Multi-wavelength polarization imaging metasurface, design method and imaging system

Abstract

The invention discloses a multi-wavelength polarization imaging metasurface, a design method and an imaging system, a plurality of dielectric cylinders placed in a rectangular array form at equal intervals are used as a metasurface unit, the whole metasurface unit is continuously repeated along an x direction and a y direction to form a matrix grating, and a diffraction image intensity calculation formula on a grating focal plane is obtained; establishing a database of dielectric cylinder size and emergent light phase distribution, and substituting the database into a diffracted image intensity calculation formula to obtain diffracted image intensity distribution on a focal plane; designing polarization detector distribution corresponding to a diffraction image on the focal plane, and taking the polarization detector distribution as a target function of diffraction image intensity distribution on the focal plane; and optimizing the structure of the metasurface unit by taking the target function as an optimization target to obtain the multi-wavelength polarization imaging metasurface.

Description

Technical Field

[0001] The present invention relates to the fields of polarization imaging and micro-nano optics, and in particular to a multi-wavelength polarization imaging metasurface, a design method, and an imaging system. Background Art

[0002] The polarization characteristics of light can carry more information about an object than its intensity characteristics. Polarization characteristics are described by the Stokes vector. Currently, many methods have been developed for measuring the Stokes vector of light, such as time-sharing measurement, split-beam measurement, amplitude measurement, and focal plane measurement. The focal plane measurement method is the primary technical approach currently used by polarization cameras. It achieves polarization imaging by covering each pixel of the camera sensor with polarizers at 0°, 45°, 90°, and 135°. Compared to time-sharing and split-beam measurement methods, this type of polarization camera has the advantages of small size and high frame rate. However, it can only capture monochromatic polarization images and cannot obtain the circular polarization component of the Stokes vector.

[0003] In the field of camouflage detection, camouflage and background objects exhibit high spectral consistency, but their polarization information differs at different wavelengths. Acquiring the polarization characteristics of an object at multiple wavelengths can provide more distinguishable features, a fact that has been validated in multiple deep learning algorithms. Currently, two main approaches are used to address the need for multi-wavelength polarization imaging. The first involves using multiple polarization cameras with filters of different wavelengths for multi-channel acquisition. This approach, due to the differing optical axes of the cameras, makes image registration difficult, making simultaneous acquisition difficult in dynamic scenes, and failing to obtain the Stokes vector's circular polarization component. The second approach involves dividing each pixel in the focal plane into multiple sub-pixels, overlaying each sub-pixel with a filter and polarizer. The polarizers include 0°, 45°, 90°, and 135°. The number of sub-pixels is the product of the filter and polarizer types. This approach results in high sensor manufacturing costs and similarly fails to obtain the Stokes vector's circular polarization component.

[0004] The use of metasurface structures can modulate the intensity and phase of polarized light, allowing different polarized lights to produce different responses. By obtaining the intensity information of the outgoing light, the polarization state of the incident light can be inferred. The intensity and phase modulation produced by this method can be designed through the geometric structure of the metasurface and is not limited to the simple modulation produced by polarizers or wave plates, so that the circular polarization component can be reflected in the measurement. By modulating the incident light through the polarization modulation characteristics of the metasurface structure, the polarization state of the incident light at a single wavelength can be measured, but dual-wavelength polarization imaging cannot be achieved. Summary of the Invention

[0005] The purpose of the present invention is to provide a multi-wavelength polarization imaging metasurface and a design method and imaging system to solve the problem that the existing single imaging system cannot achieve dual-wavelength polarization imaging.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions: A method for designing a multi-wavelength polarization imaging metasurface comprises the following steps: Step 1: Several dielectric pillars placed in an equidistant rectangular array are used as a metasurface unit. The entire metasurface unit is repeatedly formed in the x- and y-directions to form a matrix grating, and the formula for calculating the diffraction image intensity on the grating focal plane is obtained; Step 2: Create a database of dielectric column size and output light phase distribution, and substitute it into the calculation formula in step 1 to obtain the diffraction image intensity distribution on the focal plane; Step 3: Design the polarization detector distribution corresponding to the diffraction image on the focal plane, so that it serves as the target function of the intensity distribution of the diffraction image on the focal plane obtained in step 2; Step 4: Taking the objective function in step 3 as the optimization target, optimize the structure of the metasurface unit to obtain a multi-wavelength polarization imaging metasurface.

[0007] Furthermore, the dielectric column is a TiO2 column.

[0008] Furthermore, the establishment of a database of dielectric column sizes and output light phase distributions specifically includes: When light passes through a subwavelength-scale high-refractive-index dielectric column, it is mostly confined to the material and produces a birefringence effect. For a single TiO2 column, the light is transmitted along the z-axis, and its birefringence effect is shown in the following equation:

[0009] in, and represent the electric field intensity of the incident light in the x and y directions, respectively. and represent the electric field intensity of the outgoing light in the x and y directions, respectively. and They represent the phase delay caused by the dielectric column to the light in the u direction and v direction respectively. Their magnitudes are related to the geometric dimensions of the dielectric column, width w and thickness h, and are uniquely determined by w and h. represents the rotation angle of the dielectric column local coordinate system (u, v) relative to the global coordinate system (x, y), and J represents the modulation matrix of the dielectric column on the light field; The size and material of the dielectric column are input into the finite element simulation software, and a set of dimensions is set every preset distance to perform a simulation. The results of the simulation are as follows: and database.

[0010] Furthermore, the dielectric pillar geometric dimensions of width w and thickness h range from 90 nm to 330 nm, and a set of dimensions is set every 2 nm for each simulation.

[0011] Furthermore, the distribution of polarization detectors corresponding to the diffraction image on the designed focal plane specifically includes: A part of the diffraction orders on the focal plane is taken as the diffraction image position; For dual wavelengths, four diffraction images are located for wavelength 1, namely diffraction image 1, diffraction image 2, diffraction image 3, and diffraction image 4. These four diffraction images are located at four different non-zero diffraction orders, and these four diffraction orders cannot be located at 180° rotation positions of the zero-order diffraction order. For wavelength 2, four diffraction images are located, namely diffraction image 5, diffraction image 6, diffraction image 7, and diffraction image 8. Diffraction image 5 is located at 180° rotation position of diffraction image 1 at the zero-order diffraction order, diffraction image 6 is located at 180° rotation position of diffraction image 2 at the zero-order diffraction order, diffraction image 7 is located at 180° rotation position of diffraction image 3 at the zero-order diffraction order, and diffraction image 8 is located at 180° rotation position of diffraction image 4 at the zero-order diffraction order. For dual wavelengths, diffraction images 1 to 4 are formed at wavelength 1 for polarization state arrive The polarization detector, diffraction images 5 to 8 form the polarization state at wavelength 2 arrive The polarization detector, 、 、 、 These four polarization states form a regular tetrahedron on the Poincare sphere; For three wavelengths, four diffraction images are positioned for wavelength 1, namely diffraction image 1, diffraction image 2, diffraction image 3, and diffraction image 4. These four diffraction images are located at four different non-zero diffraction orders, and these four diffraction orders cannot be at the 90° rotation position of the zero-order diffraction order. For wavelength 2, four diffraction images are positioned, namely diffraction image 5, diffraction image 6, diffraction image 7, and diffraction image 8, among which diffraction image 5 is located at the 90° rotation position of diffraction image 1 at the zero-order diffraction order, diffraction image 6 is located at the 90° rotation position of diffraction image 2 at the zero-order diffraction order, and diffraction image 7 is located at the 90° rotation position of diffraction image 1 at the zero-order diffraction order. Image 3 is at a 90° rotation position of the zeroth diffraction order, and diffraction image 8 is at a 90° rotation position of diffraction image 4 at the zeroth diffraction order. For wavelength 3, four diffraction images are located, namely diffraction image 9, diffraction image 10, diffraction image 11, and diffraction image 12. Diffraction image 9 is at a 180° rotation position of diffraction image 1 at the zeroth diffraction order, diffraction image 10 is at a 180° rotation position of diffraction image 2 at the zeroth diffraction order, diffraction image 11 is at a 180° rotation position of diffraction image 3 at the zeroth diffraction order, and diffraction image 12 is at a 180° rotation position of diffraction image 4 at the zeroth diffraction order. For three wavelengths, diffraction images 1 to 4 are formed at wavelength 1 for polarization state arrive The polarization detector forms diffraction images 5 to 8 at wavelength 2 for polarization states. arrive The polarization detector, diffraction image 9 to diffraction image 12 forms the polarization state at wavelength 3 arrive The polarization detector, 、 、 、 These four polarization states form a regular tetrahedron on the Poincare sphere.

[0012] Furthermore, the optimization of the structure of the metasurface unit specifically includes: The width w, thickness h and rotation angle of each dielectric column in the A×B dielectric column array with equal spacing are For optimization, let the parameter set to be optimized be C, which is expressed as follows:

[0013] Among them, W, H and Indicates the width w, thickness h, and rotation angle of each dielectric column The array formed.

[0014] Furthermore, for dual wavelengths, the optimization conditions include: 1) For diffraction images 1 to 8, use and represents the wavelength and polarization state corresponding to the diffraction image, i takes the value of 1 or 2, j takes the value of 1, 2, 3, or 4, and the intensity of each diffraction image should be as close as possible to the target function of the diffraction image intensity distribution, as shown in the following formula:

[0015] in, Indicates correspondence and The intensity of the diffraction image of the polarization detector, Indicates correspondence and The objective function of the diffraction image intensity distribution of the polarization detector is, Represents the polarization state The transpose of represents the total intensity of the diffraction image; 2) For diffraction images 1 to 8, and The conjugate polarization The corresponding diffraction image intensity is close to 0 when input; 3) The theoretical maximum output of diffraction images 1 to 8 should be as close as possible; 4) The output of diffraction images other than diffraction images 1 to 8 should be as close to 0 as possible.

[0016] Furthermore, for the optimization conditions 1)-4), the loss function is established as follows:

[0017]

[0018]

[0019]

[0020] in, 、 and is the weight parameter in training, Indicates correspondence and The intensity of the diffraction image of the polarization detector, Indicates correspondence and The objective function of the intensity distribution of the diffraction image of the polarization detector is, express The conjugate polarization of Indicates correspondence The average intensity of all polarization detector diffraction images; The gradient descent method is used to optimize the loss function, and the optimization results of the optimization parameter set C are obtained, completing the design of the multi-wavelength polarization imaging metasurface.

[0021] A multi-wavelength polarization imaging metasurface is obtained using the above-mentioned design method of a multi-wavelength polarization imaging metasurface.

[0022] An imaging system includes a housing, an aperture provided at one end of the housing, an image acquisition sensor provided at the other end, a metal baffle and an aspheric lens provided in sequence in the housing along the direction from the aperture to the image acquisition sensor, a metasurface grating provided in the middle of the metal baffle, and the metasurface grating using the above-mentioned multi-wavelength polarization imaging metasurface.

[0023] Compared with the prior art, the present invention has the following beneficial technical effects: Taking into account the importance of obtaining multi-wavelength polarization images and the different responses of metasurface structures to different wavelengths, the present invention proposes a method for realizing multi-wavelength polarization state imaging using a single metasurface structure. The grating formed by the metasurface structure is used to polarize the light on each diffraction order. Four polarization modulations are designed for each wavelength to form a polarization detector to complete the complete measurement of the polarization state at that wavelength.

[0024] The imaging system based on the designed metasurface records diffraction images formed by the metasurface grating in one pass, enabling simultaneous measurement of polarization states at multiple wavelengths. Compared to imaging systems using multi-polarization cameras, this system can obtain complete Stokes vector imaging and simplifies image registration. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The drawings in the specification are used to provide further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0026] Figure 1 A schematic diagram showing an imaging system of a multi-wavelength polarization imaging metasurface; Wherein: 1-1, housing; 2-1, aperture; 3-1, metasurface grating; 4-1, metal baffle; 5-1, aspheric lens; 6-3, position of diffraction image 3; 6-4, position of diffraction image 4; 6-5, position of diffraction image 5; Figure 2 Schematic diagram of focal plane diffraction image, where (a) is a schematic diagram of diffraction image distribution for a dual-wavelength design, and (b) is a schematic diagram of diffraction image distribution for a triple-wavelength design; Figure 3 Schematic diagram showing the modulation of light wave transmission by a single TiO2 column; Figure 4Represents a top view of a metasurface periodic unit, where (a) is a top view of a metasurface periodic unit optimized for dual wavelengths, and (b) is a top view of a metasurface periodic unit optimized for three wavelengths. DETAILED DESCRIPTION

[0027] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0028] It should be noted that the terms "first", "second", etc. in the description and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that the numbers used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0029] Example 1 The present invention provides a design method for a multi-wavelength polarization imaging metasurface, which includes the following four parts: The first part is the design of the metasurface grating. A×B dielectric pillars are placed in an equidistant rectangular array as a metasurface unit. The overall modulation matrix of the metasurface unit is shown in formula (1).

[0030] (1) Among them, the rect function represents the rectangular window function, and represents the spacing of dielectric pillars along the x-direction and y-direction, It represents the modulation matrix J of the dielectric column in row a and column b. The modulation matrix J is shown in formula (2).

[0031] (2) Indicates the rotation angle of the dielectric cylinder local coordinate system (u, v) relative to the global coordinate system (x, y).

[0032] The entire metasurface unit is repeatedly formed along the x- and y-directions to form a matrix grating. The spatial frequency distribution of the light emitted by the matrix grating is equal to the Fourier transform of the grating modulation matrix, as shown in formula (3).

[0033] (3) Where A and B represent the number of dielectric pillar arrays along the x and y directions, and They represent the spatial frequencies of the matrix grating along the x and y directions respectively. The expression of the sinc function is shown in formula (4).

[0034] (4) By placing an aspheric lens behind the grating, the diffraction image of the grating can be obtained on the focal plane of the lens. The expression of the intensity of the diffraction image is shown in formula (5).

[0035] (5) Where E represents the Jones vector of the incident light, is the intensity of the diffraction order corresponding to the spatial frequency of the matrix grating output light in formula (3), where the diffraction order is and spatial frequency The relationship is shown in formula (6).

[0036] (6) At different wavelengths, due to the different wavelengths of light, the phase delay generated in the u direction and v direction in formula (2) is and There will be some differences, and the final grating will have different spatial frequencies. and Different diffraction image intensity distributions will also be produced under different conditions, and the same metasurface structure can be used to modulate different wavelengths of light on the diffraction image separately.

[0037] The second part is the establishment of a dielectric column database. When light passes through a sub-wavelength-scale high-refractive-index dielectric column, most of the light will be confined to the interior of the material and produce a birefringence effect. In this invention, TiO2 columns are used as the transmission medium. The transmission process of a single TiO2 column is as follows: Figure 2 As shown, the light is transmitted along the z-axis, and its birefringence effect is shown in formula (7).

[0038] (7) in, and represent the electric field intensity of the incident light in the x and y directions, respectively. and They represent the electric field intensity of the outgoing light in the x and y directions respectively. and They represent the phase delay caused by the dielectric column to the light in the u direction and v direction respectively. Their magnitudes are related to the geometric dimensions of the dielectric column, width w and thickness h, and can be uniquely determined by w and h. represents the rotation angle of the dielectric column local coordinate system (u, v) relative to the global coordinate system (x, y). For the convenience of description, J is used to represent the modulation matrix of the dielectric column on the light field.

[0039] Phase delay of dielectric pillars and The dimensions w and h of the dielectric column are related. The dimensions and materials of the dielectric column are input into the finite element simulation software, where w and h range from 90nm to 330nm. A set of dimensions is set every 2nm for a simulation, and the values from w and h to and database.

[0040] The third part is the design of the focal plane diffraction image function. For the diffraction image on the focal plane, two design schemes are proposed, one for dual wavelength and the other for triple wavelength. The schematic diagrams are as follows: Figure 2 shown. Figure 2 (a) is a dual-wavelength example, where diffraction images 1 to 4 are designed for wavelength 1, and diffraction images 5 to 8 are designed for wavelength 2. Figure 2 (b) is a three-wavelength example, where diffraction images 1 to 4 are designed for wavelength 1, diffraction images 5 to 8 are designed for wavelength 2, and diffraction images 9 to 12 are designed for wavelength 3. Figure 2 The diffraction image distribution method shown in is only one distribution scheme, and the same method can be used to obtain optimization results for other distribution schemes.

[0041] Taking the dual-wavelength result as an example, the diffraction image 1 forms a polarization state at wavelength 1. The polarization detector is similar to the effect of a polarizer, that is, under ideal conditions, for polarization states of The light can pass through completely, and the polarization state is Orthogonal polarization states The light can be completely cut off, so that the proportion of the polarization component in the light can be obtained by the collected diffraction image intensity value. Similarly, diffraction images 2 to 4 form the polarization state at wavelength 1. arrive Polarization detector. Diffraction images 5 to 8 form polarization states at wavelength 2. arrive Polarization detector.

[0042] In order to minimize the measurement error, 、 、 、 The correlation between them should be as small as possible, that is, they should maintain the largest relative distance on the Poincare sphere, so these four polarization states should form a regular tetrahedron on the Poincare sphere. According to this principle, the Stokes vectors of the four polarization states can be set as 、 、 and .

[0043] For three wavelengths, diffraction images 1 to 4 are formed at wavelength 1 for polarization state arrive The polarization detector, diffraction images 5 to 8 form the polarization state at wavelength 2 arrive The polarization detector, diffraction image 9 to diffraction image 12 forms the polarization state at wavelength 3 arrive The polarization detector, 、 、 、 These four polarization states form a regular tetrahedron on the Poincare sphere.

[0044] This design approach has the following advantages.

[0045] Easy to achieve synchronous acquisition: the grating and lens focus all diffraction images onto the focal plane to form an image. The same detector can be used to obtain detection results at the same time, and synchronous acquisition can be achieved by segmenting multiple diffraction images.

[0046] Image registration is relatively easy: the central optical axes of all segmented diffraction images are consistent with each other, and image registration can be completed only through displacement transformation.

[0047] Acquisition of circular polarization components: Calculating the Stokes vector using a linear combination of four polarization bases can obtain all four components, whereas other methods cannot.

[0048] The fourth part is the optimization method of the metasurface structure. In order to achieve the diffraction images described above that correspond to the corresponding wavelengths and have the effect of a polarization detector, it is necessary to adjust the width w, thickness h and rotation angle of each dielectric column in the A×B array. Optimize. Let the parameter set to be optimized be C, as shown in formula (8).

[0049] (8) Among them, W, H and Indicates the width w, thickness h, and rotation angle of each dielectric column Taking dual-wavelength optimization as an example, the optimization objectives include the following four points.

[0050] 1. For diffraction images 1 to 8, use and represents the wavelength and polarization state corresponding to the diffraction image, i takes the value of 1 or 2, and j takes the value of 1, 2, 3, or 4. The intensity of each diffraction image should be as close as possible to the target function of the diffraction image intensity distribution, as shown in formula (9).

[0051] (9) in, Indicates correspondence and The intensity of the diffraction image of the polarization detector, Indicates correspondence and The objective function of the diffraction image intensity distribution of the polarization detector is, Represents the polarization state The transpose of represents the total intensity of the diffraction image.

[0052] 2. For diffraction images 1 to 8, and The conjugate polarization The corresponding diffraction image intensity is close to 0 during input.

[0053] 3. The theoretical maximum outputs of diffraction images 1 to 8 should be as close as possible.

[0054] 4. The output of diffraction images other than diffraction images 1 to 8 should be as close to 0 as possible.

[0055] For the optimization conditions of points 1-4, the loss function should be established, which should include the following three items.

[0056] (10) (11) (12) in, 、 and is the weight parameter in training, Indicates correspondence and The intensity of the diffraction image of the polarization detector, Indicates correspondence and The objective function of the intensity distribution of the diffraction image of the polarization detector is, express The conjugate polarization of Indicates correspondence The average intensity of the diffraction images of all polarization detectors; the overall loss function should be the weighted sum of the above three items.

[0057] (13) According to the test, 、 and Setting them to 7, 3, and 1 respectively can achieve good training results.

[0058] The gradient descent method is used to optimize the optimization results of the optimization parameter set C, and the design of the multi-wavelength polarization imaging metasurface is completed. The results of the dual-wavelength optimization are as follows: Figure 4 Similarly, the results of the three-wavelength optimization are shown in Figure 4 As shown in (b), the dual-wavelength optimization results are optimized for wavelengths of 532nm and 671nm, and the three-wavelength optimization results are optimized for wavelengths of 471nm, 532nm, and 671nm.

[0059] Example 2 The present invention provides an imaging system that uses a metasurface to form a diffraction grating. The diffraction grating and a lens imaging system behind it generate multiple diffraction images, four of which correspond to each wavelength. A polarization image can be obtained through calculation.

[0060] Polarization imaging requires obtaining the target's Stokes vector, which can be determined through four sets of measurements. By leveraging the different birefringence effects of a subwavelength metasurface structure on polarized light of different wavelengths, a diffraction grating is designed. The diffraction orders of the diffraction grating form polarization detectors, which can be divided into multiple groups of four to measure the Stokes vector at a single wavelength. A method for designing this diffraction grating is provided, and the effectiveness of the design is verified through simulation. This invention provides a complete imaging system capable of simultaneously and completely measuring the Stokes vector of images at multiple wavelengths.

[0061] The imaging system of the present invention includes: a housing 1-1, an aperture 2-1 is provided at one end of the housing 1-1, and an image acquisition sensor is provided on the other side. A metal baffle 4-1 and an aspheric lens 5-1 are sequentially provided in the housing 1-1 along the direction from the aperture 2-1 to the image acquisition sensor. A metasurface grating 3-1 is provided in the middle of the metal baffle 4-1. The metasurface grating 3-1 is shown as a multi-wavelength polarization imaging metasurface designed in Example 1.

[0062] The shell 1-1 is made of aluminum alloy and plays the role of isolating stray light. The aperture 2-1 is used to control the size of the diffraction image. Figure 3A schematic diagram showing a TiO2 column in the periodic unit structure is shown. Figure 4 The top view of the periodic unit structure of the optimized TiO2 metasurface grating is shown. The metasurface unit structure contains a total of 121 TiO2 columns, forming an 11*11 array structure with a unit size of 4620nm×4620nm. The metasurface unit is repeatedly expanded 216 times along the x-direction and y-direction to form a metasurface grating 3-1 with a side length of 1mm. The metal baffle 4-1 is made of aluminum alloy and plays the role of supporting and fixing the metasurface grating 3-1 and isolating stray light. The purpose of using the aspheric lens 5-1 is to focus the off-axis images of diffraction image 1 to diffraction image 8 to the focal plane with a smaller aberration, forming position 6-3 of diffraction image 3, position 6-4 of diffraction image 4, and position 6-5 of diffraction image 5 at the focal plane position, where position 6-3 of diffraction image 3 corresponds to Figure 2 The diffraction image at position 3 in (a) corresponds to position 6-4 of diffraction image 4. Figure 2 The diffraction image at position 4 in (a) corresponds to the diffraction image at position 6-5 of position 5. Figure 2 The diffraction image at position 5 in (a) is Figure 1 Only the optical path diagrams of the sections where diffraction images 3, 4, and 5 are located are drawn. The imaging optical paths of other sections are the same. The schematic diagram of the diffraction image on the focal plane where the image acquisition sensor is located is shown in Figure 2 As shown in the figure, for the image acquisition sensor, a filter with a central wavelength is covered on the diffraction image corresponding to the corresponding wavelength to reduce the influence of light of other wavelengths.

[0063] In order to determine the optimization results so that diffraction images 1 to 8 can achieve the polarization detector effect for the designed polarization state, the metasurface grating 3-1 is simulated using simulation software. Taking the optimization results of dual wavelengths as an example, Table 1 summarizes the polarization state at wavelengths 1 and 2. arrive , polarization state arrive The conjugate polarization arrive The output light intensity of each diffraction order.

[0064] Table 1 Dual wavelength optimization results Output intensity of each diffraction image at different polarization inputs

[0065] It can be seen that the input polarization state at wavelength 1 and wavelength 2 is When the input polarization state is When the light is diffracted, the output light intensity of diffraction image 1 and diffraction image 5 is close to 0, which shows that diffraction image 1 can be used as the polarization state Similarly, several other diffraction images have similar properties.

[0066] Table 2 shows the optimization results of three wavelengths at wavelengths 1, 2 and 3 in polarization state. arrive , polarization state arrive The conjugate polarization arrive The output light intensity of each diffraction order.

[0067] Table 2 Output intensity of each diffraction image at different polarization states when three-wavelength optimization results are applied

[0068] For the diffraction image collected by the CMOS sensor, taking the dual-wavelength optimization result as an example, the following data processing steps are included.

[0069] Splitting the diffraction sub-images: Determine the center positions of diffraction images 1 through 8 based on the grating's diffraction angle. The diffraction images within this area are captured using these center positions as a reference. Table 3 summarizes the center positions of the eight diffraction images, calculated using a sensor with 2464 × 2056 pixels and a 2 / 3-inch target surface size.

[0070] Table 3 Center position of diffraction image

[0071] Image registration: Place a glass checkerboard calibration plate in front of the aperture, perform image registration on the split diffraction sub-images, and obtain the registration parameters of the diffraction sub-images.

[0072] Stokes vector calculation: The diffraction sub-images after registration are recorded as arrive ,in arrive Corresponding to wavelength 1. For wavelength 1, take a pixel point in the sub-image and arrive The intensity at this pixel is stacked horizontally as follows: , then the Stokes vector of the pixel is calculated according to formula (14).

[0073] (14) in The definition of is shown in formula (15).

[0074] (15) By operating each pixel point in the above method, the full Stokes vector imaging result of the target at one wavelength can be obtained. The same method can be used to calculate other wavelengths.

[0075] This imaging system has the following two advantages: first, it can obtain complete Stokes vector imaging. The Stokes vector S calculated according to formula (14) contains all four components. Second, image registration is simple. Since all diffraction images are split from the same image, there is no need for corresponding intensity adjustment. In addition, there is a basic positioning of the center position distribution, which makes it simple to obtain image registration parameters.

[0076] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit its scope of protection. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that after reading the present invention, those skilled in the art may still make various changes, modifications or equivalent substitutions to the specific implementation methods of the invention, but these changes, modifications or equivalent substitutions are all within the scope of protection of the pending claims of the invention.

Claims

1. A design method for a multi-wavelength polarization imaging metasurface, characterized in that: The steps include: Step 1: A number of dielectric pillars placed in an equidistant rectangular array are used as a metasurface unit. The entire metasurface unit is repeatedly formed in the x- and y-directions to form a matrix grating, and the formula for calculating the diffraction image intensity on the grating focal plane is obtained; Step 2: Create a database of dielectric column size and output light phase distribution, and substitute it into the calculation formula in step 1 to obtain the diffraction image intensity distribution on the focal plane; Step 3: Design the polarization detector distribution corresponding to the diffraction image on the focal plane, so that it serves as the target function of the intensity distribution of the diffraction image on the focal plane obtained in step 2; Step 4: Taking the objective function in step 3 as the optimization target, optimize the structure of the metasurface unit to obtain a multi-wavelength polarization imaging metasurface.

2. The design method of a multi-wavelength polarization imaging metasurface according to claim 1, characterized in that: The dielectric column is a TiO2 column.

3. The design method of a multi-wavelength polarization imaging metasurface according to claim 2, characterized in that: The step of establishing a database of dielectric column sizes and output light phase distributions specifically includes: When light passes through a subwavelength-scale high-refractive-index dielectric column, it is mostly confined to the material and produces a birefringence effect. For a single TiO2 column, the light is transmitted along the z-axis, and its birefringence effect is shown in the following equation: in, and represent the electric field intensity of the incident light in the x and y directions, respectively. and represent the electric field intensity of the outgoing light in the x and y directions, respectively. and They represent the phase delay caused by the dielectric column to the light in the u direction and v direction respectively. Their magnitudes are related to the geometric dimensions of the dielectric column, width w and thickness h, and are uniquely determined by w and h. represents the rotation angle of the dielectric column local coordinate system (u, v) relative to the global coordinate system (x, y), and J represents the modulation matrix of the dielectric column on the light field; The size and material of the dielectric column are input into the finite element simulation software, and a set of dimensions is set every preset distance to perform a simulation. The results of the simulation are as follows: and database.

4. The method for designing a multi-wavelength polarization imaging metasurface according to claim 3, wherein: The dielectric pillars have geometric dimensions of width w and thickness h ranging from 90 nm to 330 nm, and a set of dimensions is set every 2 nm for simulation.

5. The design method of a multi-wavelength polarization imaging metasurface according to claim 3, characterized in that: The polarization detector distribution corresponding to the diffraction image on the designed focal plane specifically includes: A part of the diffraction orders on the focal plane is taken as the diffraction image position; For dual wavelengths, four diffraction images are located for wavelength 1, namely diffraction image 1, diffraction image 2, diffraction image 3, and diffraction image 4. These four diffraction images are located at four different non-zero diffraction orders, and these four diffraction orders cannot be at the 180° rotation position of the zero-order diffraction order. For wavelength 2, four diffraction images are located, namely diffraction image 5, diffraction image 6, diffraction image 7, and diffraction image 8. Diffraction image 5 is located at the 180° rotation position of diffraction image 1 at the zero-order diffraction order, diffraction image 6 is located at the 180° rotation position of diffraction image 2 at the zero-order diffraction order, diffraction image 7 is located at the 180° rotation position of diffraction image 3 at the zero-order diffraction order, and diffraction image 8 is located at the 180° rotation position of diffraction image 4 at the zero-order diffraction order. For dual wavelengths, diffraction images 1 to 4 are formed at wavelength 1 for polarization state arrive The polarization detector, diffraction images 5 to 8 form the polarization state at wavelength 2 arrive The polarization detector, 、 、 、 These four polarization states form a regular tetrahedron on the Poincare sphere; For three wavelengths, four diffraction images are positioned for wavelength 1, namely diffraction image 1, diffraction image 2, diffraction image 3, and diffraction image 4. These four diffraction images are located at four different non-zero diffraction orders, and these four diffraction orders cannot be at the 90° rotation position of the zero-order diffraction order. For wavelength 2, four diffraction images are positioned, namely diffraction image 5, diffraction image 6, diffraction image 7, and diffraction image 8, among which diffraction image 5 is located at the 90° rotation position of diffraction image 1 at the zero-order diffraction order, diffraction image 6 is located at the 90° rotation position of diffraction image 2 at the zero-order diffraction order, and diffraction image 7 is located at the 90° rotation position of diffraction image 1 at the zero-order diffraction order. Image 3 is at a 90° rotation position of the zeroth diffraction order, and diffraction image 8 is at a 90° rotation position of diffraction image 4 at the zeroth diffraction order. For wavelength 3, four diffraction images are located, namely diffraction image 9, diffraction image 10, diffraction image 11, and diffraction image 12. Diffraction image 9 is at a 180° rotation position of diffraction image 1 at the zeroth diffraction order, diffraction image 10 is at a 180° rotation position of diffraction image 2 at the zeroth diffraction order, diffraction image 11 is at a 180° rotation position of diffraction image 3 at the zeroth diffraction order, and diffraction image 12 is at a 180° rotation position of diffraction image 4 at the zeroth diffraction order. For three wavelengths, diffraction images 1 to 4 are formed at wavelength 1 for polarization state arrive The polarization detector forms diffraction images 5 to 8 at wavelength 2 for polarization states. arrive The polarization detector, diffraction image 9 to diffraction image 12 forms the polarization state at wavelength 3 arrive The polarization detector, 、 、 、 These four polarization states form a regular tetrahedron on the Poincare sphere.

6. The method for designing a multi-wavelength polarization imaging metasurface according to claim 5, wherein: The optimization of the structure of the metasurface unit specifically includes: The width w, thickness h and rotation angle of each dielectric column in the A×B dielectric column array with equal spacing are For optimization, let the parameter set to be optimized be C, which is expressed as follows: Among them, W, H and Indicates the width w, thickness h, and rotation angle of each dielectric column The array formed.

7. The method for designing a multi-wavelength polarization imaging metasurface according to claim 6, wherein: For dual wavelengths, the optimization conditions include: 1) For diffraction images 1 to 8, use and represents the wavelength and polarization state corresponding to the diffraction image, i takes the value of 1 or 2, j takes the value of 1, 2, 3, or 4, and the intensity of each diffraction image should be as close as possible to the target function of the diffraction image intensity distribution, as shown in the following formula: in, Indicates correspondence and The intensity of the diffraction image of the polarization detector, Indicates correspondence and The objective function of the diffraction image intensity distribution of the polarization detector is, Represents the polarization state The transpose of represents the total intensity of the diffraction image; 2) For diffraction images 1 to 8, and The conjugate polarization The corresponding diffraction image intensity is close to 0 when input; 3) The theoretical maximum output of diffraction images 1 to 8 should be as close as possible; 4) The output of diffraction images other than diffraction images 1 to 8 should be as close to 0 as possible.

8. The method for designing a multi-wavelength polarization imaging metasurface according to claim 7, wherein: For optimization conditions 1)-4), the loss function is established as follows: in, 、 and is the weight parameter in training, Indicates correspondence and The intensity of the diffraction image of the polarization detector, Indicates correspondence and The objective function of the intensity distribution of the diffraction image of the polarization detector is, express The conjugate polarization of Indicates correspondence The average intensity of all polarization detector diffraction images; The gradient descent method is used to optimize the loss function, and the optimization results of the optimization parameter set C are obtained, completing the design of the multi-wavelength polarization imaging metasurface.

9. A multi-wavelength polarization imaging metasurface, obtained by using the design method of a multi-wavelength polarization imaging metasurface described in any one of claims 1-8.

10. An imaging system, characterized in that: include: A housing (1-1) is provided with an aperture (2-1) at one end of the housing (1-1), and an image acquisition sensor is provided at the other side; a metal baffle (4-1) and an aspheric lens (5-1) are sequentially provided in the housing (1-1) along a direction from the aperture (2-1) to the image acquisition sensor; a metasurface grating (3-1) is provided in the middle of the metal baffle (4-1); and the metasurface grating (3-1) adopts the multi-wavelength polarization imaging metasurface according to claim 9.

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