A metasurface polarization detection element and a method for performing broadband polarization detection

Abstract

The present invention discloses a metasurface polarization detection element and a method for performing broadband polarization detection, belonging to the technical fields of spectral encoding and measurement. The metasurface polarization detection element includes a light-transmitting substrate and a metasurface array of sub-wavelength structures integrated on its surface; the metasurface array is composed of E-type, H-type, double-Z-type metasurfaces and first and second rotated metasurfaces formed by rotating the E-type and H-type metasurfaces by 45° around their geometric centers respectively. Based on the spatial encoding arrangement of the multimodal structure, the working bandwidth of the metasurface array is successfully extended to 400~1600 nm, and the average extinction ratio reaches 2.39. At the same time, it also supports the distinction between linear polarization and circular polarization; combined with the random forest regression algorithm to correct the nonlinear response characteristics of the system, the polarization detection accuracy and anti-noise performance can be significantly improved, and finally high-precision polarization detection can be achieved in a wide spectral range, and the angular error is less than 2°.

Description

Technical Field

[0001] The present invention relates to the technical field of spectral encoding and measurement, and particularly to a metasurface polarization detection element and a method for wide-spectrum polarization detection based on the same. Background Art

[0002] Polarization detection is a core functional module of an optical system, and its existing technical solutions face multi-dimensional performance bottlenecks: one is the limitation of spectral adaptability. Traditional polarization-sensitive elements (such as wave plates and polarizers) are limited by the material dispersion characteristics and can only achieve effective polarization modulation in specific bands, making it difficult to meet the requirements of continuous wide-spectrum coverage from visible light to near-infrared. The second is the defect in polarization analysis accuracy. Existing metasurface polarization detection systems have large prediction errors for the linear polarization angle in the visible light band (usually >5°), and lack the ability to distinguish the circular polarization handedness.

[0003] Taking a typical metasurface polarization detection element as an example, its array is mostly composed of periodically arranged dielectric structure units with a single symmetric structure. This design results in a limited polarization response band, and the extinction ratio (a key index for measuring polarization resolution ability) fluctuates significantly in the wide spectrum range. At the same time, traditional algorithms (such as linear fitting) are difficult to effectively process the high-dimensional and non-linear polarization response data generated by metasurface devices, resulting in a decrease in the system-level detection accuracy.

[0004] Although the metasurface technology has realized the flexible control ability of the light field polarization, phase and amplitude through sub-wavelength structures, providing a new path for miniaturized and high-precision polarization detection, it still faces two core challenges: one is the contradiction of wide-spectrum compatibility. Metasurfaces designed based on geometric phase or resonance phase are limited by physical mechanisms and have a limited working band, making it difficult to cover the continuous spectrum from visible light to near-infrared. The second is the angle-polarization coupling error. Oblique incidence causes phase response distortion, resulting in a polarization angle detection error of more than 5°.

[0005] In order to address the dual challenges of wide-spectrum compatibility and angle sensitivity, a composite solution of heterogeneous metasurface collaborative design and intelligent algorithm correction is theoretically feasible. The working bandwidth can be extended through the spatial encoding arrangement of multi-modal structures, and machine learning algorithms can effectively correct the system non-linear response, promising to break through the existing technical bottlenecks at the device physics level and the signal processing level. However, the balance problem of algorithm generalization ability and how to achieve the perfect optimization of the metasurface array topological configuration are still the core difficulties in solving the corresponding problems, and in-depth exploration is still needed at the structural design and algorithm modeling levels, and no good collaborative optimization mechanism has been given in the existing technology. Summary of the Invention

[0006] The object of the present invention is to solve the problems existing in the prior art, and to provide a metasurface polarization detection element and a method for performing broadband polarization detection. The detection process is based on the metasurface polarization detection element. By optimizing the topological configuration of the metasurface array, the working bandwidth is extended to 400 - 1600 nm by using the spatial encoding arrangement of multimodal structures (E-type, H-type and their derivative forms). Combining with the random forest regression algorithm, the system nonlinear response is effectively corrected, high-precision polarization detection (angle error ≤ 2°) is achieved, and the distinction between linear polarization and circular polarization is supported. A collaborative optimization mechanism is successfully established between the topological configuration of the metasurface array and the algorithm regularization constraint.

[0007] To achieve the above technical objectives, the present invention is realized through the following technical solutions: A metasurface polarization detection element includes a light-transmitting substrate and a metasurface array of sub-wavelength structures integrated on its surface; the metasurface array is composed of metasurfaces of the following five configurations, specifically: an E-type structure metasurface; an H-type structure metasurface; a double-Z type structure metasurface; a first rotated metasurface formed by rotating the E-type structure metasurface by 45° around its geometric center; and a second rotated metasurface formed by rotating the H-type structure metasurface by 45° around its geometric center.

[0008] Further, the E-type structure metasurface is composed of an array arrangement of symmetric E-type dielectric structure units with gaps between the E-type dielectric structure units; the H-type structure metasurface is composed of an array arrangement of bilaterally symmetric H-type dielectric structure units with gaps between the H-type dielectric structure units; the double-Z type structure metasurface is composed of an array arrangement of non-periodic and asymmetric double-Z type dielectric structure units with gaps between the double-Z type dielectric structure units.

[0009] Further, the size of each metasurface on the metasurface array is 50 μm * 50 μm, and the overall size of the array structure is 200 μm * 150 μm.

[0010] Further, the dielectric structure material is silicon or germanium, and the light-transmitting substrate is a sapphire substrate.

[0011] Further, through the combined arrangement of metasurfaces of multiple structures, the polarization detection performance of the constructed metasurface array is optimized in the wide wavelength range of 400 - 1600 nm, and the polarization detection angle error in the whole wavelength range is less than 2°.

[0012] Further, the average extinction ratio of the metasurface array in the working wavelength range of 400 - 1600 nm is 2.39.

[0013] The method for performing broadband polarization detection based on the above metasurface polarization detection element is as follows:

[0014] 1) Calibration of the transmittance of the metasurface array: A single metasurface is vertically irradiated with a monochromatic polarized light source with known polarization state information, and the transmitted light intensity is measured by a black-and-white camera. The transmittance matrix of the metasurface array is defined as the ratio of the transmitted light intensity of the monochromatic incident light with the corresponding polarization state after passing through the metasurface to the reference value of the incident light intensity under the same conditions without the loaded metasurface, that is:

[0015]

[0016] where \(i = 1\sim5\), is the transmittance of the incident light with wavelength and polarization state after passing through the \(i\)-th metasurface, is the light intensity of the incident light with wavelength and polarization state after passing through the \(i\)-th metasurface, is the light intensity of the incident light with wavelength and polarization state ;

[0017] 2) Calibration of the polarization state response of the detector: The spectral response of the detector is calibrated by a monochromator. Specifically, a monochromatic light with a fixed polarization state is incident on the \(i\)-th metasurface, and the light intensity value of the transmitted light and the gray value collected by the CCD camera under the corresponding conditions are measured respectively. The ratio of the two is defined as the spectral responsivity of the detector to the \(i\)-th metasurface, that is:

[0018]

[0019] where, is the light intensity of the incident light with wavelength and polarization state after passing through the \(i\)-th metasurface, is the gray value of the incident light with wavelength and polarization state detected by the black-and-white CCD camera after passing through the \(i\)-th metasurface, is the spectral responsivity of the detector to the incident light with wavelength and polarization state passing through the \(i\)-th metasurface;

[0020] 3) Analyze the light intensity signal received by the black-and-white camera using the forest regression function to complete the detection of the polarization state of the incident light field;

[0021] Ⅰ. Use a light source with an unknown polarization state to irradiate the metasurface array through a lens, and use a black-and-white CCD camera to receive the gray value of the light signal passing through the metasurface array and convert it into a light intensity signal;

[0022] II. Polarization state reconstruction: An inverse problem for solving an unknown polarization state is constructed through a forest regression function, and the polarization state information of the unknown optical field is obtained using a computer. The formula in integral form is as follows:

[0023]

[0024] where is the gray-scale information of the incident light detected by the black-and-white CCD detector after passing through the i-th metasurface, and represent the range of the unknown wavelength spectrum that can be reconstructed, and represent the range of the unknown polarization angle that can be reconstructed, is the transmittance matrix of the metasurface array, is the spectral responsivity of the incident light. When the light source with unknown polarization state information passes through the metasurface array and irradiates the CCD camera, the CCD camera collects the gray-scale information , and have been experimentally calibrated. The intensity of the incident light can be deduced inversely through the spectral reconstruction formula . Since the incident light wavelength is known, the polarization state of the incident light can be solved .

[0025] Furthermore, the training set data required for training the forest regression function is obtained by simulating its theoretical polarization state, and then the polarization state of the optical field to be detected is measured by the metasurface polarization detection element. As the test set, it is put into the trained forest regression function for prediction.

[0026] The beneficial effects of the present invention are as follows:

[0027] 1. The polarization-sensitive metasurface array designed in this application not only aggregates metasurfaces of E-type, H-type and their variants obtained by rotating 45°, but also arranges double-Z-type asymmetric chiral structure metasurfaces, enabling it to support both linear / circular polarization detection. In addition, the combination of different structure metasurfaces enables the metasurface array to break through the bandwidth limitation of a single structure, enabling it to perform polarization detection in a wide wavelength range of 400 - 1600 nm;

[0028] 2. The metasurface array constructed in this application improves the extinction ratio stability through multi-structure combined design. The average extinction ratio of the obtained metasurface array reaches 2.39 in the wide spectral range of 400 - 1600 nm, which can meet the basic requirements of most application scenarios for polarization resolution;

[0029] 3. This application encodes the incident light field through a self-designed metasurface polarization detection element, combines intelligent algorithms, and uses the random forest regression algorithm to correct the nonlinear response characteristics of the system, significantly improving the polarization detection accuracy and anti-noise performance. Finally, efficient detection is achieved in the wide spectral range of 400 - 1600 nm, and the angular error is less than 2°;

[0030] 4. This application proposes that after the first encoder is fabricated using electron beam lithography (EBL) technology, subsequent nanoimprinting of the metasurface is continued using nanoimprint lithography (NIL) technology with it as a template. Compared with simply using EBL technology to fabricate the encoder, the production cost can be significantly reduced; in addition, due to the relatively simple process steps of this technology, rapid and high-throughput fabrication purposes can be achieved. Description of the Drawings

[0031] Figure 1 Curves of the transmittance of the E-type metasurface E-0° and the metasurface E-45° formed by rotating it by 45° as a function of the polarization angle;

[0032] Figure 2 Structural distribution image of the polarization-sensitive metasurface array prepared in Example 1 observed using a microscope, where 1. E-0°, 2. H-0°, 3. E-45°, 4. H-45°, 5. Z-type metasurface;

[0033] Figure 3 Scanning electron microscope (SEM) images of the E-type, H-type, and double-Z-type metasurfaces, where subfigure a is the SEM image of the E-type metasurface, subfigure b is the SEM image of the H-type metasurface, and subfigure c is the SEM image of the Z-type metasurface;

[0034] Figure 4 Schematic diagrams of the sizes of different metasurface structural units, where subfigure a is the specific size schematic diagram of the E-type metasurface structural unit, subfigure b is the specific size schematic diagram of the H-type metasurface structural unit, and subfigure c is the specific size schematic diagram of the double-Z-type metasurface structural unit;

[0035] Figure 5 Overall extinction ratio curve of the polarization-sensitive metasurface array prepared in Example 1 under a wide spectrum;

[0036] Figure 6It is a graph showing the variation law of the transmittance of different superstructures with the incident light polarization angle at a specified wavelength. The test is completed under the condition of stepwise adjustment of the polarization angle by 5°. Among them, subfigure a shows the variation of the transmittance of E-0° and E-45° with the polarization angle in the 532 nm band; subfigure b shows the variation of the transmittance of E-0° and E-45° with the polarization angle in the 638 nm band; subfigure c shows the variation of the transmittance of H-0° and H-45° with the polarization angle in the 1310 nm band; subfigure d shows the variation of the transmittance of H-0° and H-45° with the polarization angle in the 1550 nm band;

[0037] Figure 7 It is a flowchart for detecting the polarization state of the incident light field by using the metasurface polarization detection element prepared in Example 1;

[0038] Figure 8 It is a schematic diagram of the specific operation for detecting the polarization state of the incident light field by using the metasurface polarization detection element constructed in Example 1. Among them, 6 is the light source with unknown polarization state, 7 is the lens, 8 is the metasurface polarization detection element, 9 is the CCD camera, and 10 is the computer;

[0039] Figure 9 It is a scatter plot of the specific angle error distribution;

[0040] Figure 10 It is the transmittance curve of the double-Z-shaped metasurface for left-handed circularly polarized light and right-handed circularly polarized light with different wavelengths. Detailed implementation manners

[0041] The following examples further illustrate the content of the present invention, but should not be construed as limiting the present invention. Without departing from the essence of the present invention, the modifications and substitutions made to the methods, steps or conditions of the present invention all belong to the scope of the present invention.

[0042] Example 1

[0043] This example discloses a preparation method of a metasurface polarization detection element. The specific preparation process is as follows:

[0044] S1. Design and preparation of the polarization-sensitive metasurface array:

[0045] S11. In the design stage, first use the Lumerical FDTD Solutions simulation software to systematically simulate and evaluate a variety of common nanostructures (T-shaped, E-shaped, H-shaped, C-shaped, etc.), focus on examining the transmission spectra under linearly polarized light incidence at 0° and 90°, and use the average value of the absolute difference in transmittance of the two in the 400~1600 nm band as an index to measure the quality of polarization response. The results show that the E-shaped and H-shaped structures exhibit the best polarization response in the 400~1600 nm band.

[0046] In addition, it should be noted that there is a periodic relationship similar to a sine function between the transmittance of a single structure and the polarization angle. Therefore, there will be an ambiguity of "multiple polarization angles corresponding to the same transmittance value". To solve this problem, the present application proposes to additionally add a super surface with the same structure rotated by 45° (i.e., rotating the super surface with the same geometric size as a whole by 45° around its geometric center) based on the E-type super surface (denoted as E-0°) and the H-type super surface (denoted as H-0°), which are correspondingly denoted as E-45° and H-45°. See Figure 1 , Figure 1 FIG. is the curve of the transmittance of E-0° and E-45° varying with the polarization angle, which looks similar to a phase shift of the original sine curve from the figure. The curves of the transmittance of H-0° and H-45° varying with the polarization angle also show the same phase shift phenomenon. After such a design, the transmittance information corresponding to different polarization angles can be avoided from overlapping, so as to achieve a "one-to-one correspondence" relationship at a given wavelength, greatly reducing the multi-solution problem when inverting the angle.

[0047] Since the transmittances of the E-type and H-type super surfaces for left-handed circular polarization and right-handed circular polarization are almost the same, they cannot be used to distinguish the handedness of circular polarization. To further expand the detection ability of the system for multiple polarization states, the present application additionally designs and simulates a non-periodic and non-symmetric (non-centrosymmetric) super surface structure (denoted as double-Z type), so that there are obvious differences in the transmittances of left-handed and right-handed circular polarizations, in order to achieve the resolution of left-handed and right-handed states under circular polarization incidence.

[0048] The super surface array designed in the present application combines the super surfaces of three structures, namely E-type, H-type and double-Z type, and incorporates the super surface structures (E-45° and H-45°) after rotating the original E-type super surface (E-0°) and H-type super surface (H-0°) by 45° respectively, so that the entire super surface array can not only achieve high-precision linear polarization angle measurement, but also effectively distinguish the handedness of circular polarization in the wavelength range of 400~1600 nm, thus greatly improving the completeness and applicable range of polarization detection.

[0049] After that, the optimized periodic and structural parameters are drawn into a gds file by using KLayout software, and an actual super surface device is fabricated based on this design in the experiment.

[0050] S12. Draw the designed metasurface array into a GDS drawing. Each metasurface has a size of 50 μm * 50 μm, and the overall size of the array structure is 200 μm * 150 μm. Specifically, in this embodiment, the metasurface structure is designed in a form arranged in three columns. From top to bottom, the first column is E-0° and H-0°, the second column is E-45° and H-45° from top to bottom, and the double-Z-shaped metasurface is located alone in the third column ( Figure 2 ).

[0051] Deposit a 100-nanometer-thick silicon film on the surface of the sapphire substrate by chemical vapor deposition (CVD) technology through a nano-lithography machine as a master template. Electron beam lithography (EBL) is one of the main technologies for creating nanoscale patterns. There are several common resists in EBL, such as polymethyl methacrylate (PMMA) and hydrogen silsesquioxane. PMMA is a positive resist composed of a methacrylate backbone and is also the earliest developed and most widely used resist. Hydrogen silsesquioxane is used as a negative resist. Step 1: Use PMMA and hydrogen silsesquioxane as positive and negative resists and spin-coat them above the substrate with a spin-coating thickness of about 1 μm. Step 2: Use the drawn GDS drawing as a mask template basis and perform EBL on the surface of the silicon film. When primary electrons are stimulated on the surface of the resist, secondary electrons and backscattered electrons are generated. The energy of the secondary electrons is between 2 and 50 eV, and the spot size of the light beam is in the range of a few nanometers. The backscattered electrons bounce off the substrate. Because they contain a wide scattering angle, the size of the exposed area can be several micrometers. These electrons cause the formation of free radicals and radical cations and change the chemical structure of the resist. Therefore, when a positive resist is used in lithography, the exposed area can be dissolved and removed by the developer solution. In contrast, the irradiated area of the negative resist remains insoluble and persists during the development process. Step 3: Take out the sample after EBL etching, clean the surface, and obtain the processed encoder.

[0052] After that, use a micro-area spectrometer to test the transmitted light intensity of each metasurface in the processed encoder chip for incident light with different polarization states and summarize it into a transmittance matrix , and its transmission spectral range is in the 400 - 1600 nm band.

[0053] Since EBL cannot be used for large-scale production of nano-patterns or nano-structures, after obtaining the encoder template using EBL, ultraviolet-curable nanoimprint lithography (NIL) technology can be used in the subsequent process for metasurface nanoimprinting. NIL technology can be widely divided into thermal lithography technology and ultraviolet lithography technology. The ultraviolet lithography technology used in this application requires coating the ultraviolet-curable optical glue NOA63 on the substrate to be selected.

[0054] In this embodiment, sapphire is used as the substrate. A small amount of NOA63 is dropped on the surface of the sapphire, and then a spin coater is used to spin coat at a speed of 3000 r for 60 s to obtain a uniformly thick NOA63 layer with a thickness of about 1 μm. Then, the encoder obtained by EBL processing is used as a template and reversely pressed on the substrate so that NOA63 can completely cover the structure on the template. After that, the whole is fixed, and ultraviolet light is irradiated for 40 min to completely solidify NOA63. Then, the mask is removed, and the designed metasurface array pattern can be transferred to the substrate, thus quickly and stably processing the required metasurface array.

[0055] Therefore, the metasurface preparation scheme proposed in this application is to first produce the first encoder through EBL technology, and then this can be used as a template to mass-produce metasurface polarization detection elements by using the NIL technology with a simple process. Subsequent experiments are based on the metasurface polarization detection elements prepared by the NIL technology. To verify the processing accuracy, SEM scans were performed on metasurfaces with different structures, and the results are shown in Figure 3 . The results show that the processing error is about ±20 nm, meeting the requirements of the experiment for the nano-processing error. This layout design not only ensures the comprehensive detection ability of linearly polarized light and circularly polarized light in the 400~1600 nm band, but also provides a stable structural basis for subsequent experimental measurements and data analysis.

[0056] The specific dimensions of the produced E-type, H-type, and double-Z-type metasurface structural units are referred to Figure 4, the thickness of the metasurface structural units (dielectric structural units) with three morphologies is 100 nm; the E-type metasurface structural unit can be regarded as assembled by a first arm and three second arms. The length a1 of the first arm is 0.7 μm, the width b1 of the first arm is 0.2 μm, the length c1 of the second arm is 0.4 μm, the width d1 of the second arm is 0.1 μm, the distance e1 between two adjacent second arms is 0.2 μm, the distance f1 between two adjacent E-type metasurface structural units in the transverse direction is 0.3 μm, and the distance g1 between two adjacent E-type metasurface structural units in the longitudinal direction is 0.2 μm; for the H-type metasurface structural unit, it can be regarded as assembled by two first arms and a second arm. The length a2 of the first arm is 0.4 μm, the width b2 of the first arm is 0.1 μm, and the length c2 and width d2 of the second arm are both 0.2 μm. The distance e2 between two adjacent H-type metasurface structural units in the transverse direction is 0.2 μm, and the distance f2 between two adjacent H-type metasurface structural units in the longitudinal direction is 0.2 μm; for the double-Z-type metasurface structural unit, two Z-type structural nanocolumns with the same structure and arranged in parallel are used as a metasurface structural unit. A single Z-type structural nanocolumn can be regarded as assembled by two identical rectangular arms with a dislocation. The length a3 of the rectangular arm is 0.5 μm, the width b3 of the rectangular arm is 0.2 μm, the dislocation overlap width c3 is 0.2 μm, the distance d3 between two Z-type structural nanocolumns in the same group is 0.4 μm, the distance e3 between two adjacent double-Z-type metasurface structural units in the transverse direction is 0.5 μm, and the distance f3 between two adjacent double-Z-type metasurface structural units in the longitudinal direction is 0.2 μm.

[0057] The extinction ratio detection was carried out for the metasurface polarization detection element:

[0058] The extinction ratio plays a key role in measuring the ability of a device to distinguish orthogonal polarizations in polarization measurement and optical system design. Generally, the higher the extinction ratio, the more significant the difference between the transmittance of a specific polarization state and its orthogonal state, resulting in more accurate polarization detection. However, a high extinction ratio can often only be achieved within a relatively narrow wavelength band; when stable polarization performance needs to be maintained over a large range (such as 400 - 1600 nm), the requirements for structural design and material properties will increase significantly. The overall extinction ratio curve of the polarization-sensitive metasurface array designed in this application is shown in Figure 5 , and the average extinction ratio of this metasurface array is 2.39.

[0059] If only targeting a single band or a narrow frequency band, a structure with an extinction ratio far higher than 2.39 can usually be designed within the corresponding frequency band. However, this often seriously sacrifices the performance of other bands and cannot meet the requirements of wide-spectrum detection. In the design goal of this study, a wide-spectrum coverage of 400 - 1600 nm is the core requirement, and an average extinction ratio of 2.39 is sufficient to meet the basic requirements for polarization resolution in most application scenarios (such as basic polarization measurement, biomedical imaging, environmental monitoring, etc.). Coupled with subsequent machine learning algorithms and various structural combinations, not only can the deficiencies of a single structure in a specific band be compensated for, but also the comprehensive detection of multiple polarization states such as linear polarization and circular polarization can be achieved. If a higher extinction ratio is desired in a specific band, multilayer stacking or more complex nanostructures can be adopted, but at the cost of increased design and processing complexity and a decrease in performance in other bands. Therefore, to balance the requirements of processing simplicity, scalability, and full-band coverage, the average extinction ratio of 2.39 achieved in this embodiment has shown a good balance and provides sufficient polarization contrast for practical applications.

[0060] In optoelectronic detection and optical applications, the two bands of 532 nm and 638 nm are often used for precision measurement and biomedical imaging in the visible light region; while 1310 nm and 1550 nm are two key wavelengths in optical communication systems, corresponding to the O band (Original band) and the C band (Conventional band) respectively, and are widely used in optical fiber communication, lidar, and other long-distance detection scenarios. Therefore, studying the polarization responses of these four typical bands can not only verify the overall performance of the metasurface in the visible to near-infrared region but also lay a foundation for future applications in the fields of biomedical diagnosis, optical communication, and remote sensing. Based on this consideration, four lasers with wavelengths of 532, 638, 1310, and 1550 nm were selected to measure the structure, and the polarization responses of this metasurface array at different bands were systematically evaluated.

[0061] After comprehensively analyzing the transmission spectra in the range of 400 - 1600 nm, it can be found that the E-type metasurface shows a more significant polarization response at the two visible light bands of 532 nm and 638 nm, while the H-type metasurface performs better at the two communication bands of 1310 nm and 1550 nm. To illustrate this conclusion more intuitively, we measured in detail the variation of the transmittance with the polarization angle of the E-type metasurface at 532 nm and the H-type metasurface at 1310 nm at a 5° step. The results are shown in Figure 6 . From Figure 6It can be seen that as the polarization angle is scanned from 5° to 365°, the transmittance fluctuates approximately sinusoidally, and there is an obvious transmittance difference between the maximum and minimum values, which precisely reflects the selectivity of these structures for specific polarization directions. The response curve amplitude of the E-type metasurface in the 532 nm band is larger ( Figure 6 neutron figure a), indicating that it has a higher polarization resolution in the visible light region; while the H-type metasurface shows obvious transmittance oscillations in the 1310 nm ( Figure 6 neutron figure c) band, corresponding to its better polarization detection ability in the near-infrared communication band. Generally speaking, these measurement results are mutually corroborated with the spectral data, laying a sufficient basis for preferentially selecting appropriate metasurface structures in different bands in the future.

[0062] Application Example

[0063] This application example uses the metasurface polarization detection element prepared in Example 1 to perform broadband polarization detection. The specific process is as follows:

[0064] 1) Calibration of the transmittance of the metasurface array: Use a monochromatic polarized light source with known polarization state information to vertically irradiate a single metasurface, and measure the transmitted light intensity through a black and white camera. The transmittance matrix of the metasurface array is defined as the ratio of the transmitted light intensity of the monochromatic incident light with the corresponding polarization state after passing through the metasurface to the incident light intensity reference value under the same conditions without loading the metasurface, that is:

[0065]

[0066] Among them, is the transmittance of the incident light with wavelength and polarization state after passing through the i-th (i = 1 to 5, the same below) metasurface, is the light intensity of the incident light with wavelength and polarization state after passing through the i-th metasurface, is the light intensity of the incident light with wavelength and polarization state .

[0067] 2) Calibration of the polarization state response of the detector: Calibrate the spectral response of the detector through a monochromator. Specifically, it is expressed as the ratio of the light source intensity (directly read by the detector) of the monochromatic light with a fixed polarization state passing through the i-th metasurface to the gray value detected by the CCD camera after the monochromatic light with a fixed polarization state passes through the corresponding metasurface , that is:

[0068]

[0069] Among them, is the wavelength of The light intensity of incident light with a polarization state of after passing through the i-th metasurface, is the gray value of incident light with a wavelength of and a polarization state of after passing through the i-th metasurface, detected by a black-and-white CCD camera, is the spectral responsivity of the detector to incident light with a wavelength of and a polarization state of transmitted through the i-th metasurface.

[0070] 3) Analyze the light intensity signal received by the black-and-white camera using the forest regression function to complete the detection of the polarization state of the incident light field (the process reference is Figure 7 ).

[0071] Ⅰ. Measurement: The specific operation reference is Figure 8 . Use the light source 6 with an unknown polarization state to irradiate the metasurface polarization detection element 8 through the lens 7, and use the black-and-white CCD camera 9 to receive the gray value of the light signal transmitted through the metasurface polarization detection element 8, and convert it into a light intensity signal to complete the calculation process using the computer 10 and obtain the polarization information.

[0072] Ⅱ. Polarization state reconstruction: Construct an inverse problem for solving the unknown polarization state through the forest regression function, and use the computer to obtain the polarization state information of the unknown light field. The formula in integral form is:

[0073]

[0074] where, is the gray information of the incident light detected by the black-and-white CCD detector after passing through the i-th metasurface, and represent the unknown wavelength spectral range that can be reconstructed, and represent the unknown polarization angle range that can be reconstructed, is the transmittance matrix of the metasurface array, is the spectral responsivity of the incident light. When the light source with unknown polarization state information irradiates the CCD camera through the metasurface array, the CCD camera can collect the gray information , and have been calibrated through experiments. The intensity of the incident light can be deduced inversely through the spectral reconstruction formula. Since the wavelength of the incident light is known, the polarization state of the incident light can be solved.

[0075] The training set data required for forest regression function training obtains its theoretical polarization state through simulation, and then uses the metasurface polarization detection element to measure the polarization state of the optical field to be detected, and puts it into the trained forest regression function as a test set for prediction.

[0076] The selection of the forest regression algorithm to predict the polarization angle of light sources with different wavelengths is mainly based on the following considerations: First, there is often a non-linear mapping between the metasurface transmittance and the polarization angle, and forest regression can effectively handle multi-variable and high-dimensional non-linear problems; Second, this algorithm can still maintain high robustness and prediction accuracy in the face of noise, a small number of outliers, and experimental environment fluctuations; Third, forest regression does not require strict linear assumptions on input features, nor does it rely on complex prior models, and can directly learn the mapping relationship from the measured transmittance and polarization angle data, so as to better adapt to the actual experimental conditions.

[0077] To sum up, the introduction of forest regression provides a reliable and efficient modeling method for multi-band polarization detection, and significantly improves the prediction accuracy and system robustness.

[0078] In the angle prediction part, based on the previously collected transmission spectrum data and the forest regression algorithm, an inversion model from transmittance to polarization angle was constructed, and the model was verified using test data in different bands. Figure 9 Figure [ID] is a scatter plot of the specific angle error distribution, which can more intuitively observe the distribution range and concentration degree of the prediction error. From the illustrated results, it can be seen that the prediction errors of most test points are maintained within 2°, verifying the accuracy and stability of this method in different bands. The reason for achieving such high precision is mainly due to the following aspects: Multi-structure combination: By combining the E-type, H-type and the metasurface structures obtained by rotating them by 45° respectively, the multi-solution problem caused by the single "sine-type" response is significantly reduced, so that the regression algorithm can more accurately fit the relationship between the polarization angle and the transmittance. Forest regression algorithm: Forest regression has strong fault tolerance and generalization ability, can effectively handle noise and small errors in the experimental process, and automatically selects the optimal structural features to predict the polarization angle in different bands. Sufficient data sampling: The polarization light is transmitted and tested at an angle step of 5° or less in each band, and the formed training data set covers the complete polarization angle range, enabling the model to capture the full picture of the polarization response curve during regression.

[0079] Thus, the experimental results not only verify the feasibility and accuracy of the metasurface structure and regression method proposed in this study, but also lay an important foundation for further applications in the field of wide-band and high-precision polarization measurement.

[0080] In summary, the present application uses the optimized E-type and H-type metasurfaces (E-0°, H-0°, E-45°, and H-45°) to achieve polarization detection in the wide wavelength range of 400 - 1600 nm. By measuring the transmission spectra at different wavelengths and different polarization angles, and combining with the random forest regression algorithm to establish a correlation model between the transmittance and the polarization angle. The experimental results show that excellent measurement accuracy can be obtained at the wavelengths of 532 nm, 638 nm, 1310 nm, and 1550 nm, and the polarization angle error of the vast majority of the test points is within 2°. In addition, the double-Z-type metasurface designed for circular polarization detection also shows the ability to distinguish left-handed and right-handed circular polarizations. There are obvious differences in the transmittance of the left-handed circularly polarized light and the right-handed circularly polarized light with different wavelengths for this structure (as Figure 10 shown); while integrating the linear polarization light and circular polarization light detection units in a single metasurface further enhances the multi-state polarization recognition performance of the system. Overall, the above results verify the reliable detection ability of the metasurface polarization detection element for different polarization states in the wide wavelength range, and provide a solid basis for subsequent integration into various application scenarios such as optical communication and biological imaging.

[0081] The above shows and describes the basic principles, main features, and advantages of the present invention. However, the above are only specific embodiments of the present invention, and the technical features of the present invention are not limited thereto. Any other implementation manner obtained by those skilled in the art without departing from the technical solution of the present invention should be covered within the scope of the present invention.

Claims

1. A metasurface polarization detection element, characterized in that, It includes a light-transmitting substrate and a metasurface array integrated on its surface; the metasurface array is composed of the following five configurations of metasurfaces, specifically: E-type structure metasurface; H-type structure metasurface; Double-Z type structure metasurface; The first rotated metasurface formed by rotating the E-type structure metasurface by 45° around its geometric center; The second rotated metasurface formed by rotating the H-type structure metasurface by 45° around its geometric center.

2. The metasurface polarization detection element according to claim 1, wherein The E-type structure metasurface is composed of an array arrangement of symmetric E-type dielectric structure units, with gaps between the E-type dielectric structure units; the H-type structure metasurface is composed of an array arrangement of bilaterally symmetric H-type dielectric structure units, with gaps between the H-type dielectric structure units; the double-Z type structure metasurface is composed of an array arrangement of non-periodic and asymmetric double-Z type dielectric structure units, with gaps between the double-Z type dielectric structure units.

3. The metasurface polarization detection element according to claim 1, wherein The size of each metasurface on the metasurface array is 50 μm * 50 μm, and the overall size of the array structure is 200 μm * 150 μm.

4. The metasurface polarization detection element according to claim 2, wherein The dielectric structure material is silicon or germanium, and the light-transmitting substrate is a sapphire substrate.

5. The metasurface polarization detection element according to claim 1, wherein Through the combined arrangement of multiple structure metasurfaces, the metasurface array optimizes the polarization detection performance in a wide wavelength range of 400~1600 nm, and the polarization detection angle error in the full wavelength range is less than 2°.

6. The metasurface polarization detection element according to claim 1, wherein The average extinction ratio of the metasurface array in the working wavelength range of 400~1600 nm is 2.

39.

7. A method for wide-spectrum polarization detection based on the metasurface polarization detection element according to any one of claims 1 to 6, characterized in that, It includes the following steps: 1) Calibration of the transmittance of the metasurface array: Use a monochromatic polarized light source with known polarization state information to vertically irradiate a single metasurface, and measure the transmitted light intensity through a black and white camera. The transmittance matrix of the metasurface array is defined as the ratio of the transmitted light intensity of the monochromatic incident light with the corresponding polarization state after passing through the metasurface to the incident light intensity reference value under the same conditions without loading the metasurface, that is: ； where \(i = 1\sim5\), is the transmittance of the incident light with wavelength and polarization state after passing through the \(i\)-th metasurface, is the light intensity of the incident light with wavelength and polarization state after passing through the \(i\)-th metasurface, is the light intensity of the incident light with wavelength and polarization state ; 2) Calibration of the detector's polarization state response: The spectral response of the detector is calibrated through a monochromator. Specifically, monochromatic light with a fixed polarization state is incident on the i-th metasurface, and the light intensity value of the transmitted light and the gray value collected by the CCD camera under the corresponding conditions are measured respectively. The ratio of the two is defined as the spectral response rate of the detector to the i-th metasurface , that is: ； wherein, is the light intensity of incident light with wavelength and polarization state after passing through the i-th metasurface; is the gray value of incident light with wavelength and polarization state after passing through the i-th metasurface, detected by a black and white CCD camera; is the spectral responsivity of the detector to incident light with wavelength and polarization state passing through the i-th metasurface. 3) Analyze the light intensity signal received by the black and white camera using the forest regression function to complete the detection of the polarization state of the incident light field; Ⅰ. Use a light source with unknown polarization state to irradiate the metasurface array through a lens, use a black and white CCD camera to receive the gray value of the light signal passing through the metasurface array, and convert it into a light intensity signal; Ⅱ. Polarization state reconstruction: Construct an inverse problem for solving the unknown polarization state through the forest regression function, and use a computer to obtain the polarization state information of the unknown light field. The formula written in integral form is: ； Among them, is the gray-scale information of the incident light detected by the black-and-white CCD detector after passing through the i-th metasurface, and represent the unknown wavelength spectral range that can be reconstructed, and represent the unknown polarization angle range that can be reconstructed, is the transmittance matrix of the metasurface array, is the spectral responsivity of the incident light. When a light source with unknown polarization state information irradiates the CCD camera through the metasurface array, the CCD camera collects the gray-scale information , and have been calibrated through experiments. The intensity of the incident light can be deduced inversely through the spectral reconstruction formula . Since the wavelength of the incident light is known, the polarization state of the incident light can be solved .

8. The method for wide-spectrum polarization detection based on the metasurface polarization detection element according to claim 7, wherein The training set data required for training the forest regression function obtains its theoretical polarization state through simulation, and then measures the polarization state of the light field to be detected through the metasurface polarization detection element. As the test set, it is put into the trained forest regression function for prediction.

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