Metasurface polarization detection element and method for carrying out broadband spectrum polarization detection

Abstract

The invention discloses a metasurface polarization detection element and a method for carrying out broadband spectrum polarization detection, and belongs to the technical field of spectrum coding and measurement. The metasurface polarization detection element comprises a light-transmitting substrate and a sub-wavelength structure metasurface array integrated on the surface of the light-transmitting substrate. The metasurface array is composed of E-shaped, H-shaped and double-Z-shaped structure metasurfaces and first and second rotating metasurfaces formed by rotating the E-shaped and H-shaped structure metasurfaces around geometric centers of the metasurfaces by 45 degrees. The working bandwidth of the metasurface array is successfully expanded to 400-1600 nm based on space coding arrangement of a multi-mode structure, the average extinction ratio reaches 2.39, and meanwhile distinguishing of linear polarization and circular polarization is supported. And the non-linear response characteristic of the system is corrected in combination with a random forest regression algorithm, so that the polarization detection precision and the anti-noise performance can be remarkably improved, finally, high-precision polarization detection can be realized in a wide spectral range, and the angle error is less than 2 degrees.

Description

Technical Field

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

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

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

[0004] Although metasurface technology achieves flexible control of the polarization, phase, and amplitude of light fields through subwavelength structures, providing a new path for miniaturized, high-precision polarization detection, it still faces two core challenges: First, the contradiction of wide-spectrum compatibility. Metasurfaces designed based on geometric phase or resonant phase are limited by physical mechanisms, and their working bands are restricted, making it difficult to cover the continuous spectrum from visible light to near-infrared. Second, the angle-polarization coupling error. Oblique incidence leads to phase response distortion, resulting in polarization angle detection errors of more than 5°.

[0005] In order to meet the dual challenges of wide-spectrum compatibility and angle sensitivity, the composite solution of heterogeneous metasurface collaborative design and intelligent algorithm correction is theoretically feasible. The spatial coding arrangement of multimodal structures can expand the working bandwidth, and the machine learning algorithm can effectively correct the nonlinear response of the system, which is expected to break through the existing technical bottlenecks at the device physical level and signal processing level. However, the balance problem of algorithm generalization ability and how to achieve perfect optimization of metasurface array topology configuration are still the core difficulties in solving the corresponding problems. 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 purpose of the present invention is to solve the problems existing in the prior art and provide a metasurface polarization detection element and a method for performing wide-spectrum polarization detection. The detection process is performed based on the metasurface polarization detection element. By optimizing the metasurface array topology configuration, the spatial coding arrangement of the multimodal structure (E-type, H-type and its derivative forms) is used to expand the working bandwidth to 400~1600 nm. The random forest regression algorithm is combined to effectively correct the nonlinear response of the system, realize high-precision polarization detection (angle error ≤2°), and support the distinction between linear polarization and circular polarization, and successfully establish a collaborative optimization mechanism between the metasurface array topology configuration and the algorithm regularization constraint.

[0007] In order to achieve the above technical objectives, the present invention is implemented through the following technical solutions: a metasurface polarization detection element, including a light-transmitting substrate and a subwavelength structure metasurface array integrated on its surface; the metasurface array is composed of the following five configurations of metasurfaces, 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 45° around its geometric center; and a second rotated metasurface formed by rotating the H-type structure metasurface 45° around its geometric center.

[0008] Furthermore, the E-type structure metasurface is composed of a symmetrical array of E-type dielectric structure units, with gaps between the E-type dielectric structure units; the H-type structure metasurface is composed of a biaxially symmetrical array of H-type dielectric structure units, with gaps between the H-type dielectric structure units; the double Z-type structure metasurface is composed of a non-periodic, asymmetric double Z-type dielectric structure unit array, with gaps between the double Z-type dielectric structure units.

[0009] Furthermore, 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] Furthermore, the dielectric structure material is silicon or germanium, and the light-transmitting substrate is a sapphire substrate.

[0011] Furthermore, through the combination and arrangement of various metasurface structures, the constructed metasurface array achieves optimized polarization detection performance in a wide band range of 400~1600 nm, and the angle error of polarization detection in the full band is less than 2°.

[0012] Furthermore, the average extinction ratio of the metasurface array in the working band of 400~1600 nm is 2.39.

[0013] The method for performing wide spectrum polarization detection based on the above-mentioned metasurface polarization detection element is as follows:

[0014] 1) Metasurface array transmittance calibration: Use a monochromatic polarized light source with known polarization state information to vertically illuminate a single metasurface, and measure the transmitted light intensity using 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 of the corresponding polarization state after passing through the metasurface to the reference value of the incident light intensity under the same conditions when the metasurface is not loaded, that is:

[0015]

[0016] Among them, i=1~5, The wavelength is , the polarization state is The transmittance of the incident light after passing through the i-th metasurface, The wavelength is , the polarization state is The intensity of the incident light after passing through the i-th metasurface, The wavelength is , the polarization state is The intensity of the incident light;

[0017] 2) Detector polarization response calibration: The detector is calibrated for spectral response through a monochromator. Specifically, monochromatic light with a fixed polarization state is incident on the i-th metasurface, and the intensity of the transmitted light and the grayscale value collected by the CCD camera under the corresponding conditions are measured. The ratio of the two is defined as the spectral response rate of the detector to the i-th metasurface. ,Right now:

[0018]

[0019] in, The wavelength is , the polarization state is The intensity of the incident light after passing through the i-th metasurface, The wavelength detected by the black and white CCD camera is , the polarization state is The gray value of the incident light after passing through the i-th metasurface, is the wavelength of the detector pair passing through the i-th metasurface. , the polarization state is The spectral response rate of the incident light;

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

[0021] Ⅰ. Use a light source of unknown polarization state to illuminate the metasurface array through a lens, use a black and white CCD camera to receive the grayscale 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 of solving the unknown polarization state is constructed through the forest regression function, and the polarization state information of the unknown light field is obtained by computer, which is written in integral form as follows:

[0023]

[0024] in, is the grayscale information of the incident light detected by the black-and-white CCD detector after being transmitted through the i-th metasurface, and represents the unknown wavelength spectrum range that can be reconstructed, and represents the range of unknown polarization angles that can be reconstructed, is the metasurface array transmittance matrix, is the spectral response rate of the incident light. When the light source with unknown polarization state information is irradiated onto the CCD camera through the metasurface array, the CCD camera collects the grayscale information , and It has been calibrated experimentally, and the intensity of the incident light can be inferred through the spectrum reconstruction formula , due to the wavelength of the incident light Knowing that, we can solve the polarization state of the incident light .

[0025] Furthermore, the training set data required for forest regression function training is used to obtain its theoretical polarization state through simulation, and then the polarization state of the light field to be detected is measured through the metasurface polarization detection element, and is put into the trained forest regression function as a test set for prediction.

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

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

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

[0029] 3. This application encodes the incident light field through the independently designed metasurface polarization detection element, combines intelligent algorithms, and uses the random forest regression algorithm to correct the nonlinear response characteristics of the system, which significantly improves the polarization detection accuracy and anti-noise performance, and finally achieves efficient detection in the wide spectral range of 400~1600 nm, with an angle error of less than 2°;

[0030] 4. This application proposes that after the first encoder is prepared using electron beam lithography (EBL), it is subsequently used as a template to continue to use nanoimprint technology (NIL) to perform ultra-surface nanoimprinting. Compared with simply using EBL technology to prepare the encoder, the production cost can be significantly reduced; in addition, since the process steps of this technology are relatively simple, it can achieve the purpose of rapid and high-throughput preparation. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The curves of the transmittance of the E-type structure metasurface E-0° and the metasurface E-45° formed by rotating it 45° versus the polarization angle;

[0032] Figure 2 The structural distribution image of the polarization-sensitive metasurface array prepared in Example 1 observed by a microscope, wherein 1, E-0°, 2, H-0°, 3, E-45°, 4, H-45°, 5, Z-shaped structure metasurface;

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

[0034] Figure 4 Schematic diagrams of the sizes of different supersurface structure units, where sub-figure a is a schematic diagram of the specific sizes of an E-type supersurface structure unit, sub-figure b is a schematic diagram of the specific sizes of an H-type supersurface structure unit, and sub-figure c is a schematic diagram of the specific sizes of a double Z-type supersurface structure unit;

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

[0036] Figure 6The following is a graph showing how the transmittance of different superstructures at specified wavelengths changes with the polarization angle of the incident light. The test was completed under the condition of a 5° step adjustment of the polarization angle. Sub-figure a shows how the transmittance of E-0° and E-45° changes with the polarization angle in the 532 nm band; sub-figure b shows how the transmittance of E-0° and E-45° changes with the polarization angle in the 638 nm band; sub-figure c shows how the transmittance of H-0° and H-45° changes with the polarization angle in the 1310 nm band; and sub-figure d shows how the transmittance of H-0° and H-45° changes with the polarization angle in the 1550 nm band.

[0037] Figure 7 This is a flow chart of detecting the polarization state of an incident light field using the metasurface polarization detection element prepared in Example 1;

[0038] Figure 8 A schematic diagram of a specific operation of detecting the polarization state of an incident light field using the metasurface polarization detection element constructed in Example 1, wherein: 6, a light source of unknown polarization state; 7, a lens; 8, a metasurface polarization detection element; 9, a CCD camera; and 10, a computer;

[0039] Fig. 9 is a scatter plot of specific angle error distribution;

[0040] Fig.10 It is the transmittance curve of the double Z-shaped structure metasurface for left-handed circularly polarized light and right-handed circularly polarized light of different wavelengths. DETAILED DESCRIPTION

[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, modifications and substitutions made to the methods, steps or conditions of the present invention all fall within the scope of the present invention.

[0042] Example 1

[0043] This embodiment discloses a method for preparing a metasurface polarization detection element, and the specific preparation process is as follows:

[0044] S1. Design and fabrication of polarization-sensitive metasurface arrays:

[0045] S11. In the design stage, we first used Lumerical FDTD Solutions simulation software to systematically simulate and evaluate a variety of common nanostructures (T-type, E-type, H-type, C-type, etc.), focusing on the transmission spectra under 0° and 90° linear polarization incidence, and taking the average value of the absolute difference in transmittance between the two in the 400~1600 nm band as an indicator to measure the quality of polarization response. The results show that the E-type and H-type structures exhibit the best polarization response in the 400~1600 nm band.

[0046] In addition, it should be noted that the transmittance of a single structure and the polarization angle have a periodic relationship similar to that of a sine function, so there will be an ambiguity that "the same transmittance value corresponds to multiple polarization angles". In order to solve this problem, this application proposes to add an additional 45° rotated metasurface of the same structure based on the E-type structure metasurface (denoted as E-0°) and the H-type structure metasurface (denoted as H-0°) (that is, the metasurface of the same geometric size is rotated 45° around its geometric center as a whole), which are denoted as E-45° and H-45° respectively. Figure 1 , Figure 1 The curves of the transmittance at E-0° and E-45° as a function of the polarization angle are similar to the phase shift of the original sine curve. The curves of the transmittance at H-0° and H-45° as a function of the polarization angle also show the same phase shift phenomenon. This design can prevent the transmittance information corresponding to different polarization angles from overlapping, thereby achieving a "one-to-one correspondence" relationship at a given wavelength, greatly reducing the multiple solutions when inverting the angle.

[0047] Since the transmittance of E-type and H-type metasurfaces for left-handed circular polarization and right-handed circular polarization is almost the same, they cannot be used to distinguish the rotation direction of circular polarization. In order to further expand the system's detection capability for multiple polarization states, this application additionally designs and simulates a non-periodic, non-centrosymmetric metasurface structure (denoted as double Z-type), so that left-handed and right-handed circular polarization show obvious differences in transmittance, so as to achieve the distinction between left-handed and right-handed states under circular polarization incidence.

[0048] The metasurface array designed in this application combines metasurfaces of three structures: E-type, H-type and double Z-type, and brings together metasurface structures (E-45° and H-45°) that are obtained by rotating the original E-type structure metasurface (E-0°) and H-type structure metasurface (H-0°) by 45° respectively. This enables the entire metasurface array to not only achieve high-precision linear polarization angle measurement, but also effectively distinguish the handedness of circular polarization in the 400~1600 nm band, thereby greatly improving the completeness and applicability of polarization detection.

[0049] The KLayout software was then used to plot the optimized period and structural parameters into a gds file, and actual metasurface devices were made based on the design in the experiment.

[0050] S12. Draw the designed metasurface array into a gds drawing. The size of each metasurface is 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 to be arranged in three columns. The first column is E-0° and H-0° from top to bottom, the second column is E-45° and H-45° from top to bottom, and the double Z-shaped structure metasurface is located alone in the third column ( Figure 2 ).

[0051] A layer of 100-nanometer-thick silicon film is deposited on the surface of a sapphire substrate by chemical vapor deposition (CVD) technology through a nanolithography machine as a master. Electron beam lithography (EBL) is one of the main technologies for creating nanoscale patterns. There are several common resists for EBL, such as polymethyl methacrylate (PMMA) and hydrogen silsesquioxane. PMMA is a positive resist composed of a methacrylate skeleton 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, spin-coated on the substrate, and the spin-coating thickness is about 1 μm; Step 2: Use the drawn GDS drawing as a mask to perform EBL on the surface of the silicon film. When the primary electron is stimulated on the resist surface, secondary electrons and backscattered electrons are generated. The energy of the secondary electron is between 2 and 50 eV, and the spot size of the beam is in the range of several nanometers. The backscattered electrons bounce off the substrate. Because they contain a wide scattering angle, the size of the exposed area can be several microns. These electrons cause free radicals and radical cations to form and change the chemical structure of the resist. Therefore, when a positive resist is used in lithography, the exposed area can be dissolved and eliminated 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 EBL-etched sample and clean the surface to obtain the processed encoder.

[0052] The transmitted light intensity of each metasurface in the processed encoder chip for incident light of different polarization states is then tested by a micro-area spectrometer and summarized as a transmittance matrix. , and its transmission spectrum ranges from 400 to 1600 nm.

[0053] Since EBL cannot be used for mass production of nanopatterns or nanostructures, after obtaining the encoder template using EBL, ultraviolet light-cured nanoimprinting technology (NIL) can be used for ultrasurface nanoimprinting in the subsequent process. NIL technology can be broadly divided into thermal lithography technology and ultraviolet lithography technology. The ultraviolet lithography technology used in this application requires coating the ultraviolet curing optical adhesive NOA63 on the desired substrate.

[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 uniform NOA63 layer with a thickness of about 1 μm is obtained by spin coating at a rotation speed of 3000 r for 60 s using a spin coater. Then, the encoder obtained by EBL processing is used as a template and pressed on the substrate in reverse so that NOA63 can completely cover the structure on the template. After that, it is fixed as a whole and irradiated with ultraviolet light for 40 min to completely solidify NOA63. Then, the mask is removed to transfer the designed metasurface array pattern to the substrate, thereby 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 use this as a template to mass-produce metasurface polarization detection elements using the simple NIL technology. Subsequent experiments are based on metasurface polarization detection elements prepared using NIL technology. To verify the processing accuracy, SEM scans were performed on metasurfaces with different structures. The results are shown in Figure 3 The results show that the processing error is about ±20 nm, which meets the experimental requirements for nano-processing error. This arrangement design not only ensures the comprehensive detection capability of linearly polarized light and circularly polarized light in the 400-1600 nm band, but also provides a stable structural foundation 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 given in reference Figure 4The thickness of the three types of metasurface structure units (medium structure units) is 100 nm. The E-type metasurface structure unit can be regarded as 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 structure units in the horizontal direction is 0.3 μm, and the distance g1 between two adjacent E-type metasurface structure units in the vertical direction is 0.2 μm. For the H-type metasurface structure unit, it can be regarded as two first arms and one second arm. The length a2 of the first arm is 0.4 μm, the width b2 of the first arm is 0.1 μm, the length c2 and width d2 of the second arm are both 0.2 μm, and the distance e2 between two adjacent H-type metasurface structure units in the horizontal direction is 0.2 μm, and the distance f2 between two adjacent H-type metasurface structure units in the longitudinal direction is 0.2 μm; for the double Z-type metasurface structure unit, two Z-type structure nanocolumns with the same structure and arranged in parallel are regarded as a metasurface structure unit. A single Z-type structure nanocolumn can be regarded as two identical rectangular arms staggered and spliced ​​together. The length a3 of the rectangular arm is 0.5 μm, the width b3 of the rectangular arm is 0.2 μm, and the staggered overlap width c3 is 0.2 μm. The distance d3 between two Z-type structure nanocolumns in the same group is 0.4 μm, the distance e3 between two adjacent double Z-type metasurface structure units in the lateral direction is 0.5 μm, and the distance f3 between two adjacent double Z-type metasurface structure units in the longitudinal direction is 0.2 μm.

[0057] The extinction ratio test was performed on the metasurface polarization detection element:

[0058] The extinction ratio plays a key role in polarization measurement and optical system design, measuring the device's ability to distinguish orthogonal polarizations. Generally, a higher extinction ratio indicates a more significant 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 narrow band; when it is necessary to maintain stable polarization performance 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 under a wide spectrum is shown in Figure 2. Figure 5 , the average extinction ratio of the metasurface array is 2.39.

[0059] If only a single band or a narrow frequency band is targeted, a structure with an extinction ratio far higher than 2.39 can usually be designed in the corresponding frequency band, but the performance of other bands is often seriously sacrificed, and the needs of wide spectrum detection cannot be met. In the design goal of this study, 400~1600 nm wide spectrum coverage is the core requirement, and the average extinction ratio of 2.39 is sufficient to meet the basic requirements of polarization resolution in most application scenarios (such as basic polarization measurement, biological imaging, environmental monitoring, etc.). Combined with subsequent machine learning algorithms and a variety of structural combinations, it can not only make up for the shortcomings of a single structure in a specific band, but also realize the comprehensive detection of multiple types of polarization states such as linear polarization and circular polarization. If you want to pursue a higher extinction ratio in a specific band, you can use a multi-layer stack or a more complex nanostructure, but you need to pay the price of increased design and processing complexity, as well as decreased performance in other bands. Therefore, in order to take into account 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 photoelectric 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 of optical communication systems, corresponding to the O band (Original band) and C band (Conventional band), respectively, and are widely used in fiber-optic communications, lidar, and other long-distance detection scenarios. Therefore, studying the polarization response 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 the foundation for future applications in biomedical diagnosis, optical communications, and long-distance sensing. Based on this consideration, four lasers of 532, 638, 1310, and 1550 nm were selected to measure the structure, and the polarization response of the metasurface array in different bands was systematically evaluated.

[0061] After a comprehensive analysis of the transmission spectra from 400 to 1600 nm, it can be found that the E-type structure metasurface exhibits a more significant polarization response in the two visible light bands of 532 nm and 638 nm, while the H-type structure metasurface performs better in the two communication bands of 1310 nm and 1550 nm. To illustrate this conclusion more intuitively, we have made detailed measurements of the changes in the transmittance of the E-type structure metasurface in the 532 nm band and the H-type structure metasurface in the 1310 nm band with polarization angle at 5° steps. 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 in an approximately sinusoidal manner, 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 of the E-type structure metasurface in the 532 nm band is larger ( Figure 6 Neutron image a), indicating that it has higher polarization resolution in the visible light region; while the H-type structure metasurface has higher polarization resolution at 1310 nm ( Figure 6 The neutron image c) band shows obvious transmittance oscillation, which corresponds to its better polarization detection ability in the near-infrared communication band. In general, these measurement results are mutually confirmed with the spectral data, laying a sufficient basis for the subsequent selection of suitable metasurface structures in different bands.

[0062] Application Examples

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

[0064] 1) Metasurface array transmittance calibration: Use a monochromatic polarized light source with known polarization state information to vertically illuminate a single metasurface, and measure the transmitted light intensity using 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 of the corresponding polarization state after passing through the metasurface to the reference value of the incident light intensity under the same conditions when the metasurface is not loaded, that is:

[0065]

[0066] in, The wavelength is , the polarization state is The transmittance of the incident light after passing through the i-th (i=1~5, the same below) metasurface, The wavelength is , the polarization state is The intensity of the incident light after passing through the i-th metasurface, The wavelength is , the polarization state is The intensity of the incident light.

[0067] 2) Detector polarization response calibration: The detector is spectrally calibrated using a monochromator. Specifically, the detector spectral response is expressed as the ratio of the light source intensity after the fixed polarization state monochromatic light passes through the i-th metasurface (directly read by the detector) to the grayscale value detected by the CCD camera after the fixed polarization state monochromatic light passes through the corresponding metasurface. ,Right now:

[0068]

[0069] in, The wavelength is , the polarization state is The intensity of the incident light after passing through the i-th metasurface, The wavelength detected by the black and white CCD camera is , the polarization state is The gray value of the incident light after passing through the i-th metasurface, is the wavelength of the detector pair passing through the i-th metasurface. , the polarization state is The spectral response rate of the incident light.

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

[0071] Ⅰ. Measurement: Specific operation reference Figure 8 , use an unknown polarization state light source 6 to illuminate the metasurface polarization detection element 8 through a lens 7, use a black and white CCD camera 9 to receive the grayscale value of the light signal passing through the metasurface polarization detection element 8, and convert it into a light intensity signal so that a computer 10 can complete the calculation process and obtain polarization information.

[0072] II. Polarization state reconstruction: An inverse problem of solving the unknown polarization state is constructed through the forest regression function, and the polarization state information of the unknown light field is obtained by computer, which is written in integral form as follows:

[0073]

[0074] in, is the grayscale information of the incident light detected by the black-and-white CCD detector after being transmitted through the i-th metasurface, and represents the unknown wavelength spectrum range that can be reconstructed, and represents the range of unknown polarization angles that can be reconstructed, is the metasurface array transmittance matrix, is the spectral response rate of the incident light. When a light source with unknown polarization state information is irradiated onto the CCD camera through the metasurface array, the CCD camera can collect grayscale information. , and It has been calibrated experimentally, and the intensity of the incident light can be inferred through the spectrum reconstruction formula , due to the wavelength of the incident light Knowing that, we can solve the polarization state of the incident light .

[0075] The theoretical polarization state of the training set data required for forest regression function training is obtained through simulation, and then the polarization state of the light field to be detected is measured through the metasurface polarization detection element, and then put 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 of different wavelengths is mainly based on the following considerations: First, there is often a nonlinear mapping between the metasurface transmittance and the polarization angle, and forest regression can effectively handle multivariate, high-dimensional nonlinear problems; second, the algorithm can still maintain high robustness and prediction accuracy when facing noise, a small number of outliers and fluctuations in the experimental environment; third, forest regression does not require strict linear assumptions on the input features, nor does it rely on complex prior models. It can directly learn the mapping relationship from the measured transmittance and polarization angle data, thereby better adapting to actual experimental conditions.

[0077] In summary, 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, an inversion model from transmittance to polarization angle was constructed based on the previously collected transmission spectrum data and the forest regression algorithm, and the model was verified using test data in different bands. Fig. 9 The specific angle error distribution scatter diagram can more intuitively observe the distribution range and concentration of the prediction error. From the results of the diagram, it can be seen that the prediction error of most test points is maintained within 2°, which verifies the accuracy and stability of the method in different bands. The reason why such high accuracy can be achieved is mainly due to the following aspects: Multi-structure combination: By combining the E-type, H-type and the metasurface structure obtained by rotating them 45°, the multiple solution problem caused by the single "sinusoidal" response is significantly reduced, so that the regression algorithm can more accurately fit the relationship between polarization angle and transmittance. Forest regression algorithm: Forest regression has strong fault tolerance and generalization capabilities, which can effectively deal with noise and small errors in the experimental process, and automatically select the optimal structural features in different bands to predict the polarization angle. Sufficient data sampling: The transmission test of polarized light is carried out in each band with an angle step of 5° or less. The training data set formed covers the complete polarization angle range, so that the model can capture the full picture of the polarization response curve during regression.

[0079] It can be seen that 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 its further application in the field of wide-band and high-precision polarization measurement.

[0080] In summary, this application uses the optimized E-type and H-type structure metasurfaces (E-0°, H-0°, E-45° and H-45°) to achieve polarization detection in a wide band of 400~1600 nm. By measuring the transmission spectra in different bands and different polarization angles, and combining the forest regression algorithm to correlate the transmittance and polarization angle, the experimental results show that excellent measurement accuracy can be obtained in the 532 nm, 638 nm, 1310 nm and 1550 nm bands, and the polarization angle error of most test points is within 2°. In addition, the double Z-type structure metasurface designed for circular polarization detection also exhibits the ability to resolve left- and right-handed circular polarization. The structure has obvious differences in the transmittance of left-handed circularly polarized light and right-handed circularly polarized light of different wavelengths (such as Fig.10 Integrating both linearly polarized light and circularly polarized light detection units in a single metasurface further enhances the system's multi-state polarization recognition performance. Overall, the above results verify the metasurface polarization detection element's ability to reliably detect different polarization states over a wide band, and provide a solid basis for its subsequent integration into a variety of application scenarios such as optical communications and bio-imaging.

[0081] The above shows and describes the basic principles, main features and advantages of the present invention. However, the above is only a specific embodiment of the present invention, and the technical features of the present invention are not limited thereto. Any other implementation methods derived by any technician in the field without departing from the technical solution of the present invention should be included in the scope of the present invention.

Claims

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

2. The metasurface polarization detection element according to claim 1, characterized in that: The E-type structure metasurface is composed of a symmetrical array of E-type dielectric structure units, with gaps between the E-type dielectric structure units; the H-type structure metasurface is composed of a biaxially symmetrical array of H-type dielectric structure units, with gaps between the H-type dielectric structure units; the double Z-type structure metasurface is composed of a non-periodic, asymmetric double Z-type dielectric structure unit array, with gaps between the double Z-type dielectric structure units.

3. The metasurface polarization detection element according to claim 1, characterized in that: 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, characterized in that: 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, characterized in that: Through the combined arrangement of various structural metasurfaces, the metasurface array achieves optimized polarization detection performance in a wide band range of 400~1600 nm, and the angle error of polarization detection in the full band is less than 2°.

6. The metasurface polarization detection element according to claim 1, characterized in that: The average extinction ratio of the metasurface array in the 400-1600nm working band is 2.

39.

7. A method for performing wide spectrum polarization detection based on the metasurface polarization detection element according to any one of claims 1 to 6, characterized in that: The steps include: 1) Metasurface array transmittance calibration: Use a monochromatic polarized light source with known polarization state information to vertically illuminate a single metasurface, and measure the transmitted light intensity using 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 of the corresponding polarization state after passing through the metasurface to the reference value of the incident light intensity under the same conditions when the metasurface is not loaded, that is: ； Among them, i=1~5, The wavelength is , the polarization state is The transmittance of the incident light after passing through the i-th metasurface, The wavelength is , the polarization state is The intensity of the incident light after passing through the i-th metasurface, The wavelength is , the polarization state is The intensity of the incident light; 2) Detector polarization response calibration: The detector is calibrated for spectral response through a monochromator. Specifically, monochromatic light with a fixed polarization state is incident on the i-th metasurface, and the intensity of the transmitted light and the grayscale value collected by the CCD camera under the corresponding conditions are measured. The ratio of the two is defined as the spectral response rate of the detector to the i-th metasurface. ,Right now: ； in, The wavelength is , the polarization state is The intensity of the incident light after passing through the i-th metasurface, The wavelength detected by the black and white CCD camera is , the polarization state is The gray value of the incident light after passing through the i-th metasurface, is the wavelength of the detector pair passing through the i-th metasurface. , the polarization state is The spectral response rate of the incident light; 3) Use the forest regression function to analyze the light intensity signal received by the black and white camera to complete the detection of the polarization state of the incident light field; Ⅰ. Use a light source of unknown polarization state to illuminate the metasurface array through a lens, use a black and white CCD camera to receive the grayscale value of the light signal passing through the metasurface array, and convert it into a light intensity signal; II. Polarization state reconstruction: An inverse problem of solving the unknown polarization state is constructed through the forest regression function, and the polarization state information of the unknown light field is obtained by computer, which is written in integral form as follows: ； in, is the grayscale information of the incident light detected by the black-and-white CCD detector after being transmitted through the i-th metasurface, and represents the unknown wavelength spectrum range that can be reconstructed, and represents the range of unknown polarization angles that can be reconstructed, is the metasurface array transmittance matrix, is the spectral response rate of the incident light. When the light source with unknown polarization state information is irradiated onto the CCD camera through the metasurface array, the CCD camera collects the grayscale information , and It has been calibrated experimentally, and the intensity of the incident light can be inferred through the spectrum reconstruction formula , due to the wavelength of the incident light Knowing that, we can solve the polarization state of the incident light .

8. The method for performing wide spectrum polarization detection based on a metasurface polarization detection element as claimed in claim 7, characterized in that: The theoretical polarization state of the training set data required for forest regression function training is obtained through simulation, and then the polarization state of the light field to be detected is measured through the metasurface polarization detection element, which is put into the trained forest regression function as a test set for prediction.

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