Polarization mode multiplexing information encryption method and system based on metasurface

Abstract

The invention belongs to the technical field of information optics, and discloses a polarization mode multiplexing information encryption method and system based on a metasurface. The method comprises the following steps: selecting a first number of linear polarization modes with mutually orthogonal spatial distribution; generating a second number of polarization channels based on the selected second number of incident and emergent polarization state combinations; equally distributing the first number of linear polarization modes to the second number of polarization channels; combining complex amplitude distributions of a plurality of linear polarization modes contained in the polarization channel and incident and emergent polarization states as coding information; mapping target intensity distribution corresponding to the linear polarization mode is used as decoding information; and determining structure parameters and arrangement of the metasurface based on the coding information and the decoding information, and realizing polarization mode multiplexing information encryption by using the metasurface. The transmission capacity and the safety performance of the information encryption optical system can be improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of information optics, and more specifically, relates to a polarization mode multiplexing information encryption method and system based on a metasurface. Background Art

[0002] Information encryption technology is one of the research hotspots in the field of information optics. In the field of information optics, multiple physical dimensions of light waves, including working wavelength, polarization state, and spatial mode, are widely used in the encoding and decoding of information channels, so as to achieve the purpose of information encryption and decryption.

[0003] However, the information encryption method derived from a single physical dimension of light waves has the disadvantages of low design freedom and low key complexity, resulting in limited transmission capacity and security performance of the system to be improved. Therefore, it cannot be effectively integrated into information encryption and data communication platforms. Currently, there is no information encryption method based on a metasurface to achieve polarization mode multiplexing. Summary of the Invention

[0004] The present invention provides a polarization mode multiplexing information encryption method and system based on a metasurface to solve the problems of low design freedom and low key complexity of information encryption devices in the prior art.

[0005] The present invention provides a polarization mode multiplexing information encryption method based on a metasurface, including the following steps:

[0006] Select a first number of linearly polarized modes with orthogonal spatial distributions; generate a second number of polarization channels based on a second number of selected incident and outgoing polarization state combinations; evenly distribute the first number of linearly polarized modes among the second number of polarization channels;

[0007] Take the complex amplitude distribution of multiple linearly polarized modes included in the polarization channel, as well as the incident and outgoing polarization state combinations, as encoding information; take the mapped target intensity distribution corresponding to the linearly polarized mode as decoding information;

[0008] Determine the structural parameters and arrangements of the metasurface based on the encoding information and the decoding information, and use the metasurface to achieve polarization mode multiplexing information encryption.

[0009] Preferably, the complex amplitude distribution of the linearly polarized mode is obtained in the following manner: based on the transmission characteristics and characteristic equations of the linearly polarized mode, divide the mode distribution grid and calculate the amplitude distribution of the linearly polarized mode; based on the distribution characteristics of the linearly polarized mode, superimpose the amplitude distribution of the linearly polarized mode with its corresponding phase distribution to obtain the complex amplitude distribution of the linearly polarized mode.

[0010] Preferably, different regions on the same far-field plane in the transmission space of the metasurface are selected as the mapping target regions for different polarization channels; different categories of characters are selected as the mapping target character categories for different polarization channels; for a certain polarization channel, different characters under the mapping target character category of this polarization channel are respectively selected as the mapping target characters for different linear polarization modes included in this polarization channel; the image intensity distribution of the mapping target characters of the linear polarization mode is used as the mapping target intensity distribution corresponding to the linear polarization mode.

[0011] Preferably, the metasurface is composed of a plurality of periodically arranged nano-unit structures, and each nano-unit structure includes a substrate and nano-bricks arranged on the working surface of the substrate; the sizes of the working surfaces of the substrates are the same, and the heights of all the nano-bricks included in the metasurface are the same; the materials of the substrate and the nano-bricks are determined according to the working wavelength and the target amplitude transmittance.

[0012] Preferably, the length and width of the nano-bricks are scanned, the amplitude transmittance and phase modulation amount of the nano-bricks under different sizes are calculated, and a corresponding data set is obtained; according to the target phase modulation amount and the target amplitude transmittance, multiple groups of nano-bricks with different lengths and / or widths are selected from the data set as alternative nano-bricks.

[0013] Preferably, determining the structural parameters and arrangement of the metasurface includes determining the length, width and rotation angle of the nano-bricks in each nano-unit structure in the metasurface;

[0014] The complex amplitude distribution of the linear polarization mode in the encoded information is used as the input information of the working channel on the metasurface, and the mapping target intensity distribution corresponding to the linear polarization mode in the decoded information is used as the output information of the working channel on the metasurface;

[0015] Clarify the physical relationship formula between the input information and the output information under each combination of incident and outgoing polarization states; use the gradient descent algorithm to iteratively optimize the amplitude and phase modulation distribution of the metasurface to obtain the phase modulation distribution of the metasurface, and obtain the rotation angle arrangement of a plurality of nano-bricks included in the metasurface; quantize the phase modulation distribution of the metasurface into multi-order composite phase modulation, and select the nano-bricks to be arranged from the alternative nano-bricks to complete the construction of the metasurface.

[0016] Preferably, the selected combinations of incident and outgoing polarization states include the following three combinations: x-polarized incidence / x-polarized outgoing, x-polarized incidence / y-polarized outgoing, y-polarized incidence / y-polarized outgoing;

[0017] The physical relationship formula between the input information and the output information in the x-polarized incidence / x-polarized outgoing channel is:

[0018]

[0019] The physical relationship between the input information and the output information in the x-polarized incident / y-polarized output channel is as follows:

[0020]

[0021] The physical relationship between the input information and the output information in the y-polarized incident / y-polarized output channel is as follows:

[0022]

[0023] Wherein, OUT represents the output information, In represents the input information, FFT represents the Fourier transform, i represents the imaginary unit, θ represents the rotation angle of the nanobrick, P X represents the phase modulation distribution of the metasurface in the X-axis direction, and P Y represents the phase modulation distribution of the metasurface in the Y-axis direction.

[0024] Preferably, under different combinations of incident and output polarization states, different linearly polarized modes are incident on the metasurface. After passing through the metasurface, the mapped target character categories of each incident and output polarization state combination and the mapped target characters of each linearly polarized mode are respectively displayed in different regions on the same far-field plane in the transmission space of the metasurface.

[0025] On the other hand, the present invention provides a polarization mode multiplexing information encryption system based on a metasurface, including a laser, a multimode optical fiber, an incident adjustment component, a metasurface, an output adjustment component, and a detection screen arranged in sequence along the optical path;

[0026] The laser is used to provide energy for the multimode optical fiber; the multimode optical fiber supports the parallel and independent transmission of a first number of linearly polarized modes; the incident adjustment component is used to adjust the polarization state of the incident optical information; the output adjustment component is used to filter the polarization state of the output optical information; the detection screen is used to receive and display information;

[0027] The polarization mode multiplexing information encryption system based on a metasurface is used to execute the steps in the above-mentioned polarization mode multiplexing information encryption method based on a metasurface.

[0028] Preferably, the incident adjustment component includes a first polarizer and a first half-wave plate arranged in sequence along the optical path, the output adjustment component includes a second half-wave plate and a second polarizer arranged in sequence along the optical path, a collimating lens is further arranged between the multimode optical fiber and the incident adjustment component, and the laser uses an infrared laser.

[0029] One or more technical solutions provided in the present invention have at least the following technical effects or advantages:

[0030] The present invention selects a first number of linearly polarized modes that are mutually orthogonal in spatial distribution; generates a second number of polarization channels based on a selected second number of combinations of incident and outgoing polarization states; evenly distributes the first number of linearly polarized modes among the second number of polarization channels; takes the complex amplitude distributions of the multiple linearly polarized modes included in the polarization channels, as well as the combinations of incident and outgoing polarization states, as encoded information; takes the mapped target intensity distribution corresponding to the linearly polarized mode as decoded information; determines the structural parameters and arrangements of the metasurface based on the encoded information and the decoded information, and uses the metasurface to achieve polarization mode multiplexing information encryption. That is, the present invention uses a single-chip metasurface device, adopts a multiplexing method that combines two dimensions of spatial mode and polarization state, uses the first number of spatial modes (i.e., linearly polarized modes) and the second number of combinations of incident and outgoing polarization states as encoding channels, and displays the mapped target intensity distributions corresponding to different linearly polarized modes (for example, displays the mapped target characters of each spatial mode) in different regions on the same far-field plane in the transmission space of the metasurface, thereby realizing the function of information encryption.

[0031] The present invention utilizes the polarization-sensitive amplitude-phase modulation and polarization-stable high-efficiency amplitude transmission of the metasurface in the orthogonal direction to achieve amplitude-phase modulation and high-efficiency holographic display of multiple working channels, thereby realizing information encryption with dual-dimensional multiplexing of polarization and mode, and expanding the design freedom of information encryption optical devices. The present invention constructs multi-parameter encoded ciphertexts and various types of decoded plaintexts for multiple working channels by using multiple combinations of incident and outgoing polarization states and multiple sets of linearly polarized modes, thereby realizing information encryption with dual-dimensional multiplexing of polarization and mode, and increasing the key complexity of information encryption optical devices. Moreover, the present invention utilizes the nano-scale ultra-compact structure and multi-dimensional high-integration architecture of the metasurface, combined with the mature metasurface processing technology, to improve the transmission capacity and security performance of the information encryption optical system, and has potential research value and broad application prospects in the fields of information access, processing, display, encryption, and decryption. In summary, the present invention solves the problems of low design freedom and low key complexity of information encryption devices in the prior art, and improves the transmission capacity and security performance of the information encryption system. Description of the Drawings

[0032] Figure 1 Schematic diagram of the geometric structure of the metasurface in a polarization mode multiplexing information encryption method based on a metasurface provided in Embodiment 1 of the present invention;

[0033] Figure 2 Schematic diagram of the geometric structure of the nano-unit structure in a polarization mode multiplexing information encryption method based on a metasurface provided in Embodiment 1 of the present invention;

[0034] Figure 3The amplitude transmittance and phase modulation amount of the nano - unit structure in a polarization - mode - multiplexing information encryption method based on a metasurface provided in Embodiment 1 of the present invention;

[0035] Figure 4 It is the complex - amplitude distribution diagram of the incident modes on each working channel in a polarization - mode - multiplexing information encryption method based on a metasurface provided in Embodiment 1 of the present invention;

[0036] Figure 5 It is the intensity distribution diagram of the outgoing characters on each working channel in a polarization - mode - multiplexing information encryption method based on a metasurface provided in Embodiment 1 of the present invention;

[0037] Figure 6 It is the amplitude and phase modulation distribution diagram of the metasurface obtained by algorithm optimization in a polarization - mode - multiplexing information encryption method based on a metasurface provided in Embodiment 1 of the present invention;

[0038] Figure 7 It is the architecture diagram of a polarization - mode - multiplexing information encryption system based on a metasurface provided in Embodiment 2 of the present invention;

[0039] Figure 8 It is the working schematic diagram of a polarization - mode - multiplexing information encryption system based on a metasurface provided in Embodiment 2 of the present invention.

[0040] Among them, 1 - laser, 2 - multimode optical fiber, 3 - collimating lens, 4 - first polarizer, 5 - first half - wave plate, 6 - metasurface, 7 - second half - wave plate, 8 - second polarizer, 9 - detection screen. Detailed implementation manners

[0041] In order to better understand the above - mentioned technical solutions, the above - mentioned technical solutions will be described in detail below in conjunction with the accompanying drawings of the specification and specific implementation manners.

[0042] Embodiment 1:

[0043] Embodiment 1 provides a polarization - mode - multiplexing information encryption method based on a metasurface, mainly including the following steps:

[0044] Select a first number of linearly polarized modes with orthogonal spatial distributions; generate a second number of polarization channels based on a second number of selected incident and outgoing polarization state combinations; evenly distribute the first number of linearly polarized modes among the second number of polarization channels;

[0045] Take the complex - amplitude distribution of the multiple linearly polarized modes included in the polarization channel and the incident and outgoing polarization state combinations as encoding information; take the mapped target intensity distribution corresponding to the linearly polarized mode as decoding information;

[0046] Determine the structural parameters and arrangement of the metasurface based on the encoding information and the decoding information, and use the metasurface to implement polarization mode multiplexing information encryption.

[0047] For example, select different regions on the same far-field plane in the transmission space of the metasurface as the mapping target regions for different polarization channels; select different categories of characters as the mapping target character categories for different polarization channels; for a certain polarization channel, select different characters under the mapping target character category of this polarization channel as the mapping target characters for different linear polarization modes included in this polarization channel; use the image intensity distribution of the mapping target characters of the linear polarization mode as the mapping target intensity distribution corresponding to the linear polarization mode.

[0048] The following is a specific description of the present invention.

[0049] A polarization mode multiplexing information encryption method based on a metasurface provided by the present invention mainly includes the following aspects.

[0050] (1) Construct the basic structure of the metasurface.

[0051] See Figure 1 , the metasurface is composed of several periodically arranged nano-unit structures, and each nano-unit structure includes a substrate and nano-bricks arranged on the working surface of the substrate; the sizes of the working surfaces of the substrates are the same, and the heights of all the nano-bricks included in the metasurface are the same.

[0052] Determine the materials of the substrate and the nano-bricks according to the working wavelength and the target amplitude transmittance of the spatial mode.

[0053] Optimize the material and geometric size parameters of the nano-unit structure so that the metasurface has polarization-sensitive phase modulation and polarization-stable amplitude transmission characteristics for the input optical field.

[0054] Specifically, the geometric structure of the nano-unit is as Figure 2 shown. The substrates are all periodically arranged square units, and the nano-bricks are cubes located on the working surfaces of the substrates. A coordinate system XOY is established on the working surface of the substrate, and the X-axis and the Y-axis are respectively parallel to two groups of sides of the working surface of the substrate. The long side direction of the nano-brick represents the long axis direction, that is, the X-axis direction; the short side direction of the nano-brick represents the short axis direction, that is, the Y-axis direction.

[0055] The geometric size parameters of the nano-unit structure include the period CS of the nano-brick (i.e., the size of the working surface of the substrate), the length L of the nano-brick X , the width L Y and the height L Z, the geometric angle parameter includes the rotation angle θ of the nanobrick; all the nanobricks in the metasurface have the same period CS and height L Z , by optimizing the length L of the nanobrick X and width L Y , the anisotropy of the nano-unit structure is adjusted to achieve fourth-order composite phase modulation in the X and Y axis directions, while ensuring the same order of magnitude transmittance in the X and Y axis directions, so as to enable the metasurface to have polarization sensitivity to phase modulation and polarization stability of amplitude transmittance in two orthogonal directions of the X and Y axes.

[0056] The following is an example of this part in combination with the parameters.

[0057] The working wavelength of the spatial mode is 1550 nm. Based on this working wavelength, the working wavelength λ of the metasurface is 1550 nm. Therefore, both the substrate and the nanobrick of the nano-unit structure on the metasurface are made of crystalline silicon material, so that the metasurface has a high transmittance for the incident mode of this wavelength, thereby reducing the energy loss of the metasurface device and improving the display efficiency of the metasurface information encryption platform.

[0058] Use the electromagnetic simulation software COMSOL Multiphysics to construct a nano-unit structure model, and test the period CS, length L X 、width L Y and height L Z of the nanobrick, and determine that the period CS of the nanobrick is 900 nm and the height L Z = 1000 nm.

[0059] Scan the length L X and width L Y of the nanobrick in the range from 200 nm to 800 nm at a sampling interval of 10 nm, and calculate the amplitude transmittance and phase modulation amount of nanobricks of each size to obtain the corresponding data set, see Figure 3 . According to the characteristic requirements of the metasurface, that is, its polarization sensitivity to phase modulation and polarization stability of amplitude transmittance, 16 groups of geometric parameters are selected from the data set to obtain 16 sizes of nanobricks, thereby constructing 16 nano-unit structures, see Table 1.

[0060] Table 1 Geometric parameters of 16 nano-unit structures

[0061]

[0062] Among them, X-P and X-T are the phase modulation amount and amplitude transmittance of the nano-unit structure for X-polarized light, and Y-P and Y-T are the phase modulation amount and amplitude transmittance of the nano-unit structure for Y-polarized light.

[0063] These 16 types of nano-bricks or 16 types of nano-unit structures can achieve fourth-order composite phase modulation in the X and Y axis directions, that is, the fourth-order phase modulation amounts of 0, π / 2, π, and 3π / 2. At the same time, the fixed amplitude transmittances are all between 50% and 60%, enabling the metasurface based on this structure to achieve the fourth-order phase modulation amounts for each polarization mode, while ensuring the same order of magnitude of amplitude transmittance for each polarization mode.

[0064] (2) Obtain the encoded information.

[0065] Select a number of linearly polarized modes that are mutually orthogonal in spatial distribution, which can be transmitted in parallel and independently in space. Calculate the complex amplitude distribution of each linearly polarized mode, and combine a specific incident and outgoing polarization state combination as the encoded information for each working channel on the metasurface (i.e., the polarization mode multiplexing information encryption platform based on the metasurface).

[0066] For example, the following method is used to calculate the complex amplitude distribution of the incident mode on each working channel: Based on the transmission characteristics and characteristic equations of each linearly polarized mode, divide the mode distribution grid and calculate the amplitude distribution In of each mode. amp ; Based on the distribution characteristics of each linearly polarized mode, superimpose the amplitude distribution of each mode with its corresponding phase distribution In phi to generate the complex amplitude distribution In of each mode.

[0067] Based on a specific incident and outgoing polarization state combination, evenly distribute each mode working channel to each polarization working channel to determine the complex amplitude distribution In of the incident mode on each working channel.

[0068] The following is an example to illustrate this part in combination with parameters.

[0069] Select a multimode fiber that supports the parallel and independent transmission of 12 linearly polarized modes (LP 11a mode, LP 11b mode, LP 21a mode, LP 21b mode, LP 31a mode, LP 31b mode, LP 41a mode, LP 41b mode, LP 51a mode, LP 51b mode, LP 61a mode and LP 61b mode). The working wavelength λ of the multimode fiber is 1550 nm, the core diameter d is 62.5 μm, the numerical aperture NA is 0.275, and its normalized frequency V is calculated to be 34.8362, meeting the requirements for including the normalized cut-off frequencies of the 12 modes.

[0070] The transmission characteristics of the linear polarization mode satisfy the following relational expressions:

[0071] V 2 = U 2 + W 2

[0072] where V is the normalized cut-off frequency of each mode in the optical fiber, U is the normalized radial constant of each mode, and W is the attenuation constant of each mode.

[0073] The characteristic equation of the linear polarization mode is:

[0074]

[0075] where l is the logarithm of the maximum value of the amplitude distribution of each mode in the circumferential direction, and is also the number of times of the periodic transformation of the phase distribution, reflecting the pattern of the mode distribution; U and W are the normalized cut-off frequency and attenuation constant of each mode respectively, J l (U) is the Bessel function of the first kind of order l of U, J l-1 (U) is the Bessel function of the first kind of order l - 1 of U, K l (W) is the modified Bessel function of order l of W, K l-1 (W) is the modified Bessel function of order l - 1 of W.

[0076] Based on the above transmission characteristic parameters and characteristic equation, calculate the amplitude distribution In of each mode amp ; based on the above distribution characteristic parameters, perform the superposition of the phase distribution In amp of the amplitude distribution In phi to obtain the complex amplitude distribution In of each transmission mode.

[0077] Based on specific combinations of 3 incident and outgoing polarization states (x-polarized incident / x-polarized outgoing, x-polarized incident / y-polarized outgoing, and y-polarized incident / y-polarized outgoing), generate 3 polarization working channels; evenly distribute 12 modes among the 3 polarization working channels, so each polarization working channel contains 4 mode working channels. Among them, the x-polarized incident / x-polarized outgoing channel contains LP 11a mode, LP 11b mode, LP 41a mode, and LP 41b mode, the x-polarized incident / y-polarized outgoing channel contains LP 21a mode, LP 21b mode, LP 51a mode, and LP 51b mode, the y-polarized incident / y-polarized outgoing channel contains LP 31a mode, LP 31b mode, LP 61a mode, and LP61b Mode. Due to the orthogonality of the distribution of each mode, the metasurface can achieve parallel modulation of 4 modes in each polarization channel. Therefore, the complex amplitude distribution In of the incident mode on each working channel is determined. The amplitude and phase distributions of the 12 modes are as Figure 4 shown.

[0078] (3) Obtain the decoded information.

[0079] Select different regions on the same far-field plane in the transmission space of the metasurface as the mapping target regions for each polarization channel; select several categories of characters as the mapping target character categories for each polarization channel; select several characters under each character category as the mapping target characters for each mode channel, and generate the image intensity distribution of each character, which is the decoded information of each working channel on the metasurface.

[0080] For example, the following method is used to generate the intensity distribution of the outgoing characters on each working channel: Based on the area of the mode distribution and the period of the nano-unit structure, determine the size of the far-field detection plane in the transmission space of the metasurface, denoted as M×N pixels; based on the size of the far-field detection plane, determine the size of the target region corresponding to each polarization state combination, denoted as m×n pixels, and its position is evenly distributed in a triangular shape on the detection plane; select the target character class corresponding to each polarization state combination as the mapping target character class for each polarization channel, select several characters under each character category as the mapping target characters for each mode channel, generate the image intensity distribution of each character, and determine the intensity distribution Out of the outgoing characters on each working channel.

[0081] The following is an example to illustrate this part in combination with the parameters.

[0082] Based on the area of the mode distribution S = 900μm 2 and the period CS of the nano-unit structure = 900nm, determine the size parameters M = 1000, N = 1000 of the far-field detection plane in the transmission space of the metasurface, that is, the size of the detection plane is 1000×1000 pixels.

[0083] For the three polarization channels of x-polarized incidence / x-polarized outgoing, x-polarized incidence / y-polarized outgoing, and y-polarized incidence / y-polarized outgoing, select the size parameters m = 500, n = 500 of their corresponding target regions, that is, the size of the target region is 500×500 pixels, and their positions are located at the lower left, middle upper, and lower right of the detection plane respectively, and are evenly distributed in a triangular shape on the detection plane.

[0084] Select three character classes of English letters, Arabic numerals, and Greek letters, which are the mapped target character classes for three polarization channels of x-polarized incidence / x-polarized output, x-polarized incidence / y-polarized output, and y-polarized incidence / y-polarized output, respectively. In the x-polarized incidence / x-polarized output channel, select A, B, C, and D in English letters as the mapped target characters for the four mode channels of LP 11a mode, LP 11b mode, LP 41a mode, and LP 41b mode of the four mode channels; in the x-polarized incidence / y-polarized output channel, select 1, 2, 3, and 4 of Arabic numerals as the mapped target characters for the four mode channels of LP 21a mode, LP 21b mode, LP 51a mode, and LP 51b mode of the four mode channels; in the y-polarized incidence / y-polarized output channel, select α, β, γ, and δ of Greek letters as the mapped target characters for LP 31a mode, LP 31b mode, LP 61a mode, and LP 61b mode of the mapped target characters. The intensity distribution of the images of the 12 characters is as Figure 5 shown, so as to determine the intensity distribution Out of the outgoing characters on 12 working channels.

[0085] (4) Determine the structural parameters and arrangement of the metasurface to obtain the final metasurface.

[0086] Based on the above encoding information and decoding information, optimize the amplitude and phase modulation distributions of the nano-unit structures on the metasurface, determine the geometric angles and size parameters of the nano-unit structures, and arrange the nano-unit structures in an orderly manner to obtain the final metasurface.

[0087] Specifically, the following method can be used to optimize the amplitude and phase modulation distributions of the nano-unit structures on the metasurface:

[0088] Based on the encoding information, that is, the complex amplitude distributions of each mode and the specific combination of incident and outgoing polarization states, determine its mode complex amplitude distribution as the input information of each working channel on the metasurface optimization architecture.

[0089] Based on the decoding information, that is, the intensity distribution of the images of the mapped target characters of each mode, determine its character intensity distribution as the output information of each working channel on the metasurface optimization architecture.

[0090] Based on the metasurface amplitude and phase modulation principle, using the degrees of freedom of the geometric angles and size changes of the nano-bricks in the nano-unit structure, clarify the physical relationship formula between the input and output information of each working channel.

[0091] Iteratively optimize the amplitude and phase modulation distributions of the metasurface using the gradient descent algorithm to obtain the phase modulation distribution P in the X-axis direction of the metasurface X , the phase modulation distribution P in the Y-axis direction Y and the rotation angle distribution θ of the nanobricks.

[0092] Based on the phase modulation distribution P in the X-axis direction of the metasurface X and the phase modulation distribution P in the Y-axis direction Y , quantize it into a fourth-order composite phase modulation in the X and Y axis directions, select the corresponding-sized nano-unit structures, combine with the rotation angle distribution θ of the nanobricks, generate nano-unit structures with determined geometric sizes and rotation angles, arrange them orderly, and construct the final metasurface.

[0093] The following further explains this part in combination with the parameters.

[0094] Based on the encoded information, that is, the complex amplitude distributions of 12 specific patterns and 3 combinations of incident and outgoing polarization states, determine that the complex amplitude distributions of the 12 patterns are the input information for 12 working channels on the metasurface optimization architecture.

[0095] Based on the decoded information, that is, the image intensity distributions of the 12 patterns mapping target characters, determine that its character intensity distribution is the output information for 12 working channels on the metasurface optimization architecture.

[0096] Based on the metasurface amplitude and phase modulation principle, utilize the degrees of freedom of the geometric angles and sizes of the nanobricks in the nano-unit structure to clarify the physical relationship formulas between the input and output information of the 3 polarization channels.

[0097] Among them, the physical relationship formula between the input and output information of the x-polarized incident / x-polarized outgoing channel is:

[0098]

[0099] The physical relationship formula between the input and output information in the x-polarized incident / y-polarized outgoing channel is:

[0100]

[0101] The physical relationship formula between the input and output information in the y-polarized incident / y-polarized outgoing channel is:

[0102]

[0103] Among them, OUT represents the output information, In represents the input information, FFT represents the Fourier transform, i represents the imaginary unit, θ represents the rotation angle of the nanobricks, P X represents the phase modulation distribution in the X-axis direction of the metasurface, PY Represents the phase modulation distribution of the metasurface in the Y-axis direction.

[0104] Using the gradient descent algorithm to iteratively optimize the amplitude and phase modulation distributions of the metasurface, the phase modulation distribution P of the metasurface in the X-axis direction is obtained. X The phase modulation distribution P in the Y-axis direction Y And the rotation angle distribution θ of the nanobricks, as Figure 6 shown.

[0105] Based on the phase modulation distribution P of the metasurface in the X-axis direction X And the phase modulation distribution P in the Y-axis direction Y , it is quantized into 4th-order composite phase modulation in the X and Y axis directions. Select the nanobricks with corresponding sizes from the above 16 types of nano-unit structures, combine with the rotation angle distribution θ of the nanobricks, generate nano-unit structures with determined geometric sizes and rotation angles, arrange them in an orderly manner, and construct the final metasurface.

[0106] (5) Implement the information encryption function based on the metasurface.

[0107] Under different combinations of incident and outgoing polarization states, different spatial modes are incident on the metasurface. After passing through the metasurface, the mapped target character categories of each polarization combination and the mapped target characters of each spatial mode are respectively displayed in specific regions on the same far-field plane in the transmission space.

[0108] Correspondingly, the present invention can also specifically build a mode multiplexing information encryption system based on the metasurface to implement the information encryption function, which will be described below with Embodiment 2.

[0109] Embodiment 2:

[0110] Embodiment 2 provides a polarization mode multiplexing information encryption system based on the metasurface. Refer to Figure 7 , which mainly includes a laser 1, a multimode fiber 2, an incident adjustment component, a metasurface 6, an outgoing adjustment component, and a detection screen 9 arranged in sequence along the optical path.

[0111] For example, the incident adjustment component includes a first polarizer 4 and a first half-wave plate 5 arranged in sequence along the optical path, and the outgoing adjustment component includes a second half-wave plate 7 and a second polarizer 8 arranged in sequence along the optical path. In addition, a collimating lens 3 can be arranged between the multimode fiber 2 and the incident adjustment component, and the laser 1 can adopt an infrared laser.

[0112] Specifically, at the incident end of the metasurface 6, the laser 1 provides energy for the multimode fiber 2. The multimode fiber 2 supports the parallel and independent transmission of each mode. The collimating lens 3 collimates each mode distribution. The combination of the first polarizer 4 and the first half-wave plate 5 adjusts the polarization state of each incident mode distribution. The mode distribution with a specific polarization state is incident on the metasurface 6. At the exit end of the metasurface 6, the combination of the second half-wave plate 7 and the second polarizer 8 filters the polarization state of each outgoing character distribution. The character distribution with a specific polarization state is received by the infrared-sensitive detection screen 9.

[0113] That is, under a specific combination of incident and outgoing polarization states, a specific spatial mode is incident on the metasurface 6. After passing through the metasurface 6, the mapped target character categories with specific polarization combinations and the mapped target characters of specific spatial modes are respectively displayed in specific regions on the same far-field plane in the transmission space.

[0114] For example, referring to Figure 8 , under the condition of x-polarized incidence / x-polarized exit, the LP 11a mode, LP 11b mode, LP 41a mode, and LP 41b mode are incident on the metasurface. After passing through the metasurface, the English letters A, B, C, and D are respectively displayed in the lower left region on the same far-field plane in the transmission space. Under the condition of x-polarized incidence / y-polarized exit, the LP 21a mode, LP 21b mode, LP 51b mode, and LP 51b mode are incident on the metasurface. After passing through the metasurface, the Arabic numerals 1, 2, 3, and 4 are respectively displayed in the middle upper region on the same far-field plane in the transmission space. Under the condition of y-polarized incidence / y-polarized exit, the LP 31a mode, LP 31b mode, LP 61b mode, and LP 61b mode are incident on the metasurface. After passing through the metasurface, the Greek letters α, β, γ, and δ are respectively displayed in the lower right region on the same far-field plane in the transmission space.

[0115] In summary, the present invention proposes a polarization mode multiplexing information encryption method based on a metasurface. Using a single metasurface device, a multiplexing method combining two dimensions of spatial mode and polarization state is adopted. Multiple spatial modes and multiple combinations of incident and outgoing polarization states (for example, twelve spatial modes and three combinations of incident and outgoing polarization states) are used as encoding channels. The mapped target character categories with each polarization combination and the mapped target characters of each spatial mode are displayed in specific regions on the same far-field plane in the transmission space, thereby realizing the function of information encryption.

[0116] Based on the highly sensitive characteristics of metasurface devices in the polarization dimension, the precise manipulation ability of light wave characteristics, and the flexible design method in the holographic field, the present invention proposes a new information encryption method that can achieve dual multiplexing of polarization and mode. By utilizing the dual modulation function of metasurface amplitude and phase and the Fourier far-field holographic principle, a polarization-mode multiplexing information encryption system based on metasurface is built, which can realize the modulation of spatial modes of multiple polarization working channels and the display of target characters, thereby enabling the key design and information encryption of multiple polarization working channels.

[0117] Based on the polarization-sensitive characteristics, light wave modulation function, and nanoscale structure of metasurface devices, the present invention proposes a new idea for information encryption that can achieve polarization-mode multiplexing, expands the design freedom of information encryption optical devices, increases the key complexity of information encryption optical devices, improves the transmission capacity and security performance of information encryption optical systems, and has potential research value and broad application prospects in the fields of information access, processing, display, encryption, and decryption.

[0118] Finally, it should be noted that the above specific implementation manners are only used to illustrate the technical solutions of the present invention rather than to limit them. Although the present invention has been described in detail with reference to the examples, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A polarization mode multiplexing information encryption method based on a metasurface, characterized in that: The following steps are involved: Selecting a first number of linear polarization modes whose spatial distributions are mutually orthogonal; generating a second number of polarization channels based on a second number of selected incident and output polarization state combinations; and equally distributing the first number of linear polarization modes to the second number of polarization channels; The complex amplitude distribution of multiple linear polarization modes contained in the polarization channel and the incident and output polarization states are combined as encoding information; the mapping target intensity distribution corresponding to the linear polarization mode is used as decoding information; The structural parameters and arrangement of the metasurface are determined based on the encoding information and the decoding information, and polarization mode multiplexing information encryption is implemented using the metasurface.

2. The polarization mode multiplexing information encryption method based on metasurface according to claim 1 is characterized in that: The complex amplitude distribution of the linear polarization mode is obtained in the following manner: based on the transmission characteristics and characteristic equation of the linear polarization mode, the mode distribution grid is divided and the amplitude distribution of the linear polarization mode is calculated; based on the distribution characteristics of the linear polarization mode, the amplitude distribution of the linear polarization mode is superimposed on its corresponding phase distribution to obtain the complex amplitude distribution of the linear polarization mode.

3. The polarization mode multiplexing information encryption method based on metasurface according to claim 1 is characterized in that: Different areas on the same far-field plane in the transmission space of the metasurface are selected as mapping target areas for different polarization channels; different categories of characters are selected as mapping target character categories for different polarization channels; for a certain polarization channel, different characters under the mapping target character category of the polarization channel are selected as mapping target characters for different linear polarization modes contained in the polarization channel; and the image intensity distribution of the mapping target characters of the linear polarization mode is used as the mapping target intensity distribution corresponding to the linear polarization mode.

4. The polarization mode multiplexing information encryption method based on metasurface according to claim 1 is characterized in that: The metasurface is composed of a number of periodically arranged nanounit structures, each of which includes a substrate and a nanobrick arranged on the working surface of the substrate; the working surfaces of the substrates have the same size, and the heights of all the nanobricks included in the metasurface are the same; the materials of the substrate and the nanobricks are determined according to the working wavelength and the target amplitude transmittance.

5. The polarization mode multiplexing information encryption method based on metasurface according to claim 4 is characterized in that: The length and width of the nanobrick are scanned, and the amplitude transmittance and phase modulation of the nanobricks at different sizes are calculated to obtain a corresponding data set; based on the target phase modulation and target amplitude transmittance, multiple groups of nanobricks with different lengths and / or widths are screened out from the data set as candidate nanobricks.

6. The polarization mode multiplexing information encryption method based on metasurface according to claim 5 is characterized in that: Determining the structural parameters and arrangement of the metasurface includes determining the length, width and rotation angle of the nanobricks in each nanounit structure in the metasurface; Using the complex amplitude distribution of the linear polarization mode in the encoded information as input information of the working channel on the metasurface, and using the mapping target intensity distribution corresponding to the linear polarization mode in the decoded information as output information of the working channel on the metasurface; The physical relationship between input information and output information under each combination of incident and output polarization states is clarified; the amplitude and phase modulation distribution of the metasurface is iteratively optimized using a gradient descent algorithm to obtain the phase modulation distribution of the metasurface and the angular arrangement of several nanobricks contained in the metasurface; the phase modulation distribution of the metasurface is quantified into multi-order composite phase modulation, and nanobricks for arrangement are selected from the candidate nanobricks to complete the construction of the metasurface.

7. The polarization mode multiplexing information encryption method based on metasurface according to claim 6 is characterized in that: The selected incident and output polarization state combinations include the following three combinations: x-polarization incident / x-polarization output, x-polarization incident / y-polarization output, y-polarization incident / y-polarization output; The physical relationship between the input information and output information of the x-polarized incident / x-polarized output channel is: The physical relationship between the input information and the output information in the x-polarized incident / y-polarized output channel is: The physical relationship between the input information and the output information in the y-polarized incident / y-polarized exit channel is: Where OUT represents output information, In represents input information, FFT represents Fourier transform, i represents imaginary unit, θ represents the rotation angle of nanobrick, P X represents the phase modulation distribution of the metasurface in the X-axis direction, P Y Represents the phase modulation distribution of the metasurface in the Y-axis direction.

8. The polarization mode multiplexing information encryption method based on metasurface according to claim 3 is characterized in that: Under different combinations of incident and outgoing polarization states, different linear polarization modes are incident on the metasurface. After passing through the metasurface, the mapped target character categories of each combination of incident and outgoing polarization states and the mapped target characters of each linear polarization mode are displayed in different areas on the same far-field plane in the transmission space of the metasurface.

9. A polarization mode multiplexing information encryption system based on a metasurface, characterized in that: It includes a laser, a multimode optical fiber, an incident adjustment component, a metasurface, an exit adjustment component and a detection screen which are sequentially arranged along an optical path; The laser is used to provide energy to the multimode optical fiber; The multimode optical fiber supports parallel and independent transmission of a first number of linear polarization modes; the incident adjustment component is used to adjust the polarization state of the incident light information; the output adjustment component is used to filter the polarization state of the output light information; the detection screen is used to receive display information; The metasurface-based polarization mode multiplexing information encryption system is used to execute the steps in the metasurface-based polarization mode multiplexing information encryption method as described in any one of claims 1-8.

10. The polarization mode multiplexing information encryption system based on metasurface according to claim 9, characterized in that: The incident adjustment component includes a first polarizer and a first half-wave plate arranged in sequence along the optical path, the output adjustment component includes a second half-wave plate and a second polarizer arranged in sequence along the optical path, a collimating lens is also arranged between the multimode optical fiber and the incident adjustment component, and the laser is an infrared laser.

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