Multi-channel image encryption method of phase change metasurface based on degeneracy

Abstract

The invention discloses a degeneracy-based multi-channel image encryption method for a phase change metasurface. Comprising the following steps: 1) designing # imgabs0 # nano bricks which are arranged according to a certain rule and have different sizes and directions on a metasurface to obtain a phase change metasurface with two near-field printing channels and a far-field holographic channel, and performing degeneracy distribution on the two near-field printing channels of the phase change metasurface to realize energy control on the two near-field printing channels; an independent channel is provided for far-field holographic projection by using an improved annealing algorithm and additional degree of freedom given by a Malus law, and dynamic display and switching of a far-field holographic image are realized; and 2) based on the obtained phase change metasurface, optical encryption of the three-channel image is realized by switching the state of the nano brick by utilizing the property of the phase change material. Three information channels are coded on a single phase metasurface by utilizing the flexibility of the multi-channel design and by means of the reversible and adjustable characteristics of the # imgabs1 # nanostructure, so that the information capacity and the safety can be remarkably improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of image encryption and multiplexing, and particularly relates to a multi-channel image encryption method based on a degenerate phase change metasurface. Background Art

[0002] A metasurface is a sub-wavelength artificial material with the ability to precisely manipulate the phase and amplitude of a wavefront. Since Capasso's team first demonstrated the excellent phase control of the metasurface in 2011, the metasurface has attracted great attention in the academic community. By adjusting parameters such as the size and rotation angle of the meta-atoms on the metasurface and arranging the designed meta-atoms appropriately, the metasurface can exhibit different capabilities, such as holography, nanolithography, superlens, and vortex light generator. The metasurface provides a new path for traditional image display, and compared with traditional image display, the metasurface has the characteristics of high resolution and high fidelity. High-efficiency image display has always been one of the performances pursued by the academic community, and multi-channel image multiplexing on the metasurface has been proven to be an effective way.

[0003] Recently, the multiplexing of holography and nanolithography into a single metasurface has emerged as an emerging way to multiplex information. For example, the literature "3D-Integrated metasurfaces for full-colour holography" (Hu Y, Luo X, Chen Y, et al. Light: Science & Applications, 2019, 8(1): 86.) and "Full-colour nanoprint-hologram synchronous metasurface with arbitrary hue-saturation-brightness control" (Bao Y J, Yu Y, Xu H F, et al. Light: Science & Applications, 2019, 8: 95.) both use multi-nanopillar or multi-layer structures to achieve dual-channel imaging of nanolithography and holography. However, the above metasurfaces are all designed to be static, and essentially two or more structures are integrated into one metasurface, but the monomer structure is relatively complex and large, which is difficult in actual processing and application. Moreover, they are all static metasurfaces and do not have the ability to achieve dynamic image reconstruction. In the patent with the application number 20241036478.6, "A three-channel image encryption method based on dynamic regulation of phase-change metasurface", a three-channel imaging based on structural color, nano near-field printing and holography is proposed. Using the nanobricks of the unit cell structure, high-quality imaging and optical encryption are achieved. However, the imaging quality of structural color is relatively sensitive to wavelength and is easily affected when wavelength interference occurs in actual applications. Moreover, structural color occupies the length and width degrees of freedom of the nanobricks but the information capacity is limited. Therefore, this patent proposes two near-field and one far-field channels, which have good robustness to the change of the light source wavelength, and due to the characteristics of degeneracy, there are more choices in terms of structure, providing a larger information storage capacity space. Summary of the Invention

[0004] The object of the present invention is to provide a multi-channel image encryption method based on a degenerate phase change metasurface for the deficiencies of the prior art. By using double degeneracy, namely assignment degeneracy and orientation degeneracy, energy control of two channels is achieved, and the ability to precisely manipulate two near-field nanolithography channels is possessed. Then, by using an improved annealing algorithm and the additional degrees of freedom given by Malus' law, an independent far-field holographic channel is realized. Finally, by utilizing the properties of phase change materials, optical encryption of a three-channel image is achieved by switching the states of nanobricks. This method uses the principle of degeneracy to uniformly combine different combinations of gray values to achieve standardized processing of the structure. In this way, instead of selecting suitable structures one by one in the scanned database, the combination is directly completed according to the structure values, greatly improving the processing and manufacturing efficiency in practical applications. At the same time, nanolithography and holographic imaging technologies have relatively stable imaging quality and show better robustness compared to structural color imaging. The improved annealing algorithm significantly reduces the time cost of finding structures and determining rotation angles, and to a certain extent improves the imaging quality of the three channels, making the overall performance more advantageous.

[0005] To achieve the above object, the present invention adopts the following technical solutions.

[0006] A multi-channel image encryption method based on a degenerate phase change metasurface, comprising the following steps:

[0007] Step S1: Design a phase change metasurface based on the concept of degeneracy

[0008] Design Sb 2 S 3 nanobricks with different sizes and directions arranged in a certain pattern on the metasurface to obtain a phase change metasurface with two near-field lithography channels and one far-field holographic channel. Degenerate assignment is performed on the two near-field lithography channels of the phase change metasurface to achieve energy control of the two near-field lithography channels. By using an improved annealing algorithm and the additional degrees of freedom given by Malus' law, an independent channel is provided for far-field holographic projection to achieve dynamic display and switching of far-field holographic images;

[0009] Step S2: Based on the phase change metasurface obtained in Step S1, utilize the properties of the phase change material to achieve optical encryption of a three-channel image by switching the states of the nanobricks.

[0010] Specifically, the Sb 2 S 3 nanobricks in Step S1 are composed of a silicon substrate and Sb 2 S 3 nanopillars, and there are three sizes in total. The periods P of the three sizes of nanobricks in the x and y directions are the same, the heights h are the same, the widths W are the same, but they have different lengths L;

[0011] The Sb nanobricks of different sizes and orientations arranged according to a certain rule 2 S 3 are as follows:

[0012] Step S11: According to the intensity I of the nano-printing image 1 and I 2 , select an appropriate structural width of the nanobrick to ensure that the normalized structural value can cover the range of 0 - 1;

[0013] Since the images in the imaging channel are all binary images, there are four combinations of two nano-printing images, namely (0, 0), (0, 1), (1, 0), and (1, 1), and the structural value is the sum of I 1 and I 2 with the upper limit of 1. Therefore, it is necessary to find an appropriate structural width of the nanobrick to ensure that the normalized structural value can cover the range of 0 - 1;

[0014] Scan the Sb 2 S 3 nanobricks with different structural widths in the amorphous and crystalline states to obtain a structural value image. To achieve the effects of imaging and optical encryption, the structural value not only needs to meet the requirement of covering the range of 0 - 1, but also needs to meet the condition that the structural value difference between the two states is large under the same structure. Therefore, select an appropriate nanobrick width of W and lengths of L 1 , L 2 , L 3 from the image data, and the corresponding structural values need to be 1, 0.5, and 0 respectively to meet the requirements of two near-field nano-printing images;

[0015] Step S12: Calculate the corresponding nano-rotation angles θ respectively, to obtain the rotation angle matrix M 1 , θ 2 , θ 3 , θ 4 corresponding to the near-field printing image; 1 ;

[0016] Step S13: Utilize the extra degree of freedom given by Malus' law to derive the rotation angle matrix M 1 into four candidate matrices; input the far-field image into the classical Gerchberg-Saxton algorithm to calculate the corresponding phase distribution of the far-field holographic image, and obtain the rotation angle matrix M 2 corresponding to the far-field holographic channel. Iteratively optimize the four candidate matrices through an improved annealing algorithm, and keep the holographic rotation angle matrix M 2 unchanged in the length and width structure of the nanobrick. For M 1Replace the four derived corner matrices, and continuously iterate with the image pixel change rate as the evaluation factor to finally obtain a complete arrangement of the phase change metasurface.

[0017] Furthermore, the degenerate allocation of the two near-field printing channels of the phase change metasurface described in step S1 includes the following:

[0018] Irradiate the metasurface with right-handed circularly polarized light with a wavelength of 630 nm to obtain the cross-polarization conversion efficiency of the Sb 2 S 3 nanopillars in the crystalline state and the amorphous state, and then normalize the obtained cross-polarization conversion efficiency to obtain a structural value, which is derived from the concept of degeneracy. Specifically:

[0019] For nanostructures that satisfy the equality of the image intensity I 1 and the image intensity I 2 , that is, nanostructures that satisfy I 1 +I 2 =I sf relationship, the intensity state is defined as the allocation degeneracy of the nanostructure. For nanostructures of different dimensions, the value of I sf will also change accordingly;

[0020] Assume that the gray values of two images are (0,1) and (1,0) respectively. Then the images actually correspond to the same structural value intensity and nanobrick parameters, and can be degenerate allocated to achieve precise control of the amplitude in the two channels.

[0021] Specifically, the calculation method of the structural value is as follows:

[0022] When the light source is incident on the metasurface, the Jones vector of the transmitted light can be obtained:

[0023]

[0024] In formula (1), α 1 and α 2 are the angles between the transmission axis directions of the bulk polarizer and the analyzer respectively; therefore, the expression of the output transmitted light can be further converted to:

[0025]

[0026] Assume that the nanobrick is an anisotropic wave plate, and the complex transmission coefficients along its long axis and short axis are denoted as t l and t s , respectively. Place the nanobrick in an orthogonal polarization optical device, that is, the difference between the angles of α 2 and α 1 is Then the output light intensity is:

[0027]

[0028] In formula (3), is the cross-polarization conversion efficiency of the nanobrick, which is only determined by the phase difference between the long axis and the short axis of the nanobrick;

[0029] Only considering α 1 = 0 and For the two cases of, the following formulas are obtained respectively:

[0030]

[0031] Defining the normalized cross-polarization conversion efficiency as the structural value, then there is And I sf = I 1 + I 2 .

[0032] Specifically, the additional degrees of freedom given by using the improved annealing algorithm and Malus' law in step S1 provide independent channels for far-field holographic projection, where:

[0033] The improved annealing algorithm includes:

[0034] First, by introducing far-field data to replace near-field data in the neighborhood generation process, the traditional random perturbation method is replaced, thereby making full use of the global optimization potential provided by far-field information;

[0035] Second, the design of the cost function is more complex, combining the parameters of multiple imaging channels, making the optimization process not limited to minimizing the error in a single dimension, but more comprehensively measuring the quality of the solution;

[0036] The improved annealing algorithm can perform effective optimization under more complex constraint conditions and shows better performance when dealing with problems involving multi-objective and cross-domain data;

[0037] The additional degrees of freedom given by using the improved annealing algorithm and Malus' law provide independent channels for far-field holographic projection, including the following:

[0038] The realization of the holographic image is based on the principle of phase modulation: when the nanobrick is normally irradiated with LCP or RCP light, for the cross-polarized component of the transmitted light, its phase includes the geometric phase and the propagation phase:

[0039]

[0040] Here, represents the propagation phase, and ψ = 2θ represents the geometric phase;

[0041] Under different circularly polarized lights, the combined phase Can be independently modulated by the transmissive metasurface, providing a channel for far-field holographic projection.

[0042] Specifically, using the properties of phase change materials in step S2 to achieve optical encryption of three-channel images by switching the states of nanobricks includes:

[0043] By combining active structure state switching with a specific polarized light key, the encryption function of the image is achieved:

[0044] In the amorphous state, when x-polarized light is incident, by adding a phase of π / 2, a nano-printed grayscale image in the near field can be observed in the y-polarization channel; similarly, when 45° polarized light is incident, another nano-printed grayscale image can be observed in the 135° polarization channel; meanwhile, in the far field, a corresponding Fourier holographic image can be observed in the LCP channel.

[0045] The optical encryption includes two layers of encryption mechanisms: the first layer of encryption is based on the characteristic conversion of nanobrick materials; when the nanobricks are in the amorphous state, specific imaging channels are activated, enabling the display of images; while when they are switched to the crystalline state, all imaging channels are masked and the images are completely invisible; the second layer of encryption relies on the use of specific polarized light. Only when the correct polarized light is incident and received can the image be analyzed and displayed; using the wrong polarized light will introduce serious crosstalk, making the image unrecognizable, thus achieving higher-security encryption protection.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] The method of the present invention realizes an efficient image encryption function by combining active structure state switching with a specific polarized light key: by using the degeneracy principle to unify and combine combinations of different grayscale values, it not only simplifies the structure selection process but also significantly improves the processing and manufacturing efficiency; at the same time, the flexibility of the multi-channel design is combined with the reversible and adjustable characteristics of the Sb 2 S 3 nanostructure, enabling the method of the present invention to encode three information channels on a single phase metasurface, significantly improving the information capacity and security; in addition, by manipulating the phase change behavior of the Sb 2 S 3 nanostructure, the multiplexed information channels can be dynamically switched, while enhancing the robustness of information encryption, further optimizing the imaging quality and efficiency of the three channels, and the improved annealing algorithm significantly reduces the time cost of finding the structure and determining the rotation angle, demonstrating extremely high time efficiency and imaging quality. Based on its demonstrated versatility, compactness, and high security, the phase change metasurface provided by the present invention has broad application prospects in the field of active optical devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] To more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the embodiments. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0049] Figure 1 is the schematic structural diagram of Sb 2 S 3 nanobricks in the embodiments of the present invention;

[0050] Figure 2 is Sb in the embodiments of the present invention 2 S 3 structural value images obtained after scanning the nanobricks in the amorphous and crystalline states;

[0051] Figure 3 is the schematic flow chart of obtaining a complete arranged metasurface according to the method of the present invention;

[0052] Figure 4 is the schematic diagram of the phase change metasurface designed by the present invention for full-wave FDTD simulation. Detailed Embodiments

[0053] To facilitate the understanding and implementation of the present invention by those of ordinary skill in the art, the following will detail each step of the method proposed by the present invention. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0054] Embodiment

[0055] The present invention discloses a multi-channel image encryption method based on a degenerate phase change metasurface, including the following steps:

[0056] Step S1, design a phase change metasurface based on the concept of degeneracy

[0057] Design Sb with different sizes and directions arranged regularly on the metasurface 2 S 3 nanobricks to obtain a phase change metasurface with two near-field printing channels and one far-field holographic channel. Degenerate allocation is performed on the two near-field printing channels of the phase change metasurface to achieve energy control of the two near-field printing channels; by using the improved annealing algorithm and the additional degrees of freedom given by Malus' law, an independent channel is provided for the far-field holographic projection to achieve dynamic display and switching of the far-field holographic image;

[0058] Step S2: Based on the phase-change metasurface obtained in step S1, utilize the properties of the phase-change material to achieve optical encryption of the three-channel image by switching the states of the nanobricks.

[0059] Specifically, the Sb 2 S 3 nanobricks are composed of a silicon substrate and Sb 2 S 3 nanopillars, and there are three sizes in total. The periods P of the nanobricks of the three sizes in the x and y directions are the same, the heights h are the same, and the widths W are the same, but they have different lengths L; the process of arranging the Sb 2 S 3 nanobricks with different sizes and orientations according to a certain rule is as follows:

[0060] Step S11: According to the intensity I 1 and I 2 of the nano-printed image, select an appropriate nanobrick structure width to ensure that the normalized structure value can cover the range of 0 - 1;

[0061] In this embodiment, the images in the imaging channels are all binary images, and there are four possible combinations of the two nano-printed images, namely (0, 0), (0, 1), (1, 0), and (1, 1); and the structure value is the sum of I 1 and I 2 with the upper limit being 1. Therefore, it is necessary to find an appropriate nanobrick structure width to ensure that the normalized structure value can cover the range of 0 - 1.

[0062] As shown in (a) of Figure 2 and (b) of Figure 2 are the structure value images obtained after scanning the Sb 2 S 3 nanobricks in the amorphous state and the crystalline state. In order to achieve the effects of imaging and optical encryption, the structure value not only needs to meet the requirement of covering the range of 0 - 1, but also needs to meet the condition that the structure value difference between the two states is large under the same structure. Therefore, after screening the data from the figure, the nanobrick width is W, and the lengths are L 1 、L 2 、L 3 , and their corresponding structure values are 1, 0.5, and 0 respectively, which can meet the requirements of the two near-field nano-printed images, realize the energy control of the two near-field printing channels, and thus accurately manipulate the near-field continuous nano-printed grayscale image;

[0063] Step S12: Calculate the corresponding nano-rotation angles θ respectively according to 1 , θ 2 , θ 3 , θ 4, obtain the rotation angle matrix M corresponding to the near-field printed image 1 ;

[0064] Step S13: Utilize the extra degree of freedom given by Malus' law to derive the rotation angle matrix M 1 into four candidate matrices; input the far-field image into the classical Gerchberg-Saxton algorithm, calculate the corresponding phase distribution of the far-field holographic image, and obtain the rotation angle matrix M corresponding to the far-field holographic channel 2 , and perform iterative optimization on the four candidate matrices through an improved annealing algorithm. On the premise that the length and width structure of the nanobricks remains unchanged, replace the four derived rotation angle matrices of M 2 for M 1 , continuously perform iteration with the image pixel change rate as the evaluation factor, and finally obtain the complete arranged phase change metasurface.

[0065] Furthermore, the degenerate allocation of the two near-field printing channels of the phase change metasurface described in step S1 includes the following content:

[0066] Irradiate the metasurface with right-handed circularly polarized light with a wavelength of 630 nm to obtain the cross-polarization conversion efficiency of the Sb 2 S 3 nanopillars in the crystalline state and the amorphous state respectively, and then normalize the obtained cross-polarization conversion efficiency to obtain a structure value, which is derived from the concept of degeneracy. Specifically:

[0067] Degeneracy means that for the states of the same energy level, there will be multiple different options. In quantum mechanics, degenerate states are manifested as that for the same energy level state of an electron, there are options with different spin quantum numbers. In a two-dimensional plane, with the atomic core as the circle, the total energy of an electron is where E 0 represents the energy of the ground state, n x and n y represent the quantum numbers in the X-axis direction and the Y-axis direction respectively. The positive and negative of the quantum numbers, that is, the position where the electron is located, does not affect the total energy. Therefore, electrons in the same orbit can be classified into the same degenerate state;

[0068] Introduce the concept of degeneracy in the quantum field into the metasurface structure, then the same intensity image display can also be realized by nanobricks with different structural parameters, that is, one intensity state can correspond to multiple nanostructures. For nanostructures that satisfy the image intensity I 1 and the image intensity I 2 and are equal, that is, nanostructures that satisfy I 1 +I 2 =I sf relationship, the intensity state is defined as the allocation degeneracy of the nanostructure. For nanostructures of different dimensions, Isf The value will also change accordingly;

[0069] Assume that the gray values of two images are (0, 1) and (1, 0) respectively. Then the images actually correspond to the same structural value intensity and nanobrick parameters, that is, the degenerate assignment requirement is satisfied, and the precise control of the amplitude in two channels is achieved.

[0070] Specifically, the calculation method of the structural value is as follows:

[0071] When the light source is incident on the metasurface, the Jones vector of the transmitted light can be obtained:

[0072]

[0073] In formula (1), α 1 and α 2 are the angles between the transmission axis directions of the bulk polarizer and the analyzer respectively; Therefore, the expression of the output transmitted light can be further converted to:

[0074]

[0075] Assume that the nanobrick is an anisotropic wave plate, and the complex transmission coefficients along its long axis and short axis are denoted as t l and t s respectively. The nanobrick is placed in an orthogonal polarization optical device, that is, the difference between the angles of α 2 and α 1 is Then the output light intensity is:

[0076]

[0077] In formula (3), is the cross-polarization conversion efficiency of the nanobrick, which is only determined by the phase difference between the long axis and the short axis of the nanobrick;

[0078] Only considering the two cases of α 1 = 0 and , the following formulas are obtained respectively:

[0079]

[0080]

[0081] Define the normalized cross-polarization conversion efficiency as the structural value, then there is and I sf = I 1 + I 2 .

[0082] Specifically, the additional degrees of freedom provided by using the improved annealing algorithm and Malus' law in step S1 provide independent channels for far-field holographic projection, where:

[0083] The improved annealing algorithm includes:

[0084] First, by introducing far-field data to replace near-field data in the neighborhood generation process, the traditional random perturbation method is replaced, thus making full use of the global optimization potential provided by far-field information;

[0085] Second, the design of the cost function is more complex, combining the parameters of multiple imaging channels, making the optimization process not limited to minimizing the error in a single dimension, but more comprehensively measuring the quality of the solution;

[0086] The improved annealing algorithm can perform effective optimization under more complex constraint conditions and shows better performance when dealing with problems involving multi-objective and cross-domain data;

[0087] The additional degrees of freedom provided by using the improved annealing algorithm and Malus' law provide independent channels for far-field holographic projection, including the following:

[0088] The realization of the holographic image is based on the principle of phase modulation: when LCP or RCP light is normally incident on the nanobricks, for the cross-polarized component of the transmitted light, its phase includes a geometric phase and a propagation phase:

[0089]

[0090] Here, represents the propagation phase, and ψ = 2θ represents the geometric phase;

[0091] Under different circularly polarized lights, the combined phase can be independently modulated by the transmissive metasurface, providing a channel for far-field holographic projection.

[0092] Specifically, using the properties of phase change materials in step S2 to achieve optical encryption of a three-channel image by switching the state of the nanobricks includes:

[0093] By combining active structure state switching with a specific polarized light key, the encryption function of the image is realized:

[0094] In the amorphous state, when x-polarized light is incident, by adding a phase of π / 2, a near-field nanoprinted grayscale image can be observed in the y-polarization channel; similarly, when 45° polarized light is incident, another nanoprinted grayscale image can be observed in the 135° polarization channel; at the same time, in the far field, the corresponding Fourier holographic image can be observed in the LCP channel;

[0095] The optical encryption involves two layers of encryption mechanisms: The first layer of encryption is based on the property conversion of nanobrick materials; when the nanobricks are in the amorphous state, specific imaging channels are activated, enabling the display of images; while when they switch to the crystalline state, all imaging channels are masked and the images become completely invisible; The second layer of encryption relies on the use of specific polarized light. Only when the correct polarized light is incident and received can the image be parsed and displayed; using the wrong polarized light will introduce severe crosstalk, making the image unrecognizable, thus achieving a higher level of security encryption protection.

[0096] The method of the present invention will be further described below through a specific experimental process.

[0097] In this embodiment, a total of three nanobrick structures are designed, as Figure 1 shown. The nanobricks are composed of a silicon substrate and Sb 2 S 3 nanopillars, which have the same period (P = 400 nm), the same height (h = 600 nm), and the same width (W = 108 nm) in the x and y directions but have different lengths (l I = 120 nm, l II = 150 nm, l III = 177 nm); At the same time, the rotation direction of the Sb 2 S 3 nanopillars is also a physical degree of freedom that can be modulated, and the phase and amplitude are controlled by adjusting the rotation angle. Since the length L and width W of the nanobricks are determined by maximizing the optical response difference between a-Sb 2 S 3 and c-Sb 2 S 3 , the designed Sb 2 S 3 metasurface has a dynamic switching function.

[0098] Furthermore, as Figure 3 shown in the process, the phase change metasurface consists of 80×80 pixels. In this embodiment, the images of "airplane" and "tree" are respectively selected as the near-field nanolithography images, and a centrosymmetric image is selected as the far-field image, with the image content being "light". First, the two near-field lithography images are read as 80×80 binary matrices, compared pixel by pixel, and the corresponding rotation angles and size structures are matched according to the pixel categories. In order to design nanobricks with different sizes and directions, it is necessary to calculate according to the intensity I 1 and I 2, select an appropriate width of the nano - brick structure to ensure that the normalized structure values can cover the range of 0 - 1. The images in the imaging channel of the present invention are all binary images. There are four possible combinations of two nano - printed images, namely (0,0), (0,1), (1,0), and (1,1). The structure value is the sum of I 1 and I 2 . Therefore, the four combinations correspond to three structures. After calculation, the width of the nano - brick is 108 nm, and the lengths are 177 nm, 150 nm, and 120 nm respectively. Their corresponding structure values are 1, 0.5, and 0, which can meet the requirements of two near - field nano - printed images and realize the precise manipulation of the near - field continuous nano - printed grayscale images by the metasurface. Then, using the additional degrees of freedom given by Malus's law, generate four candidate matrices; pass the far - field image through the classical Gerchberg - Saxton algorithm to calculate the corresponding phase of the far - field holographic image, obtain the rotation - angle matrix corresponding to the far - field image using the relationship between the rotation angle and the geometric phase, and iteratively optimize the four candidate matrices through an improved annealing algorithm. Finally, obtain a fully arranged metasurface, which can realize the image display of two near - field printing channels and one far - field holographic channel.

[0099] Encryption and decryption process:

[0100] Apply the designed phase - change metasurface to full - wave FDTD simulation. As Figure 4 shown, the simulation results are consistent with the preset dynamic multi - functional design results: in the amorphous state, when x - polarized light is incident, after generating an additional phase of π / 2, the near - field nano - printed grayscale image "tree" can be seen in the y - polarized channel; similarly, when 45° - polarized light is incident, the near - field nano - printed grayscale image "airplane" can be observed in the 135° - polarized channel; at the same time, the Fourier holographic image can be observed in the LCP channel in the far - field. When the nano - bricks are switched between the crystalline and amorphous states, the images in the three channels will appear or be masked accordingly.

[0101] The encryption function of the present invention is mainly reflected in the dual - encryption strategy, showing significant advantages; the first - layer encryption utilizes the crystalline characteristics of the material: when the material is in the crystalline state, the optical channel is closed, and even if the correct polarized light is incident and received, the target image cannot be obtained. The second - layer encryption depends on the phase - change characteristics of the material: after the material is transformed into the amorphous state through specific external stimuli, it can only be successfully decoded when using specific polarized - light combinations. For example, when x - polarized light is incident and y - polarized light is used for reception, the near - field nano - printed grayscale image "tree" can be observed; if 45° - polarized light is incident and 135° - polarized light is used for reception, the second - channel image "airplane" can be interpreted in the near - field; and under the condition of right - hand circularly polarized light (RCP) incident and left - hand circularly polarized light (LCP) reception, the holographic image "light" can be reconstructed in the far - field.

[0102] By combining active structural state switching with specific polarized light keys, the present invention realizes the above-instantiated image encryption function, utilizes the flexibility of the multi-channel design, and with the aid of the reversible tunable characteristics of the Sb 2 S 3 nanostructure, encodes three information channels on a single-phase metasurface, and can significantly improve the information capacity and security.

[0103] The above are only the preferred embodiments of the present invention, and are not limitations on the present invention in other forms. Any person skilled in the art may use the disclosed technical content to make changes or modifications into equivalent embodiments with equivalent changes. However, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the technical solution content of the present invention still belong to the protection scope of the technical solution of the present invention.

Claims

1. A multi-channel image encryption method based on degenerate phase change metasurface, characterized in that: The following steps are involved: Step S1: Designing a phase-change metasurface based on the concept of degeneracy Sb2S3 nanobricks of different sizes and directions arranged in a certain pattern are designed on the metasurface to obtain a phase-change metasurface with two near-field printing channels and one far-field holographic channel. The two near-field printing channels of the phase-change metasurface are degenerately allocated to achieve energy control of the two near-field printing channels. An independent channel is provided for far-field holographic projection by using an improved annealing algorithm and the extra degrees of freedom granted by Malus's law to achieve dynamic display and switching of far-field holographic images. Step S2: Based on the phase change metasurface obtained in step S1, the properties of the phase change material are utilized to realize optical encryption of the three-channel image by switching the state of the nanobricks.

2. According to the multi-channel image encryption method based on degenerate phase change metasurface of claim 1, it is characterized in that: The Sb2S3 nanobricks in step S1 are composed of a silicon substrate and Sb2S3 nanocolumns, and have three sizes. The three sizes of nanobricks have the same period P in the x and y directions, the same height h, the same width W, but different lengths L; The process of designing Sb2S3 nanobricks of different sizes and directions arranged in a certain pattern is as follows: Step S11, according to the intensities I1 and I2 of the nano-printed image, select a suitable nano-brick structure width to ensure that the normalized structure value can cover the range of 0-1; Since the images in the imaging channel are all binary images, there are four combinations of two nano-printed images, namely (0,0), (0,1), (1,0), (1,1), and the structure value is the sum of I1 and I2 and the upper limit is 1. Therefore, it is necessary to find a suitable nano-brick structure width to ensure that the normalized structure value can cover the range of 0-1; Sb2S3 nanobricks with different structural widths were scanned in the amorphous and crystalline states to obtain structural value images. In order to achieve the effects of imaging and optical encryption, the structural value must not only cover the range of 0-1, but also meet the condition that the structural values ​​of the two states under the same structure are quite different. Therefore, suitable nanobricks with a width of W and lengths of L1, L2, and L3 were selected from the image data. The corresponding structural values ​​must be 1, 0.5, and 0, respectively, to meet the requirements of two near-field nanoprinting images. Step S12: The corresponding nano-angles θ1, θ2, θ3, and θ4 are calculated respectively, and the angle matrix M1 corresponding to the near-field printing image is obtained; Step S13, using the extra degrees of freedom granted by Malus's law, derive the angle matrix M1 into four candidate matrices; input the far-field image into the classic Gerchberg-Saxton algorithm, calculate the corresponding phase distribution of the far-field holographic image, and obtain the angle matrix M2 corresponding to the far-field holographic channel. The four candidate matrices are iteratively optimized through an improved annealing algorithm. On the premise that the length and width structure of the nanobrick remains unchanged, the holographic angle matrix M2 is replaced by the four derived angle matrices of M1. The image pixel change rate is used as the evaluation factor for continuous iteration, and finally a completely arranged phase change metasurface is obtained.

3. According to the multi-channel image encryption method based on degenerate phase change metasurface of claim 1, it is characterized in that: The degenerate allocation of the two near-field printing channels of the phase change metasurface described in step S1 includes the following contents: The metasurface was irradiated with right-handed circularly polarized light of 630 nm wavelength to obtain the cross-polarization conversion efficiency of Sb2S3 nanorods in the crystalline state and the amorphous state, respectively. The obtained cross-polarization conversion efficiency was then normalized to obtain the structural value, which was derived from the concept of degeneracy. Specifically: For nanostructures that satisfy the image intensity I1 and image intensity I2 are equal, that is, I1+I2=I sf The strength of the relationship is defined as the distribution degeneracy of the nanostructure. For nanostructures of different dimensions, I sf The value of will also change; Assuming that the grayscale values ​​of the two images are (0, 1) and (1, 0) respectively, the images actually correspond to the same structural value intensity and nanobrick parameters, which can be degenerately allocated to achieve precise control of the amplitude in the two channels.

4. The multi-channel image encryption method based on degenerate phase change metasurface according to claim 3, characterized in that: The calculation method of the structure value is as follows: When the light source is incident on the metasurface, the Jones vector of the transmitted light can be obtained: In formula (1), α1 and α2 are the angles between the transmission axis of the bulk light polarizer and the analyzer, respectively; therefore, the expression of the output transmitted light can be further converted to: Assuming that the nanobrick is used as an anisotropic wave plate, the complex transmission coefficients along its long axis and short axis are denoted as t l and t s , put the nanobrick into an orthogonal polarization optical device, that is, the difference between the angles of α2 and α1 is Then the output light intensity is: In formula (3), is the cross-polarization conversion efficiency of the nanobrick, which is determined only by the phase difference between the major axis and the minor axis of the nanobrick; Consider only α1=0 and The two cases are as follows: The normalized cross-polarization conversion efficiency is defined as the structural value, then And I sf =I1+I2.

5. The multi-channel image encryption method based on degenerate phase change metasurface according to claim 1, characterized in that: The step S1 provides an independent channel for far-field holographic projection by using an improved annealing algorithm and the extra degree of freedom granted by Malus's law, wherein: The improved annealing algorithm comprises: Firstly, far-field data is introduced to replace near-field data in the neighborhood generation process, replacing the traditional random perturbation method, thereby making full use of the global optimization potential provided by far-field information. Secondly, the design of the cost function is more complex, combining the parameters of multiple imaging channels, so that the optimization process is not limited to minimizing the error in a single dimension, but measures the quality of the solution more comprehensively; The improved annealing algorithm can effectively optimize under more complex constraints and show better performance when dealing with problems involving multi-objective and cross-domain data; The improved annealing algorithm and the additional degrees of freedom granted by Malus's law provide independent channels for far-field holographic projection, including the following: The realization of holographic images is based on the principle of phase control: when LCP or RCP light is used to normally illuminate the nanobrick, the phase of the cross-polarized component of the transmitted light includes the geometric phase and the propagation phase: here, represents the propagation phase, ψ=2θ represents the geometric phase; Under different circularly polarized light, the combined phase It can be independently modulated by the transmissive metasurface, providing a channel for far-field holographic projection.

6. The multi-channel image encryption method based on degenerate phase change metasurface according to claim 1, characterized in that: The optical encryption of the three-channel image is achieved by switching the state of the nanobricks using the properties of the phase change material in step S2, including: By combining active structural state switching with specific polarization light keys, image encryption is achieved: In the amorphous state, when x-polarized light is incident, a nanoprinted grayscale image in the near field can be observed in the y-polarized channel by adding a phase of π / 2; similarly, when 45° polarized light is incident, another nanoprinted grayscale image can be observed in the 135° polarized channel; at the same time, in the far field, the corresponding Fourier holographic image can be observed in the LCP channel; The optical encryption includes two layers of encryption mechanisms: the first layer of encryption is based on the property conversion of the nanobrick material; when the nanobrick is in an amorphous state, a specific imaging channel is activated so that the image can be displayed; when it switches to a crystalline state, all imaging channels are covered and the image is completely invisible; the second layer of encryption relies on the use of specific polarized light. Only when the correct polarized light is incident and received can the image be parsed and displayed; using the wrong polarized light will introduce serious crosstalk, making the image unrecognizable, thereby achieving higher security encryption protection.

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