Polarization imaging system based on super-structure micro-lens array and construction method of super-structure micro-lens array

Abstract

The invention discloses a polarization imaging system based on a super-structure micro-lens array and a construction method of the super-structure micro-lens array, and relates to the technical field of polarization imaging. The polarization imaging system comprises at least one refraction lens, a metasurface full Stokes polarization micro lens array and a pixel plane, and the pixel plane is formed by arranging four detector pixel arrays; the metasurface full-Stokes polarization micro-lens array is composed of a plurality of single micro-lens arrays, and a polychromatic light source focuses when incident to the refraction lens and then is transmitted to the metasurface full-Stokes polarization micro-lens array. The metasurface full Stokes polarization micro-lens array splits light in different polarization states and focuses the light to a pixel plane, and real-time polarization imaging is carried out on an object. According to the invention, the energy utilization efficiency of a single polarization is improved from 1 / 4 to 1 / 3, which has positive significance for polarization imaging and also has positive significance for function integration and volume miniaturization of an optical imaging system.

Description

Technical Field

[0001] The present invention belongs to the field of polarization imaging technology, and more specifically, relates to a polarization imaging system based on a metastructured microlens array and a method for constructing the metastructured microlens array. Background Art

[0002] In recent years, with the continuous development and maturity of autonomous driving, AR / VR, and drone technologies, new requirements have been put forward for information detection. Ordinary light intensity imaging can hardly meet the needs of imaging complex scenes, and higher-dimensional imaging has become increasingly important. As an electromagnetic wave, light has polarization characteristics in addition to inherent properties such as amplitude and frequency. When targets on the surface or in the atmosphere reflect, scatter, transmit, and radiate electromagnetic waves, they will generate specific polarization information determined by their own characteristics, and this polarization information can be used to analyze the target's shape, surface roughness, texture direction, and physical and chemical properties of the material. Based on its characteristics, polarization information detection has been widely used in biomedicine, remote sensing, astronomy and other fields. However, in previous imaging systems, if polarization information detection is to be achieved, the detection device usually contains multiple optical components, with a complex structure and large size, which cannot well meet the trend of miniaturization and easy integration of optical components.

[0003] Inspired by the compound eyes of insects, the concept of microlens arrays was proposed. A microlens array is an array composed of a series of tiny lenses with diameters ranging from ten microns to several millimeters arranged in a certain way. Its functions can be customized according to needs, and it can modulate the incident light by beam shaping, optical focusing, etc. Thanks to its characteristics, it is easy to achieve imaging with a large field of view, low aberration, and small distortion. It has important application potential in the miniaturization, intelligence, and integration of optical systems.

[0004] In order to improve imaging quality and obtain higher-dimensional information, a polarization grating can be added to the original imaging system microlens array to realize a polarization microlens array and obtain polarization information images. However, this poses a huge challenge in terms of processing methods and costs. Metasurfaces, which have emerged in recent years, are a type of artificial material based on subwavelength structures. They are naturally small and light. Research has proven that they have the ability to control the amplitude, phase, polarization and other characteristics of electromagnetic waves, and are considered to be a new generation of optical materials. These characteristics can perfectly adapt to the trend of miniaturization and integration of optical systems, which provides a new solution for polarization microlens arrays in traditional imaging systems. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a polarization imaging system based on a meta-microlens array and a method for constructing a meta-microlens array. Compared with traditional imaging systems, the system can obtain information of higher dimensions, thereby improving the information acquisition capability of the imaging system. The system is easy to process and realize at low cost, which has positive significance for the functional integration and miniaturization of optical systems.

[0006] To solve the above technical problems, the technical solution adopted by the present invention is: a polarization imaging system based on a metastructured microlens array, comprising at least one refractive lens, a metasurface full-Stokes polarization microlens array, and a pixel plane, wherein the pixel plane is composed of four detector pixel arrays; the metasurface full-Stokes polarization microlens array is composed of a plurality of single microlens arrays, each of which is composed of a metasurface full-Stokes polarization microlens substrate and a metasurface full-Stokes polarization microlens micro-nanostructure, and the metasurface full-Stokes polarization microlens micro-nanostructure is arranged in a regular pattern on the metasurface full-Stokes polarization microlens substrate;

[0007] When the polychromatic light source is incident on the refractive lens, it is focused and then propagates to the metasurface full-Stokes polarization microlens array. The metasurface full-Stokes polarization microlens array will split the light of different polarization states and focus it onto the pixel plane, performing real-time polarization imaging of the object.

[0008] Preferably, each single microlens forms a group with four detector pixels, and each single microlens splits polarized light of different polarization states and focuses the polarized light onto the four detector pixels in the same group. The four detector pixels in the same group receive light of different polarizations.

[0009] A method for constructing a metasurface full-Stokes polarization microlens array in a polarization imaging system, wherein the metasurface full-Stokes polarization microlens array is composed of multiple metasurface full-Stokes polarization single microlens arrays. The method for constructing the metasurface full-Stokes polarization microlens array comprises the following steps:

[0010] Step 1: First, determine the geometric parameters of the micro-nanostructure on a single metasurface full-Stokes polarization microlens based on the detector pixel size, incident light wavelength, and processing capabilities;

[0011] Step 2: Then, perform FDTD simulation based on the geometric parameters to obtain the linear polarization and circular polarization database of a single metasurface full-Stokes polarization microlens;

[0012] Step 3: Generate multiple polarization corresponding layouts according to the database, and combine the multiple polarization corresponding layouts into a layout unit;

[0013] Step 4: Then simulate the layout unit;

[0014] Step 5: After the simulation is correct, the layout units are arrayed to obtain the metasurface full-Stokes polarization microlens array layout;

[0015] Step 6: Simulate the metasurface full Stokes polarization microlens array layout obtained above to finally obtain the metasurface full Stokes polarization microlens array layout.

[0016] Preferably, in step 3, the specific steps of combining multiple polarization corresponding layouts into one layout unit are:

[0017] Step 7: Determine how many micro-nanostructures a single metasurface full-Stokes polarization microlens has. Let the number of micro-nanostructures be N. Use MATLAB software to construct a row matrix with the first N / 3 columns of the matrix having a value of 1, the middle N / 3 columns of the matrix having a value of 2, and the last N / 3 columns of the matrix having a value of 3.

[0018] Step 8: Use the randperm statement to randomly shuffle the row matrix, denoted as matrix M;

[0019] Step 9: According to the numbers in the matrix M, select different polarization layouts at each position. For example, a value of 1 selects the X-linear polarization layout and the Y-linear polarization layout, a value of 2 selects the LCP layout, and a value of 3 selects the 45° linear polarization layout. Finally, a new layout matrix is ​​created, which is recorded as Morigin.

[0020] The specific implementation content of step 5 is: expand the Morigin array to obtain the final metasurface full-Stokes polarization microlens array layout.

[0021] Preferably, in step 3, the theoretical phase value of the micro-nanostructure at each position is Determined by formula 1,

[0022]

[0023] Where λ is the wavelength, x and y are the coordinates of the micro-nanostructure, x0 and y0 are the off-axis focal length offsets of different polarizations under a single microlens, and f is the focal length of the lens;

[0024] According to Formula 2, the theoretical phase value obtained by Formula 1 is used to search the database for a suitable micro-nano structure, and finally the micro-nano structure at this position is selected.

[0025]

[0026] The micro-nanostructure that minimizes δ is selected as the micro-nanostructure at this position, where δ represents the difference between the transmittance and phase response of the selected micro-nanostructure in the complex domain and the theoretical phase, t x represents the transmittance of the micro-nanostructure, represents the phase of the micro-nanostructure in the complex domain, Indicates the theoretical complex domain phase value at that position.

[0027] The beneficial effects of adopting the above technical solution are:

[0028] 1. The metasurface full-Stokes polarization microlens array design method proposed in this invention can obtain higher-dimensional information than traditional microlens arrays, improving the information acquisition capability of the imaging system. Based on the acquired polarization information, the full Stokes parameters of the object can be obtained or light field imaging can be performed.

[0029] 2. Compared with traditional metasurface polarization imaging methods, the polarization imaging system of the present invention avoids problems such as difficult alignment and incomplete imaging. It also improves the energy utilization efficiency of a single polarization from 1 / 4 to 1 / 3, an increase of 33%, which has a positive effect on imaging.

[0030] 3. The method for constructing a metasurface full-Stokes polarization microlens array proposed in the present invention is simple and easy to understand, easy to program and implement, has low processing cost, and can be implemented at any wavelength, such as visible light, near-infrared, and far-infrared. This has positive significance for the functional integration and miniaturization of optical systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Schematic diagram of a full-Stokes polarization microlens array of a metasurface used in an imaging system according to the present invention;

[0032] Figure 2 The responses of the micro-nano structures of the full-Stokes polarization microlens on the metasurface of the present invention to phase, amplitude and polarization conversion efficiency are shown.

[0033] Figure 3 A schematic diagram of generating a full-Stokes polarization microlens layout for the metasurface of the present invention;

[0034] Figure 4 This is a simulation effect diagram of a single microlens of the full-Stokes polarization microlens on the metasurface of the present invention;

[0035] Figure 5 This is a simulation effect diagram of the full-Stokes polarization microlens array on the metasurface of the present invention;

[0036] Figure 6 This is a flow chart of the method for designing a full-Stokes polarization microlens array based on a metasurface of the present invention;

[0037] In the figure: 1. Polychromatic light source, 2. Refractive lens, 3. Metasurface full-Stokes polarization microlens array, 4. Pixel plane, 5. Metasurface full-Stokes polarization microlens substrate, 6. Metasurface full-Stokes polarization microlens micro-nanostructure, 7. Detector pixel, 8. Focusing point. DETAILED DESCRIPTION

[0038] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0039] like Figure 1 The figure shows a three-dimensional schematic diagram of the metasurface full-Stokes polarization microlens array used in the imaging system of the present invention. The polarization imaging system includes two refractive lenses 2, a metasurface full-Stokes polarization microlens array 3, and a pixel plane 4. The metasurface full-Stokes polarization microlens array 3 is composed of a plurality of single microlens arrays. A single microlens is the abbreviation of a single metasurface full-Stokes polarization microlens. The structure of the single microlens is shown in the enlarged figure on the right, including a metasurface full-Stokes polarization microlens substrate 5 and a metasurface full-Stokes polarization microlens micro-nanostructure 6. The pixel plane 4 is composed of an array of four detector pixels 7. When the polychromatic light source 1 is incident on the refractive lens 2, it is focused and then propagates to the metasurface full-Stokes polarization microlens array 3. The metasurface full-Stokes polarization microlens array 3 splits the light of different polarization states and focuses them onto the pixel plane 4, performing real-time polarization imaging of the object.

[0040] A single microlens and four detector pixels 7 form a group. The single microlens splits the incident X-polarized light, Y-polarized light, 45° linear polarized light, and left-hand circularly polarized light LCP into four directions and focuses them to the upper left corner, upper right corner, lower left corner, and lower right corner of the four detector pixels 7, forming four focusing points 8 with different polarization states, namely I x , I y , I 45 , I LCP , thereby realizing polarization microlens array imaging based on the original imaging system.

[0041] The construction process of the metasurface full-Stokes polarization microlens array can be roughly divided into four parts, such as Figure 6As shown, it includes determining the geometric parameters of the micro-nano structure according to the detector pixel size, incident wavelength, and processing capability, performing FDTD simulation based on the geometric parameters, obtaining a linear polarization and circular polarization database, generating multiple polarization corresponding layouts, and then combining them into one layout, simulating the layout content, and then expanding it to the array, simulating again, and finally realizing the design.

[0042] The design of a metasurface full-Stokes polarization microlens array requires a clear understanding of the size of individual microlenses and their ability to detect all Stokes parameters. This invention uses a medium-wave infrared detector as an example to illustrate its specific implementation. Common medium-wave infrared detectors have a pixel size of 15 μm and a sensing band of 3 to 5 μm. Therefore, the size of a single metasurface full-Stokes polarization microlens is 30 × 30 μm, and the wavelength is selected to be 3.7 μm, a relatively common wavelength. To ensure the acquisition of all Stokes parameters, four polarization states are selected: X-linear polarization, Y-linear polarization, 45° linear polarization, and left-handed circular polarization (LCP). These four polarization states can be solved to obtain the full Stokes parameters S0, S1, S2, and S3. Based on the operating wavelength, individual microlens size, and processing capabilities, the metasurface micro-nanostructure arrangement period is determined to be 1.5 μm. The minimum linewidth of the structure is 0.3 μm, the height is 6 μm, and the F-number is determined to be 1.

[0043] Based on the above-mentioned micro-nanostructure geometric parameters, the finite-difference time-domain (FDTD) method is used to scan the database of the metasurface full-Stokes polarization microlens micro-nanostructure (hereinafter referred to as the micro-nanostructure). The shape of the micro-nanostructure is an anisotropic structure - elliptical. Of course, other shapes can be added to further expand the database range. It is necessary to scan the database for linear polarization and circular polarization to obtain the micro-nanostructure geometric parameters and the phase delay and transmittance they provide, such as Figure 2 As shown, for linear polarization, we focus on its phase and transmittance, and for circular polarization, we focus on its conversion efficiency, as shown in Figure 2 As shown in (a)(b)(c). A full layout of a single microlens is then designed for each polarization, and used for the entire format. Here, the birefringence characteristics of the anisotropic micro-nanostructure are used to reuse a pair of orthogonal polarizations, X-linear polarization and Y-linear polarization, in one micro-nanostructure, that is, one micro-nanostructure is used to simultaneously regulate X-linear polarization and Y-linear polarization. For 45° linear polarization, a single micro-nanostructure is used to match the corresponding X-linear polarization, and then the structure is rotated 45° counterclockwise. For LCP, the structural unit with the highest polarization conversion efficiency is selected, and the geometric phase principle is used to rotate the structure counterclockwise by half the required phase value. The micro-nanostructure phase value required for each position Determined by this formula:

[0044]

[0045] Where λ is the wavelength, x and y are the coordinates of the micro-nanostructure, x0 and y0 are the off-axis focal length offsets of different polarizations under a single microlens, and f is the focal length of the lens.

[0046] The phase values ​​of each position calculated according to the above focusing phase formula are as follows:

[0047]

[0048] The micro-nanostructure that minimizes δ is selected as the micro-nanostructure at this position, where δ represents the difference between the transmittance and phase response of the selected micro-nanostructure in the complex domain and the theoretical phase, t x represents the transmittance of the micro-nanostructure, represents the phase of the micro-nanostructure in the complex domain, Represents the complex domain phase value of the theoretical position. Match all positions, select the micro-nano structure with the smallest difference at each position, and finally determine the layout of each polarization. A total of three layouts are obtained, which are used to focus the X and Y linear polarization to the upper left and upper right corners, focus the 45° linear polarization to the lower left corner, and focus the LCP to the lower right corner. The difference between the layouts is that different layouts have different micro-nano structures at the same position. The three layouts are then combined into one layout, and the new layout takes 1 / 3 of the above three layouts, as shown in the schematic diagram. Figure 3 As shown, the specific operation method is:

[0049] 1. Determine how many structures a single microlens has. Let the number of structures be N, which is (30 / 1.5) here. 2 =400, use MATLAB to construct a row matrix, the first N / 3 is the value 1, the middle N / 3 is the value 2, and the last N / 3 is the value 3;

[0050] Second, use the randperm statement to randomly shuffle the row matrix, denoted as matrix M;

[0051] 3. According to the numbers in the matrix M, a different polarization layout is selected at each position. For example, a value of 1 selects the X, Y linear polarization layout, a value of 2 selects the LCP layout, and a value of 3 selects the 45° linear polarization layout. Finally, a new layout matrix is ​​created, which is recorded as Morigin.

[0052] 4. Then expand the Morigin array to obtain the final metasurface full-Stokes polarization microlens array layout.

[0053] Before expanding the Morigin array, first origin The constructed simulation model and simulation results are as follows: Figure 4As shown in the figure, we can see that the expected results are produced when different polarization states are incident. For example, for the incident Y polarization, no focus is produced at the design focus of X polarization, which is in line with the design expectation. origin The layout array will get the metasurface full Stokes polarization microlens array layout. Here we consider the computational complexity of the simulation of the metasurface full Stokes polarization microlens array layout. The simulation of the metasurface full Stokes polarization microlens array layout uses four single microlenses. The simulation effect is shown in the figure below. Figure 5 As shown in the figure, the designed metasurface full-Stokes polarization microlens array meets expectations. Of course, the method is not limited to this wavelength band and can be further improved according to needs, such as expanding to visible light and performing achromatic design.

[0054] The above are only preferred specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with this technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solutions and inventive concepts of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A polarization imaging system based on a metastructured microlens array, characterized in that: The invention comprises at least one refractive lens (2), a metasurface full-Stokes polarization microlens array (3), and a pixel plane (4), wherein the pixel plane (4) is composed of four detector pixel elements (7) arranged in an array; the metasurface full-Stokes polarization microlens array (3) is composed of a plurality of single microlens arrays, each single microlens is composed of a metasurface full-Stokes polarization microlens substrate (5) and a metasurface full-Stokes polarization microlens micro-nanostructure (6), and the metasurface full-Stokes polarization microlens micro-nanostructure (6) is regularly arranged on the metasurface full-Stokes polarization microlens substrate (5); When the polychromatic light source (1) is incident on the refractive lens (2), it is focused and then propagates to the metasurface full-Stokes polarization microlens array (3). The metasurface full-Stokes polarization microlens array (3) splits the light of different polarization states and focuses them onto the pixel plane (4), performing real-time polarization imaging of the object.

2. The polarization imaging system based on a meta-microlens array according to claim 1, characterized in that: Each single microlens forms a group with four detector pixels (7). Each single microlens splits polarized light of different polarization states and focuses the light onto the four detector pixels (7) in the same group. The four detector pixels (7) in the same group receive light of different polarizations.

3. A method for constructing a metasurface full-Stokes polarization microlens array in a polarization imaging system according to any one of claims 1 to 2, characterized in that: The metasurface full-Stokes polarization microlens array is composed of multiple metasurface full-Stokes polarization single microlens arrays. The method for constructing the metasurface full-Stokes polarization microlens array includes the following steps: Step 1: First, determine the geometric parameters of the micro-nanostructure on a single metasurface full-Stokes polarization microlens based on the detector pixel size, incident light wavelength, and processing capabilities; Step 2: Then, perform FDTD simulation based on the geometric parameters to obtain the linear polarization and circular polarization database of a single metasurface full-Stokes polarization microlens; Step 3: Generate multiple polarization corresponding layouts according to the database, and combine the multiple polarization corresponding layouts into a layout unit; Step 4: Then simulate the layout unit; Step 5: After the simulation is correct, the layout units are arrayed to obtain the metasurface full-Stokes polarization microlens array layout; Step 6: Simulate the metasurface full Stokes polarization microlens array layout obtained above to finally obtain the metasurface full Stokes polarization microlens array layout.

4. The method for constructing a metasurface full-Stokes polarization microlens array in a polarization imaging system according to claim 3, wherein: In step 3, the specific steps of combining multiple polarization corresponding layouts into one layout unit are: Step 7: Determine how many micro-nanostructures a single metasurface full-Stokes polarization microlens has. Let the number of micro-nanostructures be N. Use MATLAB software to construct a row matrix with the first N / 3 columns of the matrix having a value of 1, the middle N / 3 columns of the matrix having a value of 2, and the last N / 3 columns of the matrix having a value of 3. Step 8: Use the randperm statement to randomly shuffle the row matrix, denoted as matrix M; Step 9: According to the numbers in the matrix M, select different polarization layouts at each position. For example, a value of 1 selects the X-linear polarization layout and the Y-linear polarization layout, a value of 2 selects the LCP layout, and a value of 3 selects the 45° linear polarization layout. Finally, a new layout matrix is ​​created, which is recorded as Morigin. The specific implementation content of step 5 is: expand the Morigin array to obtain the final metasurface full-Stokes polarization microlens array layout.

5. The method for constructing a metasurface full-Stokes polarization microlens array in a polarization imaging system according to claim 4, characterized in that: In step 3, the theoretical phase value of the micro-nanostructure at each position Determined by formula 1, Where λ is the wavelength, x and y are the coordinates of the micro-nanostructure, x0 and y0 are the off-axis focal length offsets of different polarizations under a single metasurface full-Stokes polarization microlens, and f is the focal length of the lens; According to formula 2, the theoretical phase value obtained by formula 1 is used to search the database for a suitable micro-nano structure, and finally the micro-nano structure at this position is selected. The micro-nanostructure that minimizes δ is selected as the micro-nanostructure at this position, where δ represents the difference between the transmittance and phase response of the selected micro-nanostructure in the complex domain and the theoretical phase, t x represents the transmittance of the micro-nanostructure, represents the phase of the micro-nanostructure in the complex domain, Indicates the theoretical complex domain phase value at that position.

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