectrum of the metasurface and use mechanical methods to change the geometric parameters of the metasurface to achieve reconstruction of the metasurface:

[0060] 1-① Set the incident light to x-polarized light and change the air gap (gap). Each time the gap changes, measure the complex amplitude of the x-polarized component in the reflected light to obtain rx (S22).

[0061] By changing the gap → measuring the complex amplitude → changing the gap, we can get a set of rx, [rx] = [rx gap1 , rx gap2 , rx gap3 ,........rx gapn ](When the incident light is x-polarized, the reflected light is basically x-polarized, and the y-polarization component is almost 0 and can be ignored).

[0062] 1-② Set the incident light to y-polarized light and change the air gap gap. Each time the gap changes, measure the complex amplitude of the y-polarized component in the reflected light to obtain ry (S22).

[0063] By changing the gap → measuring the complex amplitude → changing the gap, we can get a set of ry, [ry] = [ry gap1 ,ry gap2 ,ry gap3 ,........ry gapn ](When the incident light is y-polarized, the reflected light is basically y-polarized, and the x-polarization component is almost 0 and can be ignored).

[0064] The rxgapi (S22) and ry gapi (S11) is the complex amplitude of the two orthogonal polarization components x and y of the metasurface under the same gapi;

[0065] (2) Change the polarization state of the incident light, measure the reflection spectrum of the metasurface, verify the polarization conversion function, and actively control the polarization state of the reflected light by changing the gap. That is, the gap can be controlled as needed to obtain the target polarization state, which is highly flexible:

[0066] 2-① Change the incident light to linearly polarized light with a polarization angle of θ, change the air gap, and measure the complex amplitudes rx and ry of the corresponding x-polarization component and y-polarization component of the reflected light respectively;

[0067] 2-② Once the complex amplitudes rx and ry of the two orthogonal components of the reflected light x and y are known, the specific polarization state and Jones vector of the reflected light are known. To more intuitively demonstrate the polarization conversion effect, the Stokes vector of the reflected light is obtained based on the conversion relationship between the Jones vector and the Stokes vector. Points are plotted on the Poincare sphere with the Stokes parameters as the coordinate axes to visually see the polarization conversion effect (a closed curve can be formed on the surface of the Poincare sphere). A gap corresponds to a set of rx and ry, corresponds to a polarization state, and corresponds to a point on the Poincare sphere.

[0068] Preferably, the above-mentioned detection method of active terahertz polarization conversion based on gap-controlled metasurface has the following polarization detection functions:

[0069] (1) Calibrate the metasurface

[0070] The reflection spectra of x-polarized light and y-polarized light incident on the metasurface are measured to obtain rx and ry, that is, the Muller matrix of the metasurface is obtained, that is, the coefficient matrix of the linear equation group (22) is obtained.

[0071] (2) Measuring the intensity of reflected light

[0072] The polarization state of the incident light is unknown. In order to facilitate the detection of elliptical polarization, a 45° linear polarizer is placed in front of the light detector, the gap is changed, and the intensity of the reflected light is measured (one gap corresponds to one reflected light intensity), thus obtaining the augmented matrix of the linear equation system (22);

[0073] (3) Solve equation group (22) to obtain the polarization state of the incident light

[0074] From steps (1) and (2), a complete set of linear equations containing four unknowns (four Stokes parameters) is obtained. The Stokes vector of the incident light, that is, the polarization state of the incident light, can be obtained using the SQP algorithm.

[0075] The above-mentioned polarization detection function is different from other polarization detectors that need to rely on complex optical systems to measure spectral lines. After the device is calibrated, the method described in the present invention only requires a single detector and simple optical elements. By measuring the intensity of the reflected light, the polarization state of the incident light can be directly inferred.

[0076] Beneficial effects:

[0077] The above-mentioned active terahertz polarization conversion and detection method based on gap-controlled metasurface, combined with technologies such as electrical control, mechanics and metasurface design, can realize active control of reflected terahertz polarization and detection of incident terahertz polarization. The control and conversion method has the characteristics of short response time, high modulation efficiency and strong stability. The detection method only requires a single detector and simple optical elements, and does not rely on the terahertz time-domain spectroscopy system to measure the polarization state of the incident light, reducing hardware complexity and detection costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 This is a diagram of the FP resonant cavity structure.

[0079] Figure 2 is a schematic diagram of the super surface microstructure, where L in (a) x =L y , n x =n y ;L x ≠L y , n x ≠n y .

[0080] Figure 3 Schematic diagram of an active metasurface based on gap regulation.

[0081] Figure 4 It is a schematic diagram of the unit structure.

[0082] Figure 5 is the amplitude and phase spectrum.

[0083] Figure 6 It is the phase difference change diagram in the resonance area.

[0084] Figure 7 This is a graph showing how the amplitude, phase, and phase difference of the two orthogonal components change with the gap.

[0085] Figure 8 This is a diagram of the polarization conversion effect.

[0086] Figure 9 It's S out Curve comparison chart. DETAILED DESCRIPTION

[0087] A basic temperature-pressure dual-mode sensing unit of the ionization multi-modal tactile sensor of the present invention will be described below with reference to the embodiments and drawings.

[0088] Example 1

[0089] An active terahertz polarization conversion and detection method based on a gap-controlled metasurface, comprising the following steps:

[0090] (1) Polarization conversion

[0091] Any completely polarized light can be decomposed into two orthogonal polarization components. 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, an arbitrary 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 beam of linearly polarized light enters the wave plate, it is divided into two orthogonal components, ordinary light (o light) and extraordinary light (e light), due to the birefringence effect of the crystal. The refractive indices correspond to 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 the crystal with a thickness of d, a certain phase difference δ will be generated:

[0092]

[0093] This phase difference is the phase delay of the light wave in the slow axis relative to the light wave in the fast axis. By superimposing two linearly polarized beams with mutually perpendicular vibration directions and a certain phase difference, polarized light of any polarization state can be obtained depending on the phase difference and amplitude ratio.

[0094] Metasurfaces, as the two-dimensional counterpart of three-dimensional metamaterials, possess similar electromagnetic properties to metamaterials. When used to achieve polarization conversion of terahertz waves, it can be understood that the metasurface interacts with the electromagnetic wave to control the amplitude and phase of the incident light, thereby achieving the required polarization response. The normalized incident and outgoing light can be expressed using the Jones matrix as follows:

[0095]

[0096] Where 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 between the two orthogonal polarization components. Similarly, a'1 and a'2 are the amplitudes of the two corresponding polarization components of the reflected (transmitted) light, a' = a'2 / a'1 is the amplitude ratio of the two components of the outgoing (transmitted) light, and δ' is the phase difference between the two components of the reflected (transmitted) light.

[0097] To elaborate on the principle of how the metasurface regulates the phase difference of two orthogonal components, we take the incident light as an example, which is linearly polarized at 45° to the X-axis. Figure 2 As shown, Figure 2 (a) is a square metal microstructure. Figure 2 (b) is a rectangular metal microstructure. The linear polarization 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 Figure 2 In the structure shown in (a), since the lengths in the X and Y directions are equal and completely symmetrical, the two components have the same equivalent refractive index in the X and Y 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. Figure 2 In the structure shown in (b), due to the unequal lengths in the X and Y directions, the equivalent refractive indices of the two components are also different, resulting in different propagation phases. The phase difference also changes accordingly, thereby achieving the conversion of the polarization state of the outgoing light.

[0098] It is worth noting that for passive metasurfaces, when the unit structure is determined, the phase difference between the two orthogonal components of the electric field is also fixed, thus limiting the polarization conversion capability of electromagnetic waves. To achieve more comprehensive and flexible control of the polarization state, the metasurface is combined with a plane mirror to form an FP resonant cavity. By adjusting the air gap between the two, the polarization control function from zero-wave plate to full-wave plate can be achieved.

[0099] The FP resonant cavity, also known as the plane parallel cavity, is a passive optical resonant cavity invented by Charles Fabry and Alfred Perot. Figure 1 As shown, the FP cavity is usually composed of two parallel plates M1 and M2 with certain reflectivity r and r':

[0100] If the amplitude of the incident light is A, and the refractive indices of surfaces M1 and M2 are t and t' respectively, then the amplitude of the reflected light is:

[0101]

[0102] The amplitude of the transmitted light is:

[0103]

[0104] When the two surfaces remain parallel, the optical path difference between adjacent reflected and transmitted lights is ΔL = 2ndcosθ, and the phase difference is The complex amplitudes of the reflected light and the transmitted light are:

[0105]

[0106] From the above formula, by summing the geometric series, we can get the complex amplitudes of the total reflected light and the total transmitted light respectively:

[0107]

[0108] Therefore, by varying the FP cavity length d, the phase difference between adjacent light beams can be altered, thereby tuning the complex amplitudes of the reflected and transmitted light. Combined with the metasurface's ability to manipulate the phase difference between the two orthogonal components of the incident electromagnetic wave, the phase difference between the two orthogonal components can be freely adjusted from 0 to 2n, thus achieving active polarization control from a zero-wave plate to a full-wave plate.

[0109] (2) Polarization detection

[0110] 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.

[0111] First, for the incident light with unknown polarization state, the Stokes vector is used to describe its polarization state as follows:

[0112]

[0113] 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:

[0114] S out =M(gap)·S in #(11)

[0115] Where M is the 4×4 Muller matrix of the measurement system, which is a function of the metasurface gap:

[0116] M(gap)=M P ·M meta (gap)#(12)

[0117] 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.

[0118] First, the FP-type metasurface is described by the Jones matrix. The Jones matrix of the entire measurement system is:

[0119] J=J P ·J meta #(13)

[0120] Among them, J P and J meta are the Jones matrices of the polarizer and the metasurface respectively. If the angle between the polarizer transmission axis and the X axis is θ P , then the Jones matrix of the polarizer is:

[0121]

[0122] The designed metasurface has no chirality and its cross polarization is ignored, so the metasurface Jones matrix is a diagonal matrix, that is,

[0123]

[0124] Among them, r x and r y Respectively represent the complex amplitudes of X and Y polarized light reflected by the metasurface. Therefore,

[0125]

[0126] According to the conversion formula of Jones matrix and Muller matrix:

[0127]

[0128] Where * represents complex conjugate, represents the tensor product,

[0129]

[0130] Since the photoelectric detector can only detect light intensity, equation (11) can be rewritten as:

[0131] S out，0 =M0(gap)·S in #(19)

[0132] Among them, M0 is the first row of M, that is:

[0133] M0(gap)=[m 00 (gap)m 01 (gap)m 02 (gap)m 03 (gap)]#(20)

[0134] Substituting equations (16) and (18) into equation (17), we can obtain the first row of the Muller matrix of the measurement system:

[0135]

[0136] in,

[0137] By discretely changing the gap of the metasurface through the motor, the following linear equations can be obtained:

[0138]

[0139] 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.

[0140] By solving S in The full Stokes parameters of the incident light can be obtained by solving the linear equation system (22) with as the independent variable.

[0141] It is worth noting that since the above linear equations have four independent variables, in order to obtain a 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.

[0142] Example 2

[0143] The active terahertz polarization conversion and detection method based on the gap-controlled metasurface described in Example 1 are shown in FIG. Figure 3 As shown in the figure, different grayscale colors are used to distinguish different materials. The dark gray substrate is quartz with a dielectric constant of 3.75, and the light gray C-shaped split ring is gold. A terahertz wave is incident vertically along the +z direction, undergoes multiple reflections in the FP cavity formed by the C-shaped split ring and the flat gold mirror, and then exits along the -z direction through the quartz substrate. Figure 4 The schematic diagram of the unit structure is shown, and the geometric parameters used in the present invention are marked, and the period P x =P y =170μm, substrate thickness , the outer radius of the C-type open ring R = 75μm, the inner radius r = 55μm, and the opening angle is Metal thickness t Au = 200 nm. The gap between the structure and the reflector varies from 0 to 300 μm.

[0144] In order to analyze the control effect of the unit structure, CST Microwave Studio electromagnetic simulation software was used for parameter scanning, device design and simulation. In order to obtain the amplitude and phase distribution of the reflected light, modeling was performed in the frequency domain and time domain solvers according to the above geometric parameters. Unit cell boundary conditions were used in the x and y directions of the unit structure, and a perfectly matched open boundary condition was set in the z direction. By performing parameter scanning with gap as the variable, the scanning range was determined to be 5-300μm, and the scanning interval was 5μm, the following spectrum can be obtained, where Figure 5 (a) Figure 5 (b) S11(ry) amplitude and phase spectra for y-polarized incident and y-polarized reflected, respectively. Figure 5 (c) Figure 5 (d) The amplitude and phase spectra of S22(rx) for x-polarized incidence and x-polarized reflection, respectively.

[0145] The above S11 and S22 are S parameters, which describe the impact of the network on the incident wave and the reflected wave. They are widely used in RF and microwave engineering. Their corresponding physical indicators are ry and rx, that is, y-polarized incident, y-polarized reflection and x-polarized incident, x-polarized reflection.

[0146] Depend on Figure 6 An obvious resonance region is visible. Based on the phase change, 0.65 THz in the resonance frequency band is selected as the target frequency point, and the phase difference between the polarization in the x-direction and the y-direction is observed as the gap changes. Figure 6 As shown in FIG, as the gap changes, the distance between the two phase curves also changes, and the phase difference between the two orthogonal components also changes with the change of the gap.

[0147] By processing the time domain simulation data with Matlab, we can get the changes of the amplitude, phase and phase difference of the two orthogonal components of the structure at 0.65THz with the gap as follows: Figure 7 (a) Figure 7 (b) Figure 7 As shown in (c), the phase difference between the two orthogonal components can cover 0-2π, and the reflectivity is greater than 95% when the gap ≥ 10μm. This shows that the designed metasurface can realize the polarization conversion function from zero-wave plate to full-wave plate with high efficiency.

[0148] In order to intuitively demonstrate the polarization conversion effect of the designed metasurface, taking 45° linearly polarized light as an example, the above data are plotted on the Poincare sphere according to the point drawing method, and the effect is as follows: Figure 8 It can be seen that the simulation results are in good agreement with the theoretical situation.

[0149] In order to verify the polarization detection effect of the designed metasurface, right-handed circularly polarized light is taken as an example. The above simulation results S22 and S11 (rx and ry) are used as the Jones matrix of the metasurface, and the reflected light is made to pass through a 45° linear polarizer before reaching the light detector. The first row of the Muller matrix of the test system is obtained by equation (21). Finally, the optimal solution of the linear equation group (22) with n = 60 is fitted by the optimization method, and the solution is:

[0150]

[0151] The angle between the solid angle of perfect right-handed circularly polarized light and the solid angle on the Poincare sphere is 0.049°, which means the error is extremely small. In order to more intuitively demonstrate the accuracy of polarization detection, the solved S in Multiplying by 60 different measurement system Muller matrices, we can get the inverse solution S that changes with the gap. out curve, and the actual S out The curves are compared to obtain Figure 9 Results shown.

[0152] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. An active terahertz polarization conversion method based on a gap-controlled metasurface, characterized by: A linearly polarized light is incident on a wave plate and is divided into two orthogonal components by the birefringence effect of the crystal, the ordinary light o and the extraordinary light e, with refractive indices 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 the crystal with a thickness of d, a certain phase difference δ will be generated: According to the phase difference and amplitude ratio, polarized light of arbitrary polarization state can be obtained.

2. The active terahertz polarization conversion method based on gap-controlled metasurface according to claim 1, characterized in that: The metasurface interacts with electromagnetic waves to control the amplitude and phase of the incident light to achieve the required polarization response. The incident light and the outgoing light are normalized and expressed using the Jones matrix 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 between 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 between the two components of the reflected light.

3. A detection method for active terahertz polarization conversion based on a gap-controlled metasurface, characterized by: The device with polarization conversion function 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 for active terahertz polarization conversion based on a gap-controlled metasurface according to claim 3, characterized in that: The specific steps are as follows: ① By measuring the complex amplitude rx(S22), different M0(gap) are obtained using formula (21): in, Then measure the corresponding reflected light intensity under different gaps, that is We get a system of equations (22) with four unknowns, 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, n represents that M0(gap) and the reflected light power under n different gaps are measured, and the four values of the Stokes parameters are obtained through equation group (22).

5. The detection method for active terahertz polarization conversion based on gap-controlled metasurface according to claim 4, characterized in that: It has the following polarization conversion functions: (1) Measure the reflection spectrum of the metasurface and use mechanical methods to change the geometric parameters of the metasurface to achieve reconstruction of the metasurface: 1-① Set the incident light to x-polarized light and change the air gap (gap). Each time the gap changes, measure the complex amplitude of the x-polarized component in the reflected light to obtain rx (S22). 1-② Set the incident light to y-polarized light and change the air gap gap. Each time the gap changes, measure the complex amplitude of the y-polarized component in the reflected light to obtain ry (S22). The rx gapi (S22) and ry gapi (S11) is the complex amplitude of the two orthogonal polarization components x and y of the metasurface under the same gapi; (2) Change the polarization state of the incident light, measure the reflection spectrum of the metasurface, verify the polarization conversion function, and actively control the polarization state of the reflected light by changing the gap: 2-① Change the incident light to linearly polarized light with a polarization angle of θ, change the air gap, and measure the complex amplitudes rx and ry of the corresponding x-polarization component and y-polarization component of the reflected light respectively; 2-② Once the complex amplitudes rx and ry of the two orthogonal components of the reflected light x and y are known, the specific polarization state and Jones vector of the reflected light are known. Based on the conversion relationship between the Jones vector and the Stokes vector, the Stokes vector of the reflected light is obtained. The points are plotted on the Poincare sphere with the Stokes parameters as the coordinate axes to visually see the polarization conversion effect. A gap corresponds to a set of rx and ry, a polarization state, and a point on the Poincare sphere.

6. The detection method for active terahertz polarization conversion based on gap-controlled metasurface according to claim 4, characterized in that: It has the following polarization detection functions: (1) Calibrate the metasurface The reflection spectra of x-polarized light and y-polarized light incident on the metasurface are measured to obtain rx and ry, and the Muller matrix of the metasurface is obtained, that is, the coefficient matrix of the linear equation system (22) is obtained; (2) Measuring the intensity of reflected light The polarization state of the incident light is unknown. A 45° linear polarizer is placed in front of the light detector, the gap is changed, and the intensity of the reflected light is measured to obtain the augmented matrix of the linear equation system (22); (3) Solve equation group (22) to obtain the polarization state of the incident light From steps (1) and (2), a complete set of linear equations containing four unknowns is obtained. The Stokes vector of the incident light, that is, the polarization state of the incident light, can be obtained using the SQP algorithm.

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