Snapshot terahertz polarization detection method based on circular polarization multiplexing metasurface

Abstract

The present invention discloses a snapshot terahertz polarization detection method based on a circular polarization multiplexing metasurface, which belongs to the field of polarization state detection and includes the following steps: S1, designing the phase of a metasurface composed of anisotropic cross-shaped super units, and calculating the theoretical value of all Stokes parameters; S2, constructing a simulation model of the metasurface, and calculating the simulation value of all Stokes parameters; S3, comparing the theoretical value of the Stokes parameters of incident light in different polarization states with the simulation value, and verifying the theoretical value of the all Stokes parameters; S4, preparing a metasurface sample, and calculating the experimental value of the all Stokes parameters; S5, comparing the theoretical value of the all Stokes parameters of incident light in different polarization states with the experimental value, to verify the theoretical value of the all Stokes parameters again. The present invention adopts the above-mentioned snapshot terahertz polarization detection method based on a circular polarization multiplexing metasurface, and effectively realizes the accurate detection of polarization information in the terahertz band.

Description

Technical Field

[0001] The present invention relates to the field of polarization state detection, and in particular to a snapshot terahertz polarization detection method based on a circular polarization multiplexing metasurface. Background Art

[0002] Polarization state is a physical quantity used to describe the vibration state of light waves in a plane perpendicular to the direction of light propagation. Since polarized light with different polarization states can be used to record, process and store information, it is widely used in biomedicine, remote sensing navigation, information coding and encryption, etc. Among them, in biomedicine, polarized light produced by fluorescent labeled substances can be used for the determination of genes, drugs, and hormones; in remote sensing navigation, polarized light compasses, polarized light sensors and other devices are used to imitate the mode of biological positioning using polarized light, providing new ideas for navigation; in information coding and encryption, applications such as optical fiber polarization encoders and optical encryption systems improve the speed and accuracy of information coding and reduce information leakage.

[0003] At the same time, compared with radio waves and microwaves, terahertz waves between microwaves and infrared rays have the advantages of high frequency and strong transmission ability, and the energy they carry is not enough to cause ionizing radiation, so they have good characteristics. Since the research on the terahertz band, terahertz imaging technology and terahertz spectrum analysis technology have been derived, which have played a pivotal role in the exploration of non-destructive testing of materials, biomedical testing, photon communication and other fields.

[0004] The frequency of the terahertz band is high, and it is difficult for ordinary equipment to capture the polarization information in this frequency range. In addition, the existing detection technology can only detect the intensity of each component but cannot directly measure its phase difference. Therefore, how to effectively detect the polarization state of the terahertz band is very challenging.

[0005] The common polarization detection methods in the existing terahertz band are as follows:

[0006] 1. For single-frequency continuous terahertz waves, the time-sharing measurement method uses a rotating linear polarizer and a λ / 4 wave plate to measure the power multiple times, and then calculate the Stokes parameters to characterize the polarization state.

[0007] 2. The time-domain terahertz spectroscopy system based on electro-optical sampling can utilize the anisotropy of optical crystals to measure orthogonal linearly polarized terahertz wave components.

[0008] 3. The indirect measurement method based on coordinate rotation can obtain the polarization spectrum information of the photoconductive time-domain spectroscopy system.

[0009] However, the above method still requires time segmentation, takes a long time to test and debug, is not intuitive, and requires multiple measurements.

[0010] Metasurfaces have the characteristics of low profile, miniaturization, and integration, and can manipulate the phase, amplitude, and polarization state of light in a compact form. In order to reduce the complexity of the test system and make the test results more intuitive, Ali Basiri et al. designed a chiral optical circular polarization filter metasurface and integrated it with a linear polarization filter to perform full Stokes polarization measurement of light in any polarization state. However, in essence, it still achieves a single measurement by integrating multiple independent devices, and this design is only applicable to the near-infrared band.

[0011] Xuqian zhang et al. designed a polarization multiplexing metasurface, combined with holographic technology, to achieve direct polarization detection in one measurement, and can read the amplitude and phase difference of polarized light from the generated orthogonal polarization holographic image, thereby extracting the stokes parameters. This design can work in the near infrared to visible light band, but requires holographic image capture and reconstruction, which has high requirements on the algorithm.

[0012] Chenglong Zheng and others designed a terahertz polarization detection metasurface based on the analysis of longitudinal polarization vortex patterns. By analyzing the intensity, phase and mode purity of the longitudinal polarization vortex field, complete polarization parameters were obtained. This design method introduces a mode purity spectrum to determine the handedness of elliptical polarization, and the determination method is relatively complex. Summary of the invention

[0013] In order to solve the above problems, the present invention provides a snapshot terahertz polarization detection method based on circular polarization multiplexing metasurface, which effectively realizes the accurate detection of polarization information in the terahertz band.

[0014] To achieve the above object, the present invention provides a snapshot terahertz polarization detection method based on circular polarization multiplexing metasurface, comprising the following steps:

[0015] S1. Design the phase of an all-silicon dielectric metasurface composed of anisotropic cross-shaped supercells and calculate the theoretical value of all Stokes parameters;

[0016] S2, constructing a simulation model of the metasurface described in step S1, and calculating simulation values ​​of all Stokes parameters;

[0017] S3, comparing the theoretical values ​​of the Stokes parameters of incident light with different polarization states with the simulated values ​​of the Stokes parameters, and drawing the unit Poincare sphere to verify the theoretical values ​​of the full Stokes parameters;

[0018] S4, preparing the metasurface sample described in step S1 by using ultraviolet etching technology or inductively coupled plasma etching technology, and calculating the experimental value of the full Stokes parameter;

[0019] S5. Compare the theoretical values ​​of the full Stokes parameters of incident light in different polarization states with the experimental values ​​of the full Stokes parameters to verify the theoretical values ​​of the full Stokes parameters again.

[0020] Preferably, the metasurface described in step S1 is composed of a plurality of anisotropic cross-shaped superunits, and the superunit includes a cross-shaped column at the top and a rectangular parallelepiped base at the bottom;

[0021] And step S1 specifically includes the following steps:

[0022] S11. Introduce the Jones matrix and set the Jones matrix of the super unit Among them, t x represents the transmission coefficient of the electric field propagating along the x-axis, and A x represents the transmission amplitude of the electric field propagating along the x-axis, represents the phase of the electric field propagating along the x-axis; t y represents the transmission coefficient of the electric field propagating along the y-axis, and A y The transmission amplitude of the electric field propagating along the y-axis, Represents the phase of the electric field propagating along the y-axis;

[0023] S12. Assumption A x =A y , the rotation angle of the super-unit structure is θ, then the linear Jones matrix of the super-unit after coordinate transformation is J￠(θ)=R(θ)′J′R T (θ), where R(θ) represents the rotation matrix, and The linear Jones matrix expression of the super-element after coordinate transformation is as follows:

[0024]

[0025] S13. Set the output electric field E after passing through the metasurface out The expression is:

[0026]

[0027] In the formula, represents the Jones vector of circularly polarized light waves, σ represents the unit coefficient, and when σ=1, represents the Jones vector of left-handed circularly polarized light wave. When σ=-1, The Jones vector represents the right-handed circularly polarized light wave;

[0028] S14. From formula (2), when the super unit rotates by θ, the generated cross-polarization component produces a phase change of ±2θ. The phase difference between the two cross-polarization components is set to π, and we get Then, the dynamic phase and geometric phase principle generated by the super-unit anisotropic structure are combined to realize circular polarization multiplexing, and the phase design principle expression is as follows:

[0029]

[0030] In the formula, It indicates the phase of the process in which the incident left-handed polarized light is completely converted into right-handed polarized light through the transmitted field; It indicates the phase of the process in which the incident right-hand polarized light is completely converted into left-hand polarized light through the transmitted field;

[0031] S15. Principles for constructing metasurface phase design:

[0032] The expression of the scalar beam focusing phase distribution function is as follows:

[0033]

[0034] In the formula, and They represent the phases required for the target to generate a scalar beam when right-handed circularly polarized light and left-handed circularly polarized light are incident, respectively; d represents the distance of the scalar beam from the origin of the focal plane; f represents the focal length; λ represents the wavelength of the incident wave;

[0035] Based on the topological charge l = ±1, the expression of the focusing phase distribution function of the vector vortex beam is as follows:

[0036]

[0037] In the formula, and They represent the phases required for the target to generate a vector vortex beam when right-handed circularly polarized light and left-handed circularly polarized light are incident, respectively;

[0038] S17. Set the Stokes parameter expression under circular polarization basis:

[0039]

[0040] In the formula, s 0 Indicates the total light intensity; s 1 Indicates the intensity difference between X-polarized light and Y-polarized light; s 2 Represents the intensity difference between +45° linear polarized light and -45° linear polarized light; s 3 represents the intensity difference between right-handed circularly polarized light and left-handed circularly polarized light; E represents the complex electric field vector, and E = E R ε R +E L ε L , ε R and ε LThey represent the unit vectors of right-handed circularly polarized light and left-handed circularly polarized light respectively; E R and E L denote the right-handed and left-handed polarization components of the complex electric field, respectively, and i represents the imaginary unit; and Represent the conjugate of the unit vector of right-handed circularly polarized light and left-handed circularly polarized light respectively; a R and a L represent the amplitudes of right-handed circularly polarized light and left-handed circularly polarized light respectively; and They represent the chiral phases of right-handed circularly polarized light and left-handed circularly polarized light, respectively.

[0041] Preferably, step S2 specifically includes the following steps:

[0042] S21. Constructing a simulation model of the metasurface:

[0043] S211, parameter setting: set the width and length of the circumscribed rectangle of the cross section of the top cross-shaped column to dx and dy respectively, and dx = 58 μm, dy = 104 μm, the thickness of the top cross-shaped column D = 40 μm; the height T = 200 μm; and set the cross section of the bottom rectangular base to be a square, and the side length of the square P = 140 μm; the height of the bottom rectangular base H = 40 μm; set the dielectric constant of the metasurface to 11.9, and the operating frequency to 1 Thz;

[0044] S212, using simulation software to scan each super unit from 40 μm to 136 μm with dx and dy as a step of 2 μm, to obtain transmission coefficients and dynamic phase parameters, and to form a unit structure library with full phase coverage;

[0045] S213, selecting a plurality of spin decoupling structures with a phase gradient of 22.5° from a unit structure library with full phase coverage to form a unit library;

[0046] S214, according to the phase distribution function formula (4) and formula (5), the screened unit library is called to automatically arrange the array to generate a metasurface composed of multiple superunits;

[0047] S22, detecting whether the metasurface structure meets the set standard: incident LCP light, RCP light, X-polarized light, and Y-polarized light on the metasurface respectively, and deriving a focal plane electric field intensity image, judging whether the metasurface structure meets the set standard based on the focal plane electric field intensity image, if yes, executing step S23, if no, returning to step S21;

[0048] S23. Detect the polarization function of the metasurface: Use simulation software to test the metasurface with +45° linear polarized light, Y linear polarized light, -45° linear polarized light, and elliptically polarized light when the axial ratio is 2 and the phase differences of the two vertical components are +45°, +90°, +135°, -45°, -90°, and -135°, respectively, and derive the focal plane electric field intensity image to determine whether the polarization function of the metasurface meets the set requirements.

[0049] Preferably, step S3 specifically includes the following steps:

[0050] S31, using formula (6) to calculate the Stokes parameter under the incident condition of the polarized light described in step S23, to obtain a theoretical value of the Stokes parameter;

[0051] S32, using the electric field intensity diagram of the x component of the focal plane in step S23, find the average value within the 3dB range of the maximum intensity of the two scalar light spots, and use the average value as the intensity of the left-hand circular polarization component or the right-hand circular polarization component, and then calculate the Stokes parameter using formula (6) to obtain the simulation value of the Stokes parameter;

[0052] S33, compare the theoretical value of the Stokes parameter of incident light with different polarization states with the simulated value, and draw the unit Poincare sphere according to the comparison result, and 1 、s 2 、s 3 Marked as coordinate values ​​on the unit Poincare sphere;

[0053] S34. The theoretical values ​​of all Stokes parameters are verified based on the drawn unit Poincare sphere.

[0054] Preferably, step S4 specifically includes the following steps:

[0055] S41, preparing the metasurface sample described in step S1 by using ultraviolet etching technology or inductively coupled plasma etching technology;

[0056] S42, using a near-field optical scanning system equipped with a terahertz probe to scan the metasurface sample, when emitting 1THz X-linear polarized light, its left-handed circularly polarized component or right-handed circularly polarized component will generate a vortex beam focused to the center of the focal plane and a scalar beam with a symmetrical focal position, and the electric field distribution at the focus is obtained using the THz probe;

[0057] S43. Calculate the Stokes parameters by using the electric field intensities of different components in the focal plane, and use the calculated Stokes parameters as experimental values ​​of the Stokes parameters.

[0058] The present invention has the following beneficial effects:

[0059] The snapshot design significantly shortens the measurement time and makes it more convenient to detect the instantaneous polarization state in this band, making it suitable for situations where the phase changes rapidly in the terahertz band.

[0060] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 It is a flow chart of a snapshot terahertz polarization detection method based on circular polarization multiplexing metasurface of the present invention;

[0062] Figure 2 : is a simulation experiment result diagram of an embodiment of the present invention; wherein, Figure 2 (a) is a schematic diagram of the super unit structure; Figure 2 (b) is the profile of transmission coefficient and electric field intensity in the x and y directions of the super unit; Figure 2 (c) is the dynamic phase diagram of the supercell in the x and y directions; Figure 2 (d) is a cross-sectional view of the top cross-shaped column;

[0063] Figure 3 This is a scanning result diagram of the unit structure library of an embodiment of the present invention; wherein, Figure 3 (a) and Figure 3 (d) Transmission coefficient diagrams of all structures in x and y directions respectively; Figure 3 (b) and Figure 3 (e) Dynamic phase diagrams of all structures in x and y directions respectively; Figure 3 (c) is the transmission coefficient diagram of the unit library in the x and y directions; Figure 3 (f) is the dynamic phase diagram of the unit library in the x and y directions;

[0064] Figure 4 is a transmission field simulation result diagram of an embodiment of the present invention; wherein, Figure 4 (a) is the electric field diagram of LCP incident focal plane; Figure 4 (b) is the electric field diagram of the incident focal plane with X-ray polarization; Figure 4 (c) is the electric field diagram of LCP incident focal plane; Figure 4 (d) is the electric field diagram of the incident focal plane with y linear polarization

[0065] Figure 5 : is a function detection simulation result diagram of an embodiment of the present invention; wherein Figure 5 (a)- Figure 5 (i) The simulation results under nine polarizations of incident light;

[0066] Figure 6 : is a comparison result diagram of the theoretical value of the Stokes parameter of the embodiment of the present invention and the simulation calculation value; wherein, Figure 6 (a)- Figure 6 (c) Comparison of theoretical and simulated Stokes parameter values ​​for +46°, Y, and -46° linear polarization incidence; Figure 6 (d)- Figure 6 (i) Axial ratio = 2 and The comparison diagrams of the theoretical values ​​of Stokes parameters and the simulation calculated values ​​for elliptical vibration incidence of +46°, +90°, +136°, -46°, -90°, and -136° respectively; Figure 6 (j) is a comparison diagram of the theoretical coordinates of the Poincare sphere and the coordinates calculated by simulation for three types of linearly polarized incident light; Figure 6 (k) is a comparison diagram of the theoretical coordinates of the Poincare sphere and the coordinates calculated by simulation for three types of right-handed elliptically polarized incident light; Figure 6 (l) is a comparison diagram of the theoretical coordinates of the Poincare sphere and the coordinates calculated by simulation for three types of left-handed elliptically polarized incident light;

[0067] Figure 7 : is an experimental verification result diagram of an embodiment of the present invention; wherein, Figure 7 (a) Figure 7 (d) SEM images of the circular polarization multiplexing metasurface at 420 times and 1300 times magnification; Figure 7 (b) Figure 7 (e) The electric field strength test results of the x and y components of the focal plane, respectively; Figure 7 (c) Figure 7 (f) are the theoretical and experimental calculation results of Stokes parameters for X-ray polarized light; Figure 7 (g) Schematic diagram of the structure of the terahertz near-field scanning system equipped with a micro-probe. DETAILED DESCRIPTION

[0068] In order to make the purpose, technical scheme and advantages disclosed in the embodiments of the present invention clearer, the embodiments of the present invention are further described in detail in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the embodiments of the present invention and are not intended to limit the embodiments of the present invention. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application. Examples of the embodiments are shown in the accompanying drawings, where the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions.

[0069] It should be noted that the terms "include", "have" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or server comprising a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to these processes, methods, products or devices.

[0070] Like reference numerals and letters denote similar items in the following drawings, and thus, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0071] Completely polarized light can be regarded as the superposition of two orthogonal circularly polarized components. According to the terminal trajectory equation of polarized light, the amplitude ratio and phase difference of the two components can determine the polarization state.

[0072] like Figure 1 As shown, based on the above analysis, the present invention is designed: a snapshot terahertz polarization detection method based on circular polarization multiplexing metasurface, comprising the following steps:

[0073] S1. Design the phase of an all-silicon dielectric metasurface composed of anisotropic cross-shaped supercells and calculate the theoretical value of all Stokes parameters;

[0074] The metasurface described in step S1 is composed of a plurality of anisotropic cross-shaped superunits, each of which includes a cross-shaped column at the top and a rectangular parallelepiped base at the bottom;

[0075] And step S1 specifically includes the following steps:

[0076] S11. Introduce the Jones matrix and set the Jones matrix of the super unit Among them, t x represents the transmission coefficient of the electric field propagating along the x-axis, and A x represents the transmission amplitude of the electric field propagating along the x-axis, represents the phase of the electric field propagating along the x-axis; t y represents the transmission coefficient of the electric field propagating along the y-axis, and A y The transmission amplitude of the electric field propagating along the y-axis, Represents the phase of the electric field propagating along the y-axis;

[0077] S12. Assumption A x =A y , the rotation angle of the super-unit structure is θ, then the linear Jones matrix of the super-unit after coordinate transformation is J￠(θ)=R(θ)′J′R T (θ), where R(θ) represents the rotation matrix, and The linear Jones matrix expression of the super-element after coordinate transformation is as follows:

[0078]

[0079] S13. Set the output electric field E after passing through the metasurface out The expression is:

[0080]

[0081] In the formula, represents the Jones vector of circularly polarized light waves, σ represents the unit coefficient, when σ=1, represents the Jones vector of left-handed circularly polarized light wave. When σ=-1, The Jones vector represents the right-handed circularly polarized light wave;

[0082] S14. According to formula (2), after the left-handed or right-handed circularly polarized component interacts with the anisotropic metasurface structure, its outgoing electric field contains a complex amplitude of The co-polarized component of The cross-polarization component carries an additional geometric phase of 2σθ. Therefore, when the super unit rotates by θ, the generated cross-polarization component produces a phase change of ±2θ. In order to achieve the highest efficiency of polarization conversion, the phase difference between the two cross-polarization components is set to π, and we get Then, the dynamic phase and geometric phase principle generated by the super-unit anisotropic structure are combined to realize circular polarization multiplexing, and the phase design principle expression is as follows:

[0083]

[0084] In the formula, It indicates the phase of the process in which the incident left-handed polarized light is completely converted into right-handed polarized light through the transmitted field; It indicates the phase of the process in which the incident right-hand polarized light is completely converted into left-hand polarized light through the transmitted field;

[0085] S15. In order to generate different focused beams with different circularly polarized components of the incident wave, it is necessary to design the phase distribution and construct the metasurface phase design principle:

[0086] The expression of the scalar beam focusing phase distribution function is as follows:

[0087]

[0088] In the formula, and They represent the phases required for the target to generate a scalar beam when right-handed circularly polarized light and left-handed circularly polarized light are incident, respectively; d represents the distance of the scalar beam from the origin of the focal plane; f represents the focal length; λ represents the wavelength of the incident wave;

[0089] Based on the topological charge l = ±1, the expression of the focusing phase distribution function of the vector vortex beam is as follows:

[0090]

[0091] In the formula, and They represent the phases required for the target to generate a vector vortex beam when right-handed circularly polarized light and left-handed circularly polarized light are incident, respectively;

[0092] S17. Set the Stokes parameter expression under circular polarization basis:

[0093]

[0094] In the formula, s 0 Indicates the total light intensity; s 1 Indicates the intensity difference between X-polarized light and Y-polarized light; s 2 Represents the intensity difference between +45° linear polarized light and -45° linear polarized light; s 3 represents the intensity difference between right-handed circularly polarized light and left-handed circularly polarized light; E represents the complex electric field vector, and E = E R ε R +E L ε L , ε R and ε L They represent the unit vectors of right-handed circularly polarized light and left-handed circularly polarized light respectively; E R and E L denote the right-handed and left-handed polarization components of the complex electric field, respectively, and i represents the imaginary unit; and Represent the conjugate of the unit vector of right-handed circularly polarized light and left-handed circularly polarized light respectively; a R and a L represent the amplitudes of right-handed circularly polarized light and left-handed circularly polarized light respectively; and They represent the chiral phases of right-handed circularly polarized light and left-handed circularly polarized light, respectively.

[0095] S2, constructing a simulation model of the metasurface described in step S1, and calculating simulation values ​​of all Stokes parameters;

[0096] Step S2 specifically includes the following steps:

[0097] S21. Constructing a simulation model of the metasurface:

[0098] S211, parameter setting: set the width and length of the circumscribed rectangle of the cross section of the top cross-shaped column to dx and dy respectively, and dx = 58 μm, dy = 104 μm, the thickness of the top cross-shaped column D = 40 μm; the height T = 200 μm; and set the cross section of the bottom rectangular base to be a square, and the side length of the square P = 140 μm; the height of the bottom rectangular base H = 40 μm; set the dielectric constant of the metasurface to 11.9, and the operating frequency to 1 Thz;

[0099] S212, using simulation software to scan each super unit from 40 μm to 136 μm with dx and dy as a step of 2 μm, to obtain transmission coefficients and dynamic phase parameters, and to form a unit structure library with full phase coverage;

[0100] In this embodiment, according to the design principle of formula (3), 15 spin decoupling structures with a phase gradient of 22.5° are selected to form a unit library. The scanning results are as follows: Figure 2 As shown, it can be seen that the transmission coefficients of the above 15 structures are all around 0.7, and the phase difference in the x and y directions is close to π, and full coverage of the dynamic phase 2π is achieved in both the x and y directions.

[0101] S213, selecting a plurality of spin decoupling structures with a phase gradient of 22.5° from a unit structure library with full phase coverage to form a unit library;

[0102] S214, according to the phase distribution function formula (4) and formula (5), the screened unit library is called to automatically arrange the array to generate a metasurface composed of multiple superunits;

[0103] S22, detecting whether the metasurface structure meets the set standard: incident LCP light, RCP light, X-polarized light, and Y-polarized light on the metasurface respectively, and deriving a focal plane electric field intensity image, judging whether the metasurface structure meets the set standard based on the focal plane electric field intensity image, if yes, executing step S23, if no, returning to step S21;

[0104] In this embodiment, if Figure 3 As shown, the transmission coefficients of the designed metasurface in the x and y directions are both around 0.7, and the phase difference of the electric field in the x and y directions is close to π after the incident wave passes through the super unit. In the range of 0.1 Thz above and below the operating frequency, the dynamic phase in the x and y directions shows that the phase difference of the two components at the same frequency point is close to π, proving that the design meets the requirements.

[0105] In this embodiment, since the phase distribution of the supercell has been quantized, this process produces errors, so the focal plane is not exactly at the preset f=7000μm, but at 6300μm, which is closer to the metasurface. After the different circularly polarized components are transmitted through the metasurface, a vortex beam focused to the center of the focal plane and two scalar beams symmetrical about the y-axis can be formed, which can basically achieve the preset function; when the X and Y linear polarizations are incident, the intensity of the focal plane scalar beam is close, and the central vortex beam has obvious dark stripes.

[0106] S23. Detect the polarization function of the metasurface: Use simulation software to test the metasurface with +45° linear polarized light, Y linear polarized light, -45° linear polarized light, and elliptically polarized light when the axial ratio is 2 and the phase differences of the two vertical components are +45°, +90°, +135°, -45°, -90°, and -135°, respectively, and derive the focal plane electric field intensity image to determine whether the polarization function of the metasurface meets the set requirements.

[0107] In this embodiment, the simulation results are as follows: Figure 4 As shown, the x component of the electric field intensity in the focal plane is uniformly observed. When the linear polarization incident light is 45°, Y, and -45°, the scalar spot intensity on both sides of the electric field intensity in the X direction in the focal plane is close, which means that the S3 parameter is close to 0, and it can be basically determined that the incident light is linearly polarized light, and the dark stripes of the central vortex beam have a directional effect.

[0108] like Figure 5 As shown, Axial ratio = 2, cross-polarization component phase difference The contrast of the scalar light spots on the left and right sides of the x-component electric field intensity image of the focal plane at +45°, +90°, and +135° is very obvious, and the line connecting the strongest points of the middle vector vortex beam (or dark stripes) has obvious angular directivity. When right-handed elliptically polarized light is incident, the intensity of the scalar light spot on the right is greater, that is, the S3 parameter after quantization is greater than 0, so it can be preliminarily determined that the above three types of incident light are all right-handed elliptically polarized light. Axialratio=2, vertical component phase difference The contrast of the scalar light spots on the left and right sides of the x-component electric field intensity image of the focal plane at -45°, -90°, and -135° is very obvious, and the line (or dark stripe) connecting the strongest points of the middle vector vortex beam has obvious angular directivity, which can preliminarily determine that the three incident lights are left-handed elliptically polarized light. In summary, when the axial ratio is the same, the absolute value of the phase difference is equal but the sign is opposite, the scalar light spots on both sides of the x-component electric field intensity image are almost symmetrical, and the deflection angle of the dark stripe of the central vortex beam is almost the same.

[0109] S3, comparing the theoretical values ​​of the Stokes parameters of incident light with different polarization states with the simulated values ​​of the Stokes parameters, and drawing the unit Poincare sphere to verify the theoretical values ​​of the full Stokes parameters;

[0110] like Figure 6 As shown, the Stokes parameters of the three linearly polarized lights are the same as the Axial ratio = 2 and the vertical component phase difference The simulated calculated value of the Stokes parameter when is closest to the theoretical value, and the simulated calculated values ​​of the incident light in other polarization states are different from the theoretical values, but the extraction of basic handedness information is obvious and correct.

[0111] Step S3 specifically includes the following steps:

[0112] S31, using formula (6) to calculate the Stokes parameter under the incident condition of the polarized light described in step S23, to obtain a theoretical value of the Stokes parameter;

[0113] S32, using the electric field intensity diagram of the x component of the focal plane in step S23, find the average value within the 3dB range of the maximum intensity of the two scalar light spots, and use the average value as the intensity of the left-hand circular polarization component or the right-hand circular polarization component, and then calculate the Stokes parameter using formula (6) to obtain the simulation value of the Stokes parameter;

[0114] S33, compare the theoretical value of the Stokes parameter of the incident light with different polarization states with the simulated value, and draw the unit Poincare sphere according to the comparison result. The normalized Stokes parameter value corresponds to the coordinate of the unit Poincare sphere one by one, and each coordinate corresponds to a uniquely determined polarization state, so s 1 、s 2 、s 3 Marked as coordinate values ​​on the unit Poincare sphere;

[0115] S34. The theoretical values ​​of all Stokes parameters are verified based on the drawn unit Poincare sphere.

[0116] S4, preparing the metasurface sample described in step S1 by using ultraviolet etching technology or inductively coupled plasma etching technology, and calculating the experimental value of the full Stokes parameter;

[0117] Step S4 specifically includes the following steps:

[0118] S41, preparing the metasurface sample described in step S1 by using ultraviolet etching technology or inductively coupled plasma etching technology;

[0119] S42, using a near-field optical scanning system equipped with a terahertz probe to scan the metasurface sample, when emitting 1THz X-linear polarized light, its left-handed circularly polarized component or right-handed circularly polarized component will generate a vortex beam focused to the center of the focal plane and a scalar beam with a symmetrical focal position, and the electric field distribution at the focus is obtained using the THz probe;

[0120] S43. Calculate the Stokes parameters by using the electric field intensities of different components in the focal plane, and use the calculated Stokes parameters as experimental values ​​of the Stokes parameters.

[0121] S5. Compare the theoretical values ​​of the full Stokes parameters of incident light in different polarization states with the experimental values ​​of the full Stokes parameters to verify the theoretical values ​​of the full Stokes parameters again.

[0122] In this embodiment, the scanning electron microscope (SEM) image is as follows Figure 7 As shown, by introducing a near-field scanning system equipped with a terahertz probe, when the THz emitter emits 1THz X-linear polarized light, its left-handed / right-handed circularly polarized components will generate a vortex beam focused to the center of the focal plane and a scalar beam with a symmetrical focal position. The electric field distribution at the focal point can be obtained using the THz probe. It can be seen that the electric field intensity at the focal point of the scalar beams on the left and right sides is close, and the dark stripes after the coherence of the central vortex beam are obvious, with a directional indication effect. It can be seen that the experimental value is close to the theoretical value, which further verifies the effectiveness of the present invention.

[0123] In summary, the present invention uses anisotropic cross-shaped silicon pillars as a single unit to form a metasurface, combines geometric phase and dynamic phase to realize circular polarization multiplexing metasurface, and realizes the control of left-handed circular polarization component and right-handed circular polarization component to form a focused beam. When the incident light beam passes through the metasurface, it automatically splits into two left-handed and right-handed circular polarization components, one of which generates a vortex beam focused to the center of the focal plane with a topological charge number l=+1, and a scalar beam with a focus on the x-axis; the other circular polarization component generates a vortex beam focused to the center of the focal plane with a topological charge l=-1, and a scalar beam with a focus at a symmetrical position. Then, the scalar light spot reflects the intensity of the left-handed polarization component or the right-handed polarization component, and the interference dark fringes of the central vector beam reflect the chiral phase difference, so that the full Stokes parameters can be calculated, and the polarization state of the incident light can be uniquely determined.

[0124] Therefore, the present invention adopts the above-mentioned snapshot terahertz polarization detection method based on circular polarization multiplexing metasurface, which effectively realizes the accurate detection of polarization information in the terahertz band.

[0125] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A snapshot terahertz polarization detection method based on circular polarization multiplexing metasurface, characterized by: The following steps are involved: S1. Design the phase of an all-silicon dielectric metasurface composed of anisotropic cross-shaped supercells and calculate the theoretical value of all Stokes parameters; The metasurface described in step S1 is composed of a plurality of anisotropic cross-shaped superunits, each of which includes a cross-shaped column at the top and a rectangular parallelepiped base at the bottom; And step S1 specifically includes the following steps: S11. Introduce the Jones matrix and set the Jones matrix of the super unit Among them, t x represents the transmission coefficient of the electric field propagating along the x-axis, and A x represents the transmission amplitude of the electric field propagating along the x-axis, represents the phase of the electric field propagating along the x-axis; t y represents the transmission coefficient of the electric field propagating along the y-axis, and A y The transmission amplitude of the electric field propagating along the y-axis, Represents the phase of the electric field propagating along the y-axis; S12. Assumption A x =A y , the rotation angle of the super-unit structure is θ, then the linear Jones matrix of the super-unit after coordinate transformation is J′(θ)=R(θ)×J×R T (θ), where R(θ) represents the rotation matrix, and The linear Jones matrix expression of the super-element after coordinate transformation is as follows: S13. Set the output electric field E after passing through the metasurface out The expression is: In the formula, represents the Jones vector of circularly polarized light waves, σ represents the unit coefficient, and when σ=1, represents the Jones vector of left-handed circularly polarized light wave. When σ=-1, The Jones vector represents the right-handed circularly polarized light wave; S14. From formula (2), when the super unit rotates by θ, the cross-polarization component produces a phase change of ±2θ. The phase difference between the two cross-polarization components is set to π, and we get Then, the dynamic phase and geometric phase principle generated by the super-unit anisotropic structure are combined to realize circular polarization multiplexing, and the phase design principle expression is as follows: In the formula, It indicates the phase of the process in which the incident left-handed polarized light is completely converted into right-handed polarized light through the transmitted field; It indicates the phase of the process in which the incident right-hand polarized light is completely converted into left-hand polarized light through the transmitted field; S15. Principles for constructing metasurface phase design: The expression of the scalar beam focusing phase distribution function is as follows: In the formula, and They represent the phases required for the target to generate a scalar beam when right-handed circularly polarized light and left-handed circularly polarized light are incident, respectively; d represents the distance of the scalar beam from the origin of the focal plane; f represents the focal length; λ represents the wavelength of the incident wave; Based on the topological charge l = ±1, the expression of the focusing phase distribution function of the vector vortex beam is as follows: In the formula, and They represent the phases required for the target to generate a vector vortex beam when right-handed circularly polarized light and left-handed circularly polarized light are incident, respectively; S17. Set the Stokes parameter expression under circular polarization basis: Where s0 represents the total light intensity; s1 represents the intensity difference between the X-polarized light and the Y-polarized light; s2 represents the intensity difference between the +45° linear polarized light and the -45° linear polarized light; s3 represents the intensity difference between the right-handed circularly polarized light and the left-handed circularly polarized light; E represents the complex electric field vector, and E = E R ε R +E L ε L , ε R and ε L They represent the unit vectors of right-handed circularly polarized light and left-handed circularly polarized light respectively; E R and E L denote the right-handed and left-handed polarization components of the complex electric field, respectively, and i represents the imaginary unit; and Represent the conjugate of the unit vector of right-handed circularly polarized light and left-handed circularly polarized light respectively; a R and a L represent the amplitudes of right-handed circularly polarized light and left-handed circularly polarized light respectively; and represent the chiral phases of right-handed circularly polarized light and left-handed circularly polarized light respectively; S2, constructing a simulation model of the metasurface described in step S1, and calculating simulation values ​​of all Stokes parameters; Step S2 specifically includes the following steps: S21. Constructing a simulation model of the metasurface: S211, parameter setting: set the width and length of the circumscribed rectangle of the cross section of the top cross-shaped column to dx and dy respectively, and dx = 58 μm, dy = 104 μm, the thickness of the top cross-shaped column D = 40 μm; the height T = 200 μm; and set the cross section of the bottom rectangular base to be a square, and the side length of the square P = 140 μm; the height of the bottom rectangular base H = 40 μm; set the dielectric constant of the metasurface to 11.9, and the operating frequency to 1 THz; S212, using simulation software to scan each super unit from 40 μm to 136 μm with dx and dy as a step of 2 μm, to obtain transmission coefficients and dynamic phase parameters, and to form a unit structure library with full phase coverage; S213, selecting a plurality of spin decoupling structures with a phase gradient of 22.5° from a unit structure library with full phase coverage to form a unit library; S214, according to the phase distribution function formula (4) and formula (5), the screened unit library is called to automatically arrange the array to generate a metasurface composed of multiple superunits; S22, detecting whether the metasurface structure meets the set standard: incident LCP light, RCP light, X-polarized light, and Y-polarized light on the metasurface respectively, and deriving a focal plane electric field intensity image, judging whether the metasurface structure meets the set standard based on the focal plane electric field intensity image, if yes, executing step S23, if no, returning to step S21; S23. Detecting the polarization function of the metasurface: using simulation software to test the metasurface for incident +45° linear polarized light, Y linear polarized light, -45° linear polarized light, and elliptically polarized light when the axial ratio is 2 and the phase differences of the two vertical components are +45°, +90°, +135°, -45°, -90°, and -135°, respectively, and deriving a focal plane electric field intensity image to determine whether the polarization function of the metasurface meets the set requirements; S3, comparing the theoretical values ​​of the Stokes parameters of incident light with different polarization states with the simulated values ​​of the Stokes parameters, and drawing the unit Poincare sphere to verify the theoretical values ​​of the full Stokes parameters; S4, preparing the metasurface described in step S1 by using ultraviolet etching technology or inductively coupled plasma etching technology, and calculating the experimental value of the full Stokes parameter; Step S4 specifically includes the following steps: S41, preparing the metasurface described in step S1 by using ultraviolet etching technology or inductively coupled plasma etching technology; S42, using a near-field optical scanning system equipped with a terahertz probe to scan the metasurface, when emitting 1THz X-linear polarized light, its left-handed circularly polarized component or right-handed circularly polarized component will generate a vortex beam focused to the center of the focal plane and a scalar beam with a symmetrical focal position, and the electric field distribution at the focus is obtained using the THz probe; S43, calculating the Stokes parameters by the electric field strength of different components in the focal plane, and using the calculated Stokes parameters as experimental values ​​of the Stokes parameters; S5. Compare the theoretical values ​​of the full Stokes parameters of incident light in different polarization states with the experimental values ​​of the full Stokes parameters to verify the theoretical values ​​of the full Stokes parameters again.

2. The snapshot terahertz polarization detection method based on circular polarization multiplexing metasurface according to claim 1 is characterized in that: Step S3 specifically includes the following steps: S31, using formula (6) to calculate the Stokes parameter under the incident condition of the polarized light described in step S23, to obtain a theoretical value of the Stokes parameter; S32, using the electric field intensity diagram of the x component of the focal plane in step S23, find the average value within the 3dB range of the maximum intensity of the two scalar light spots, and use the average value as the intensity of the left-hand circular polarization component or the right-hand circular polarization component, and then calculate the Stokes parameter using formula (6) to obtain the simulation value of the Stokes parameter; S33, comparing the theoretical value of the Stokes parameter of incident light in different polarization states with the simulation value, and drawing a unit Poincare sphere according to the comparison result, and marking s1, s2, and s3 as coordinate values ​​on the unit Poincare sphere; S34. The theoretical values ​​of all Stokes parameters are verified based on the drawn unit Poincare sphere.

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