A multi-image encryption method based on metasurface, a metasurface and its design method

Abstract

The present invention belongs to the field of optical technologies, and discloses a multi-image encryption method based on a metasurface, a metasurface and a design method thereof. The present invention utilizes the metasurface to present a near-field nano-printed image with three-channel independent coding and a far-field vector holographic image with three-channel independent coding; by overlapping multiple beams and introducing phase-difference interference, the metasurface additionally presents an interference-assisted vector encrypted image with six channels including an additional far-field vector holographic image with four channels and an additional near-field nano-printed image with two channels, so that the metasurface can present a twelve-channel optical image. The present invention extends encryption to twelve-channel near / far-field optical images, expands the capacity of vector encryption, and has high hiding security.

Description

Technical Field

[0001] The present invention belongs to the field of optical technologies, and more specifically, relates to a multi-image encryption method based on a metasurface, a metasurface, and a design method thereof. Background Art

[0002] Vector holography based on multi-parameter control of the Jones matrix has been proposed and widely studied to increase the modulation space and information capacity. Conventional holography only controls the amplitude part of the Jones matrix parameters for holographic intensity construction, and the phase and polarization parts are not constrained. In contrast, vector holography introduces additional degrees of freedom for optical encoding by manipulating the phase or polarization part in the Jones matrix. For a single-layer metasurface, independent amplitude and phase encoding is basically achieved by only three of the four components in the Jones matrix. This is because the two-dimensional plane with mirror symmetry restricts the two non-diagonal components in the Jones matrix to be the same. Therefore, the programmable degrees of freedom of the Jones matrix of a single-layer metasurface are limited to 6, and by controlling different input / output polarizations, it is encrypted to the limit of three-channel holography and three-channel nano-printing. However, in the face of the limit of the basic degrees of freedom, it is difficult for vector holography to further exceed the state-of-the-art information channels within a single-layer metasurface. For example, most previous vector holographic encryptions can only achieve a few storage channels (≤6), indicating a capacity bottleneck for high-demand practical applications. In addition, some current image encryptions can be easily decrypted, and it is very necessary to enhance the security and fidelity of information encryption. Summary of the Invention

[0003] The present invention solves the problem of low capacity of vector encryption in the prior art by providing a multi-image encryption method based on a metasurface, a metasurface, and a design method thereof.

[0004] In a first aspect, the present invention provides a multi-image encryption method based on a metasurface, which uses the metasurface to present a near-field nano-printing image with three-channel independent encoding and a far-field vector holographic image with three-channel independent encoding; uses overlapping multi-beams and introduces phase-difference interference to enable the metasurface to additionally present an interference-assisted vector encrypted image with six channels, so that the metasurface can present a twelve-channel optical image; the interference-assisted vector encrypted image with six channels includes an additional far-field vector holographic image with four channels and an additional near-field nano-printing image with two channels.

[0005] Preferably, the metasurface is composed of a substrate and a number of nano-bricks with the same size located on the substrate, and the target Jones matrix is reconstructed by using nano-bricks at different positions and rotation angles to achieve optical response decoupling;

[0006] The near-field nano-printing image with three-channel independent encoding is generated by three amplitude variables in the target Jones matrix; by using interference-assisted vector encryption, additional near-field encryption channels are created to encrypt and decrypt the additional two nano-printing images.

[0007] Generate the three-channel independently encoded far-field vector holographic image from three phase variables in the target Jones matrix; use the overlapping region of the three-channel independently encoded far-field vector holographic image as an additional encoding space, and encrypt and decrypt four additional vector holographic images by controlling the phase difference in the additional encoding space through multi-beam interference.

[0008] Preferably, the multi-image encryption method based on metasurface includes the following steps:

[0009] Step 1: Design meta-molecules for achieving optical response decoupling; several of the nanobricks form periodically arranged meta-molecules, one meta-molecule contains a first number of the nanobricks, the first number is greater than 3, and the positions and rotation angles of the nanobricks determine the optical response based on the detour phase.

[0010] Step 2: Select the encryption target image, use the GS algorithm to generate the corresponding near-field / far-field image and the vector distribution in the overlapping region, and obtain the target Jones matrix.

[0011] Step 3: Use the optimized simulated annealing algorithm integrating the genetic algorithm to optimize the phase distribution of the metasurface based on the target Jones matrix and the Jones matrix of the meta-molecule coherence, and find the optimal encoding parameters of each nanobrick, where the encoding parameters include position parameters and rotation angle parameters.

[0012] Preferably, use the GS algorithm to encode three phase components in the Jones matrix to achieve far-field vector holography, and control the encrypted information encoding of the far-field x polarization and y polarization corresponding to the polarization at any point on the holographic image by controlling their phase differences; superimpose the electric field distributions in the far field so that the non-overlapping regions in the far-field hologram do not affect each other, and the phases in the overlapping regions are assimilated; by further superimposing the phase differences on different overlapping regions, optimize the fixed-phase difference vector holography in the overlapping regions, generate the corresponding near-field / far-field image and the vector distribution in the overlapping region, obtain three optimized vector distributions, and form the target Jones matrix.

[0013] Preferably, the Jones matrix of the meta-molecule coherence is expressed as:

[0014]

[0015] where J represents the Jones matrix of the meta-molecule coherence, n represents the number of nanobricks contained in one meta-molecule, P represents the meta-molecule period, x i represents the x-axis position of the i-th nanobrick in the meta-molecule, and θ i represents the rotation angle of the i-th nanobrick in the meta-molecule;

[0016] Use the difference between the target Jones matrix and the Jones matrix of the meta - molecule coherence as the fitness function, and use the genetic algorithm to iteratively optimize to find the optimal coding parameters of each of the nanobricks.

[0017] Preferably, by selecting the polarization states of the incident light beam and the outgoing light beam, the reproduction of a twelve - channel optical image is achieved.

[0018] In a second aspect, the present invention provides a design method for a metasurface, including the following steps:

[0019] Step 1: Construct meta - molecules in the metasurface, where the meta - molecules are used to achieve optical response decoupling; the metasurface includes a substrate and a number of nanobricks of the same size located on the substrate. The number of nanobricks forms a periodically arranged meta - molecule. One meta - molecule contains a first number of nanobricks, and the first number is greater than 3. The positions and rotation angles of the nanobricks determine the optical response based on the detour phase.

[0020] Step 2: Based on the selected encrypted target image, use the GS algorithm to generate the corresponding near - field / far - field image and the vector distribution of the overlapping region, and obtain the target Jones matrix.

[0021] Step 3: Use the optimized simulated annealing algorithm integrated with the genetic algorithm to optimize the phase distribution of the metasurface based on the target Jones matrix and the Jones matrix of the meta - molecule coherence, and find the optimal coding parameters of each of the nanobricks. The coding parameters include position parameters and rotation angle parameters.

[0022] The designed metasurface can present a twelve - channel optical image, including a three - channel independently encoded near - field nanoprinting image, a three - channel independently encoded far - field vector holographic image, and a six - channel interference - assisted vector encryption image; the six - channel interference - assisted vector encryption image includes a four - channel additional far - field vector holographic image and a two - channel additional near - field nanoprinting image.

[0023] Preferably, one meta - molecule contains 4 nanobricks. The y - axis positions of the 4 nanobricks are fixed and evenly distributed along the y - axis; the coding parameter of each nanobrick is (x, θ), where x represents the x - axis position of the nanobrick and θ represents the rotation angle of the nanobrick. One meta - molecule provides 8 variables.

[0024] In a third aspect, the present invention provides a metasurface, including a substrate and a number of nanobricks of the same size located on the substrate; the metasurface is obtained by using the design method of the metasurface as described above.

[0025] Preferably, the material of the substrate is silica, and the material of the nanobricks is silicon.

[0026] One or more technical solutions provided in the present invention have at least the following technical effects or advantages:

[0027] Utilize a metasurface to present a near-field nano-printed image with three-channel independent encoding and a far-field vector holographic image with three-channel independent encoding; utilize overlapping multi-beams and introduce phase-difference interference to enable the metasurface to additionally present a six-channel interference-assisted vector encrypted image including a four-channel additional far-field vector holographic image and a two-channel additional near-field nano-printed image, so that the metasurface can present a twelve-channel optical image. The present invention, based on a multi-channel metasurface with interference-assisted vector encryption, utilizes the phase difference between beams in the overlapping region to successfully encrypt and decrypt the additional encrypted vector image. It will not be decrypted unless a specific input / output polarization state joint key is set, and has a high hidden security. The four-channel additional far-field vector holographic image is successfully encrypted / decrypted, with low crosstalk of the encrypted image and obvious contrast. Near-field nano-printing based on interference-assisted vector encryption is realized for the first time, realizing multi-channel additional encrypted nano-printing information. In summary, the present invention extends encryption to twelve-channel near / far-field optical images, expands the capacity of vector encryption, and has a high hidden security. Description of the Drawings

[0028] Figure 1 is a schematic structural diagram of the substrate and nano-bricks constituting the metasurface in an embodiment of the present invention;

[0029] Figure 2 is a schematic diagram of the decoupled Jones matrix of a meta-molecule in an embodiment of the present invention;

[0030] Figure 3 is a schematic diagram of five selected arbitrary Jones matrix targets in an embodiment of the present invention, and a theoretical comparison of the three components of the Jones matrix with the generated meta-molecules;

[0031] Figure 4 is a flow chart of an improved G-S algorithm for an interference-assisted vector encryption strategy based on overlapping phase-difference encoding in an embodiment of the present invention;

[0032] Figure 5 is a flow chart of optimizing the phase distribution of an interference-assisted vector encryption metasurface using an optimized simulated annealing algorithm integrating a genetic algorithm in the present invention;

[0033] Figure 6 is a schematic diagram of a far-field holographic encryption measurement system in an embodiment of the present invention;

[0034] Figure 7 is a diagram of the simulation results and experimental results corresponding to a three-channel independently encoded far-field vector holographic image in an embodiment of the present invention;

[0035] Figure 8It is the simulation result and experimental result diagram corresponding to the four-channel additional far-field vector holographic image in the embodiment of the present invention;

[0036] Figure 9 It is the experimental result diagram of the near-field nano-printing image in the embodiment of the present invention. Detailed implementation manners

[0037] In order to better understand the above technical solution, the above technical solution will be described in detail below in conjunction with the specification drawings and specific implementation manners.

[0038] Embodiment 1:

[0039] Embodiment 1 provides a multi-image encryption method based on a metasurface, which uses the metasurface to present a near-field nano-printing image with three-channel independent encoding and a far-field vector holographic image with three-channel independent encoding; by overlapping multiple beams and introducing a phase difference interference, the metasurface additionally presents an interference-assisted vector encryption image with six channels, so that the metasurface can present a twelve-channel optical image; the six-channel interference-assisted vector encryption image includes a four-channel additional far-field vector holographic image and a two-channel additional near-field nano-printing image.

[0040] Specifically, the metasurface is composed of a substrate and a number of nano-bricks with the same size located on the substrate, and the target Jones matrix is reconstructed by using nano-bricks at different positions and rotation angles to achieve optical response decoupling; three amplitude variables in the target Jones matrix are used to generate the near-field nano-printing image with three-channel independent encoding; by using interference-assisted vector encryption, additional near-field encryption channels are created to encrypt and decrypt the additional two nano-printing images; three phase variables in the target Jones matrix are used to generate the far-field vector holographic image with three-channel independent encoding; the overlapping region of the three-channel independent encoded far-field vector holographic image is used as an additional encoding space, and by controlling the phase difference of the multi-beam interference in the additional encoding space, the additional four vector holographic images are encrypted and decrypted.

[0041] The principle of the present invention is to reconstruct the target Jones matrix by arranging nano-bricks to achieve optical response decoupling, generate near-field three-channel nano-printing by three amplitude variables in the Jones matrix, and use interference-assisted encryption to achieve two-channel additional encrypted nano-printing; the far-field holography is generated by three-channel holography of three phase variables in the Jones matrix, and a new encoding space is also generated in the overlapping region of the two holographic channels to achieve four-channel encrypted information hiding. By using the mutual interference in the overlapping region and precisely controlling the phase difference between the two beams, the additional encrypted vector image is successfully encrypted and decrypted. Based on the near-field image, the holographic phase is encoded by using computer-generated holography technology and optimization algorithms, the position and rotation angle of each nano-brick are determined, and broadband far-field holography is introduced while maintaining near-field display.

[0042] The following is an illustration in combination with specific steps.

[0043] The multi-image encryption method based on metasurface includes the following steps:

[0044] Step 1: Design meta-molecules for realizing optical response decoupling; several of the nanobricks form periodically arranged meta-molecules, one meta-molecule contains a first number of the nanobricks, the first number is greater than 3, and the positions and rotation angles of the nanobricks determine the optical response based on the detour phase.

[0045] In the present invention, the Jones matrix is decoupled in six degrees of freedom by encoding the position parameters and rotation angle parameters of the nanobricks, so as to realize the optical response of the metasurface based on the detour phase.

[0046] For example, one meta-molecule contains 4 of the nanobricks, the y-axis positions of the 4 nanobricks are fixed and evenly distributed along the y-axis; the encoding parameter of each nanobrick is (x, θ), x represents the x-axis position of the nanobrick, θ represents the rotation angle of the nanobrick, and one meta-molecule provides 8 variables.

[0047] Step 2: Select an encrypted target image, and use the GS algorithm to generate the corresponding near-field / far-field image and the vector distribution of the overlapping region, so as to obtain the target Jones matrix.

[0048] Select a target image, and use the GS algorithm (Gerchberg-Saxton algorithm) to encode three phase components in the Jones matrix to realize far-field vector holography, and control the encrypted information encoding of the far-field x polarization and y polarization corresponding to the polarization of any point on the holographic image by controlling their phase differences; superimpose the electric field distributions in the far field so that the non-overlapping regions in the far-field hologram do not affect each other, and the phases of the overlapping regions are assimilated; by further superimposing the phase differences on different overlapping regions, optimize the fixed-phase difference vector holography in the overlapping region, generate the corresponding near-field / far-field image and the vector distribution of the overlapping region, obtain three optimized vector distributions, and form the target Jones matrix.

[0049] Step 3: Use the optimized simulated annealing algorithm integrating the genetic algorithm to optimize the phase distribution of the metasurface based on the target Jones matrix and the Jones matrix of the meta-molecule coherence, and find the optimal encoding parameters of each nanobrick, where the encoding parameters include position parameters and rotation angle parameters.

[0050] Use the optimized simulated annealing algorithm to further optimize the phase distribution of the metasurface on the basis of the above GS optimization, obtain the nanobrick parameters (x, θ) corresponding to the target Jones matrix, and realize the dual functions of nano-printing and holographic display encryption.

[0051] The Jones matrix representation of the meta - molecule coherence is as follows:

[0052]

[0053] where J represents the Jones matrix of the meta - molecule coherence, n represents the number of nanobricks included in a meta - molecule, P represents the meta - molecule period, x i represents the x - axis position of the i - th nanobrick in the meta - molecule, and θ i represents the rotation angle of the i - th nanobrick in the meta - molecule. The difference between the target Jones matrix and the Jones matrix of the meta - molecule coherence is used as the fitness function, and the genetic algorithm is used to iteratively optimize to find the optimal coding parameters of each nanobrick.

[0054] A multi - image encryption method based on metasurface provided in Embodiment 1 can realize the reproduction of a twelve - channel optical image by selecting the polarization states of the incident beam and the outgoing beam.

[0055] Embodiment 2:

[0056] Embodiment 2 provides a design method of a metasurface, including the following steps:

[0057] Step 1, construct the meta - molecules in the metasurface, and the meta - molecules are used to achieve optical response decoupling; the metasurface includes a substrate and a plurality of nanobricks with the same size located on the substrate, and the plurality of nanobricks form periodically arranged meta - molecules. A meta - molecule contains a first number of the nanobricks, and the first number is greater than 3. The positions and rotation angles of the nanobricks determine the optical response based on the phase - delay.

[0058] Step 2, based on the selected encrypted target image, use the GS algorithm to generate the corresponding near - field / far - field image and the vector distribution of the overlapping region to obtain the target Jones matrix.

[0059] Step 3, use the optimized simulated annealing algorithm integrated with the genetic algorithm to optimize the phase distribution of the metasurface based on the target Jones matrix and the Jones matrix of the meta - molecule coherence, and find the optimal coding parameters of each nanobrick. The coding parameters include position parameters and rotation angle parameters.

[0060] The designed metasurface can present a twelve - channel optical image, including a three - channel independently encoded near - field nano - printed image, a three - channel independently encoded far - field vector holographic image, and a six - channel interference - assisted vector encrypted image; the six - channel interference - assisted vector encrypted image includes a four - channel additional far - field vector holographic image and a two - channel additional near - field nano - printed image.

[0061] Example 2 provides a design method for a metasurface. The finally obtained metasurface corresponds to the function of the metasurface in a multi-image encryption method based on a metasurface provided in Example 1, that is, the metasurface designed in Example 2 is used to implement the multi-image encryption method. Therefore, in the description of Example 2, the relevant content that is the same as the details described in Example 1 will not be repeated.

[0062] Example 3:

[0063] Example 3 provides a metasurface, including a substrate and a plurality of nanobricks with the same size located on the substrate; the metasurface is obtained by using the design method of the metasurface as described in Example 2.

[0064] Among them, the material of the substrate is silicon dioxide, and the material of the nanobrick is silicon.

[0065] The following further describes the present invention.

[0066] Implementation principle of the metasurface decoupled Jones matrix: In the x-z plane, a plane wave with a wavelength of λ is incident on the metasurface from one side of the substrate at an angle of to ensure that the outgoing first-order diffraction is perpendicular to the metasurface. Due to the anisotropic characteristics of the rectangular nanobricks, the optical response only occurs on the polarized light component along the long axis of the nanobricks. Here, the electric field propagation E t etc. can be expressed as:

[0067]

[0068] Among them, E xr / E yr represents the electric field component of the refracted light at the air / substrate interface. Therefore, the Jones matrix of the meta-molecule coherence can be expressed as:

[0069]

[0070] Among them, J represents the Jones matrix of the meta-molecule coherence, n represents the number of nanobricks included in a meta-molecule, P represents the meta-molecule period, x i represents the x-axis position of the i-th nanobrick in the meta-molecule, and θ i represents the rotation angle of the i-th nanobrick in the meta-molecule.

[0071] Since the non-diagonal elements of the single-layer metasurface have mirror symmetry as shown in formula (2), there are a total of 6 degrees-of-freedom parameters in the Jones matrix It can be encoded arbitrarily. To completely decouple the degrees of freedom and satisfy the 6 equations in formula (2) (one linear equation for each degree-of-freedom parameter, 6 independently controllable degrees of freedom, a total of 6 equations), at least 3 nanobricks need to be included to provide 6 variables in the metamolecule (i.e., each nanobrick provides two variables: the x position and the rotation angle θ). Therefore, when the metamolecule contains 4 nanoblocks and a total of 8 variables, it can provide greater flexibility to improve the encoding efficiency and the resolvability of the Jones matrix.

[0072] First, the material of the nanobrick 102 is selected as silicon, and the material of the substrate 101 is silica. The geometric dimensions of the nanobrick 102 are uniform. For example, the length L of the nanobrick 102 is 160 nm, the width W is 80 nm, and the height H is 380 nm. The structure is as Figure 1 shown.

[0073] The designed metamolecule contains four such nanobricks, which can achieve the decoupling of three parameters (J xx , J xy / J yx , J yy ) of the Jones matrix. x i represents the x-axis position of the i-th nanobrick in the metamolecule, and θ i represents the rotation angle of the i-th nanobrick in the metamolecule, where i = 1, 2, 3, 4. As Figure 2 shown. In the preferred solution, the y-axis position of the nanobrick is fixed and evenly distributed along the y-axis, which can avoid spatial overlap and influence between adjacent nanobricks to resolve each amplitude / phase component in the Jones matrix. For example, the period P of the metamolecule is 800 nm.

[0074] As a simplified proof-of-concept demonstration of the accuracy and reliability of the calculated Jones matrix variables, we randomly selected five arbitrary Jones matrix targets obtained from the calculation and compared each independent component of the three parameters of the Jones matrix with the position of the metamolecule we generated in the Poincaré sphere. As Figure 3 shown, the calculation results are consistent with the Jones matrix targets, confirming the basic mechanism of metasurface multiplexing based on Jones matrix encoding.

[0075] In the design of image encryption, the present invention uses an improved Gerchberg-Saxton (GS) algorithm to generate the corresponding near-field / far-field images and the vector distribution in the overlapping region. See Figure 4 . For far-field encryption, the fast Fourier transform (FFT) generates a complex amplitude distribution from a random phase (three complex amplitudes calculated by the fast Fourier transform Add them together); convert the vector distributions corresponding to two additional far-field vector holographic images located in the first overlapping region of the three-channel independently encoded far-field vector holographic image into two opposite phase distributions (±Φ / 2), and also convert the vector distributions corresponding to another two additional far-field vector holographic images located in the second overlapping region of the three-channel independently encoded far-field vector holographic image into two opposite phase distributions (±Φ / 2); multiply the above vector distributions by , and extract the corresponding phase from the obtained result Encode it into each image channel and its overlapping region, thereby obtaining the respective three Jones matrix components (J xx , J xy / J yx , J yy ). Among them, A1, A2, and A3 are the amplitudes of the three-channel independently encoded far-field vector holographic image respectively, and 's final phase differences respectively determine the image patterns in the first overlapping region and the second overlapping region. Extract the image vector distribution through the inverse fast Fourier transform (IFFT). When the residual decreases to the threshold or exceeds the maximum number of cycles, it is determined that the iterative optimization process of the improved GS algorithm stops, and finally three optimized vector phase distributions are obtained for reconstructing a single-channel hologram including the overlapping region. In terms of near-field encryption, denote the amplitudes of the three-channel independently encoded near-field nanoprinting images as A 11 ("W"), A 21 ("H"), A 31 ("U"), and encode a fixed phase difference to implement additional near-field nanoprinting images for two channels ("1893" corresponds to "2023" corresponds to ). These near-field images appear as constraints on the near-field distribution in the above GS algorithm, see Figure 4 , Figure 9 .

[0076] The present invention optimizes the phase distribution of the interference-assisted vector encryption metasurface by using a typical general optimization simulated annealing algorithm, and finally realizes the dual functions of nanoprinting and holographic display encryption. See Figure 5 , first randomly generate a Jones matrix with 8-step phase and 2-step amplitude as the optimization target (J xx , J xy / J yx , J yy) Using formula (2), the corresponding Jones matrix is calculated based on the position parameters and rotation angle parameters of the nanobricks. The difference between the two (the target Jones matrix and the calculated Jones matrix) is used as the fitness function, and the optimal nanobrick parameter distribution is iteratively optimized to finally achieve the dual functions of nanoprinting and holographic display encryption.

[0077] The present invention encodes three independently encoded far-field holographic displays from the three phase parameters of the Jones matrix. By coordinately tuning the polarization incident state and the polarization analysis state, three independently encoded vector holographic images are directly captured by a mobile phone camera as Figure 7 shown. Figure 8 It shows that by manipulating Key 1 and Key 2, four different encrypted holograms are successfully encrypted / decrypted. The experimental images match well with the simulated images, the graphic shapes are clear, there is no obvious crosstalk, and the contrast is obvious. On the one hand, by precisely setting the input / output linear polarization, the "favorite" and "start" symbol images are decrypted; on the other hand, by setting a left / right circular polarization analyzer to obtain the "attention" and "pause" symbol images, at this time, a quarter-wave plate needs to be inserted in the post-sampling optical path. Correspondingly, the schematic diagram of the system for far-field holographic encryption measurement is as Figure 6 shown, and a laser, an aperture 1, a polarizer 2, a lens 3, a metasurface 4, a quarter-wave plate 5, an analyzer 6, and a camera 7 (such as a CCD) are sequentially arranged along the optical path.

[0078] The present invention also proposes to implement an interference-assisted encryption mechanism in near-field encryption to create additional encryption channels. In addition to the above holographic functions, three independently encoded nanoprintings ( Figure 9 a-c in) and two interference-assisted vector nanoprintings ( Figure 9 d-e in) are stored on a single metasurface, where Figure 9 the scale in a-e in is 80 μm (the white horizontal line in the figure is a schematic scale annotation). An optical microscope (see Figure 9 f in) is used to insert two polarizers for nanoprinting characterization. Specifically, referring to Figure 9 , three dark letter images on a bright background are encoded by the three amplitude components in the Jones matrix, that is, "W" represents J xx , "H" represents J xy / J yx , "U" represents J yy ; the additional encrypted images "1893" and "2023" can only be decrypted under the correct input and output polarization combinations. When conducting experimental verification, refer to Figure 9f, 610nm monochromatic beam is generated by supercontinuum laser, incident obliquely from the substrate side into the xz plane, adjusted by a linear polarizer, and another linear polarizer is placed behind the sample to selectively analyze the x or y polarization passing through. For circularly polarized light, a quarter wave plate is inserted in the optical path after sampling as a left / right circular polarization analyzer. The holographic image is taken by a charge coupled device (CCD). The nanoprinted image is observed under the 10× / 0.25 objective lens of the optical microscope system, combining the CCD and supercontinuum laser source.

[0079] In summary, the present invention is based on the interference-assisted vector encryption strategy and adopts the method of multi-beam interference. It cleverly creates additional coding space on the basis of independently encoded near-field / far-field images, and precisely controls the phase difference. It successfully encrypts and decrypts the additional encrypted vector images, expands the encryption to twelve-channel near-field / far-field optical images, and significantly enhances the hidden security.

[0080] Finally, it should be noted that the above specific implementation methods are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A multi-image encryption method based on metasurface, characterized in that, Using a metasurface to present a near-field nano-printed image with three-channel independent encoding and a far-field vector holographic image with three-channel independent encoding; using overlapping multi-beams and introducing phase-difference interference to enable the metasurface to additionally present a six-channel interference-assisted vector encrypted image, so that the metasurface can present a twelve-channel optical image; the six-channel interference-assisted vector encrypted image includes a four-channel additional far-field vector holographic image and a two-channel additional near-field nano-printed image; The metasurface is composed of a substrate and a number of nano-bricks of the same size located on the substrate, and the target Jones matrix is reconstructed by using nano-bricks at different positions and rotation angles to achieve optical response decoupling; The three-channel independent encoded near-field nano-printed image is generated by three amplitude variables in the target Jones matrix; Using interference-assisted vector encryption to create an additional near-field encryption channel for encrypting and decrypting two additional nano-printed images; The three-channel independent encoded far-field vector holographic image is generated by three phase variables in the target Jones matrix; the overlapping region of the three-channel independent encoded far-field vector holographic image is used as an additional encoding space, and by controlling the phase difference of multi-beam interference in the additional encoding space, four additional vector holographic images are encrypted and decrypted; The multi-image encryption method based on the metasurface includes the following steps: Step 1, design meta-molecules for realizing optical response decoupling; a number of the nano-bricks form a periodically arranged meta-molecule, one meta-molecule contains a first number of the nano-bricks, the first number is greater than 3, and the position and rotation angle of the nano-bricks determine the optical response based on the detour phase; Step 2, select an encrypted target image, use the GS algorithm to generate corresponding near-field / far-field images and vector distributions in the overlapping region to obtain the target Jones matrix; Step 3, use an optimized simulated annealing algorithm integrating a genetic algorithm to optimize the phase distribution of the metasurface based on the target Jones matrix and the Jones matrix of the meta-molecule coherence, and find the best encoding parameters of each nano-brick, where the encoding parameters include position parameters and rotation angle parameters.

2. The multi-image encryption method based on metasurface according to claim 1, wherein Use the GS algorithm to encode three phase components in the Jones matrix to achieve far-field vector holography, and control the encrypted information encoding of the far-field x polarization and y polarization corresponding to the polarization of any point on the holographic image by controlling their phase differences; superimpose the electric field distributions in the far field so that the non-overlapping regions in the far-field hologram do not affect each other and the phases of the overlapping regions are assimilated; by further superimposing the phase differences on different overlapping regions, optimize the fixed-phase-difference vector holography in the overlapping region to generate corresponding near-field / far-field images and vector distributions in the overlapping region, obtain three optimized vector distributions, and form the target Jones matrix.

3. The multi-image encryption method based on metasurface according to claim 1, characterized in that The Jones matrix of the meta-molecule coherence is expressed as: where J represents the Jones matrix of the meta-molecular coherence, n represents the number of nanobricks contained in a meta-molecule, P represents the meta-molecular period, x i represents the x-axis position of the i-th nanobrick in the meta-molecule, and θ i represents the rotation angle of the i-th nanobrick in the meta-molecule; Take the difference between the target Jones matrix and the Jones matrix of the meta-molecule coherence as the fitness function, and use the genetic algorithm to iteratively optimize to find the best encoding parameters of each nano-brick.

4. The multi-image encryption method based on metasurface according to claim 1, characterized in that, By selecting the polarization states of the incident beam and the outgoing beam, the reproduction of the twelve-channel optical image is realized.

5. A design method of a metasurface, characterized in that, It includes the following steps: Step 1: Construct the meta - molecules in the metasurface, where the meta - molecules are used to achieve optical response decoupling; the metasurface includes a substrate and a number of nanobricks with the same size located on the substrate. The number of nanobricks forms a periodically arranged meta - molecule. One meta - molecule contains a first number of the nanobricks, and the first number is greater than 3. The positions and rotation angles of the nanobricks determine the optical response based on the detour phase; Step 2: Based on the selected encrypted target image, use the GS algorithm to generate the corresponding near - field / far - field images and the vector distribution in the overlapping region, and obtain the target Jones matrix; Step 3: Use the optimized simulated annealing algorithm integrated with the genetic algorithm to optimize the phase distribution of the metasurface based on the target Jones matrix and the Jones matrix of the meta - molecule coherence, and find the best coding parameters for each of the nanobricks. The coding parameters include position parameters and rotation angle parameters; The designed metasurface can present a twelve - channel optical image, including a three - channel independently encoded near - field nano - printed image, a three - channel independently encoded far - field vector holographic image, and a six - channel interference - assisted vector encrypted image; the six - channel interference - assisted vector encrypted image includes a four - channel additional far - field vector holographic image and a two - channel additional near - field nano - printed image.

6. The design method of the metasurface according to claim 5, characterized in that One meta - molecule contains 4 nanobricks. The y - axis positions of the 4 nanobricks are fixed and evenly distributed along the y - axis; the coding parameter of each nanobricks is (x, θ), where x represents the x - axis position of the nanobrick and θ represents the rotation angle of the nanobrick. One meta - molecule provides 8 variables.

7. A metasurface, characterized in that, It includes a substrate and a number of nanobricks with the same size located on the substrate; the metasurface is obtained by using the design method of the metasurface as described in Claim 5 or 6.

8. The metasurface according to claim 7, wherein The material of the substrate is silica, and the material of the nanobricks is silicon.

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