Metasurface encryption device and design method thereof

Abstract

The invention belongs to the technical field of micro-nano optics, and discloses a metasurface encryption device and a design method thereof. The method comprises the following steps: firstly, constructing a basic structure of a metasurface encryption device, then selecting an encrypted image, scrambling the encrypted image by using a scrambling algorithm, and taking the obtained scrambled image as a first target image; taking transformation parameters and iteration times in the scrambling algorithm as a first secret key, and taking an image containing first secret key information as a second target image; taking observation conditions of the first target image and the second target image as a second secret key, and taking an image containing second secret key information as a third target image; then, phase regulation and control quantities corresponding to the three target images under the three wavelengths are obtained respectively, and finally, size selection and arrangement of a plurality of nano bricks in the super-surface encryption device are determined according to the three phase regulation and control quantities. According to the invention, wavelength decoupling can be realized, the structure is simple, and the reliability and security of information encryption can be improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of micro-nano optics, and more specifically, relates to a metasurface encryption device and a design method thereof. Background Art

[0002] With the continuous deepening of informatization and digitalization, the importance of information security has become increasingly prominent. Different from traditional software-based information encryption methods, metasurfaces, which are artificial sub-wavelength materials, are expected to become new types of encryption devices due to their small size, easy integration, and powerful and diverse modulation capabilities for light waves.

[0003] However, previous metasurface encryption devices usually perform multiplexing based on amplitude, angle, polarization, etc. Most of the metasurface devices based on wavelength multiplexing have complex structures, and the design methods of staggered or stacked arrangements cannot achieve wavelength decoupling and have high processing complexity. At the same time, most of the previous metasurface encryption devices also have problems of low security and reliability of information encryption because they only change the incident conditions for encryption. Summary of the Invention

[0004] The present invention provides a metasurface encryption device and a design method thereof to solve the problems in the prior art that the metasurface encryption device based on wavelength multiplexing cannot achieve wavelength decoupling, has a complex structure, and has poor reliability and security of information encryption.

[0005] The present invention provides a design method of a metasurface encryption device, including the following steps: Construct the basic structure of the metasurface encryption device, where the metasurface encryption device includes a substrate and a nano-brick array located on the substrate, and the nano-brick array is composed of two sizes of nano-bricks; Select an encrypted image, scramble the encrypted image using a scrambling algorithm, and use the obtained scrambled image as the first target image; use the transformation parameters and the number of iterations in the scrambling algorithm as the first key, and use the image containing the first key information as the second target image; use the observation conditions of the first target image and the second target image as the second key, and use the image containing the second key information as the third target image; Obtain the first phase modulation amount corresponding to the first target image at the first wavelength, the second phase modulation amount corresponding to the second target image at the second wavelength, and the third phase modulation amount corresponding to the third target image at the third wavelength; Determine the size selection and arrangement of several nano-bricks in the metasurface encryption device according to the three phase modulation amounts.

[0006] Preferably, the first target image and the second target image are encoded into the metasurface encryption device for near-field holographic display; the third target image is encoded into the metasurface encryption device for far-field holographic display.

[0007] Preferably, the observation conditions include the diffraction distance and the polarization state of the incident light.

[0008] Preferably, the substrate is divided into a plurality of unit structures with the same size. The working surface of the unit structure is square. One unit structure and one nanobrick located on the working surface of the unit structure form a nano-unit structure. The size of the nano-unit structure is sub-wavelength; the two sizes of nanobricks have the same height, different lengths and different widths.

[0009] Preferably, the phase modulation amount includes the modulation amount of the transmission phase and the modulation amount of the geometric phase; The modulation amount of the transmission phase is expressed as:

[0010] The modulation amount of the geometric phase is expressed as:

[0011] In the formula, is the modulation amount of the transmission phase, is the equivalent refractive index of the nanobrick, is the wavelength, is the height of the nanobrick, is the modulation amount of the geometric phase, is the rotation angle of the nanobrick.

[0012] Preferably, LCP light is incident at the first wavelength and the second wavelength, and RCP light is incident at the third wavelength; the phase modulation amounts at the three wavelengths are expressed as:

[0013]

[0014]

[0015] In the formula, is the phase modulation amount at the first wavelength, is the phase modulation amount at the second wavelength, is the phase modulation amount at the third wavelength, is the modulation amount of the transmission phase of the first-size nanobrick at the first wavelength, is the modulation amount of the transmission phase of the second-size nanobrick at the first wavelength, The regulation amount of the transmission phase of the nanobrick of the first size at the second wavelength, The regulation amount of the transmission phase of the nanobrick of the second size at the second wavelength, The regulation amount of the transmission phase of the nanobrick of the first size at the third wavelength, The regulation amount of the transmission phase of the nanobrick of the second size at the third wavelength.

[0016] Preferably, the simulated annealing algorithm is used to obtain the phase regulation amounts at three wavelengths.

[0017] Preferably, the scrambling algorithm uses the Arnold scrambling algorithm.

[0018] On the other hand, the present invention provides a metasurface encryption device, and the metasurface encryption device is obtained based on the above design method of the metasurface encryption device.

[0019] Preferably, at the third wavelength, a third target image is observed in the far field of the metasurface encryption device; according to the second wavelength and the second key information included in the third target image, a second target image and a first target image are observed in the near field of the metasurface encryption device; the first target image is inversely transformed according to the first key information included in the second target image to obtain an encrypted image.

[0020] One or more technical solutions provided in the present invention have at least the following technical effects or advantages: The present invention first constructs the basic structure of the metasurface encryption device, then selects an encrypted image, scrambles the encrypted image using a scrambling algorithm, and uses the scrambled image as the first target image; uses the transformation parameters and the number of iterations in the scrambling algorithm as the first key, and uses the image containing the first key information as the second target image; uses the observation conditions of the first target image and the second target image as the second key, and uses the image containing the second key information as the third target image; then obtains the phase regulation amounts corresponding to the three target images at three wavelengths respectively, and finally determines the size selection and arrangement of several nanobricks in the metasurface encryption device according to the three phase regulation amounts. The present invention realizes near three-wavelength decoupling through a design method of arranging only two sizes of nanobricks, and solves the problem of high processing complexity. The present invention has the advantages of flexible design and simple structure. At the same time, the present invention adopts a design idea of combining geometric phase and transmission phase to regulate light waves, combines traditional information encryption algorithms and the non-direct observation method of Fresnel holography to realize multi-key encryption, and can improve the reliability and security of information encryption. The present invention uses the wavelength information of three wavelength channels as the encryption and decryption keys, which also adds to the security of information encryption. Description of the Drawings

[0021] Figure 1 Schematic diagram of the nano - unit structure in a design method of a metasurface encryption device provided in Embodiment 1 of the present invention; Figure 2 Spectral response diagrams of two sizes of nanobricks selected in a design method of a metasurface encryption device provided in Embodiment 1 of the present invention; wherein, Figure 2 In (a), it is a diagram of the polarization conversion efficiency varying with wavelength, Figure 2 In (b), it is a diagram of the relative phase varying with wavelength; Figure 3 Encryption images and images after Arnold transformation adopted in a design method of a metasurface encryption device provided in Embodiment 1 of the present invention; wherein, Figure 3 In (a), it is the encryption image, Figure 3 In (b), it is the image after Arnold transformation; Figure 4 Target images containing key information adopted in a design method of a metasurface encryption device provided in Embodiment 1 of the present invention; wherein, Figure 4 In (a), it is the second target image containing the first key information, Figure 4 In (b), it is the third target image containing the second key; Figure 5 Simulation images of three target images adopted in a design method of a metasurface encryption device provided in Embodiment 1 of the present invention after being optimized by the simulated annealing algorithm; wherein, Figure 5 In (a), it is the simulation image corresponding to the third target image, Figure 5 In (b), it is the simulation image corresponding to the second target image, Figure 5 In (c), it is the simulation image corresponding to the first target image; Figure 6 The encryption image obtained after decoding in a design method of a metasurface encryption device provided in Embodiment 1 of the present invention; Figure 7 Schematic diagram of a metasurface encryption device for realizing information encoding and decoding provided in Embodiment 2 of the present invention. Detailed implementation manners

[0022] To better understand the above - mentioned technical solutions, the above - mentioned technical solutions will be described in detail below in combination with the accompanying drawings of the specification and specific implementation manners.

[0023] Embodiment 1: Embodiment 1 provides a design method of a metasurface encryption device, including the following steps: Step 1, construct the basic structure of the metasurface encryption device. The metasurface encryption device includes a substrate and a nanobrick array located on the substrate. The nanobrick array is composed of two sizes of nanobricks.

[0024] Among them, the substrate is divided into a plurality of unit structures with the same size. The working surface of the unit structure is square. One unit structure and one nanobrick located on the working surface of the unit structure form a nano-unit structure, and the size of the nano-unit structure is sub-wavelength level; the nanobricks of two sizes have the same height, different lengths and different widths.

[0025] Step 2: Select an encrypted image, scramble the encrypted image using a scrambling algorithm, and use the obtained scrambled image as the first target image; use the transformation parameters and the number of iterations in the scrambling algorithm as the first key, and use the image containing the first key information as the second target image; use the observation conditions of the first target image and the second target image as the second key, and use the image containing the second key information as the third target image.

[0026] Among them, the observation conditions include the diffraction distance and the polarization state of the incident light.

[0027] The present invention encodes the first target image and the second target image into the metasurface encryption device for near-field holographic display; encodes the third target image into the metasurface encryption device for far-field holographic display.

[0028] The scrambling algorithm can adopt the Arnold scrambling algorithm or other existing algorithms.

[0029] Step 3: Obtain the first phase modulation amount corresponding to the first target image at the first wavelength, the second phase modulation amount corresponding to the second target image at the second wavelength, and the third phase modulation amount corresponding to the third target image at the third wavelength.

[0030] Among them, the phase modulation amount includes the modulation amount of the transmission phase and the modulation amount of the geometric phase.

[0031] The modulation amount of the transmission phase is expressed as:

[0032] The modulation amount of the geometric phase is expressed as:

[0033] In the formula, is the modulation amount of the transmission phase, is the equivalent refractive index of the nanobrick, is the wavelength, is the height of the nanobrick, is the modulation amount of the geometric phase, is the rotation angle of the nanobrick.

[0034] To increase the difference between the three wavelength channels and reduce crosstalk, left-handed circularly polarized (LCP) light is incident at both the first wavelength and the second wavelength, and right-handed circularly polarized (RCP) light is incident at the third wavelength. Correspondingly, the phase modulation amounts at the three wavelengths are respectively expressed as:

[0035]

[0036]

[0037] wherein, is the phase modulation amount at the first wavelength, is the phase modulation amount at the second wavelength, is the phase modulation amount at the third wavelength, is the modulation amount of the transmission phase of the nanobricks of the first size at the first wavelength, is the modulation amount of the transmission phase of the nanobricks of the second size at the first wavelength, is the modulation amount of the transmission phase of the nanobricks of the first size at the second wavelength, is the modulation amount of the transmission phase of the nanobricks of the second size at the second wavelength, is the modulation amount of the transmission phase of the nanobricks of the first size at the third wavelength, is the modulation amount of the transmission phase of the nanobricks of the second size at the third wavelength.

[0038] Specifically, the simulated annealing algorithm can be used to obtain the phase modulation amounts at the three wavelengths.

[0039] Step 4: Determine the size selection and arrangement of several nanobricks in the metasurface encryption device according to the three phase modulation amounts.

[0040] Taking the Arnold scrambling algorithm as an example of the scrambling algorithm and combining parameters, the following further illustrates several main steps of the present invention.

[0041] (1) Construct the basic structure of the metasurface encryption device.

[0042] The metasurface encryption device is composed of a substrate and two different sizes of nanobricks etched on the substrate and carefully arranged. According to the height of the selected structural material and the roughly determined working wavelength, the length and width of the nanobricks are scanned with electromagnetic simulation software, and two nanobricks whose transmission phases at three specific wavelengths meet the design conditions and are approximately half-wave plates are selected. To improve the effect of three-wavelength decoupling, the difference in the transmission phase differences of the two nanobricks at the three specific wavelengths should be obvious. See Figure 1, the size parameters of the selected nano-unit structure include the length L, width W, and height H of the nano-brick, as well as the side length of the working surface of the unit structure (i.e., the period CS), and the corner of the nano-brick is denoted as .

[0043] (2) Obtain three target images.

[0044] Select an encrypted image and scramble the encrypted image using the image Arnold scrambling algorithm. Arnold scrambling is a shear transformation. Let the pixel coordinates of an N×N image be ( , ), , and the Arnold transformation is:

[0045] The extended generalized Arnold transformation is:

[0046] The Arnold matrix is invertible, and the Arnold inverse transformation is:

[0047] where, ( , ) are the transformed pixel coordinates, a and b are transformation parameters less than N, and N is the image size.

[0048] Iterating the Arnold transformation K times can disrupt the relationship between the original image pixels and turn it into another image indistinguishable to the human eye. Only when the parameters a and b and the iteration number K are known can the transformed image be inversely transformed back to the original image. Therefore, the parameters a and b and the iteration number K can be used as keys to further improve the security of information encryption.

[0049] Take the parameters a, b of the Arnold transformation and the iteration number K as the first key, encode them into the metasurface, and display them holographically using Fresnel. When the diffraction distance z is much larger than the size of the diffraction screen, the complex amplitude of the Fresnel diffracted light wave can be approximately expressed by the following formula:

[0050] where, is the complex amplitude distribution of the incident field, and z is the diffraction distance.

[0051] Obviously, the incident wavelength and diffraction distance will both cause changes in the Fresnel diffraction pattern. In addition, the change in the polarization state of the incident light wave will cause a change in the incident light field, thus changing the diffraction pattern. Therefore, the diffraction distance and the polarization state of the incident light can be used as the second key to encrypt the first key and the information after the Arnold transformation.

[0052] (3) Obtain three phase modulation quantities.

[0053] Adopt a phase modulation method combining geometric phase and transmission phase, and design the phase modulation quantities ( , , ) of three wavelength channels. , , ).

[0054] The modulation quantity of the geometric phase is only related to the rotation angle of the nanobrick, and has nothing to do with the wavelength and the geometric parameters of the nanostructure. Its phase modulation quantity is: . In the above formula, "+" is the phase modulation quantity when LCP light is incident, and "-" is the phase modulation quantity when RCP light is incident.

[0055] The phase modulation quantity of the transmission phase is:

[0056] Among them, is the equivalent refractive index of the nanobrick, is the wavelength, and H is the height of the nanobrick. Usually, by changing the geometric size or shape of the nanostructure, the equivalent refractive index between it and the surrounding medium is changed, so that the phase delay quantity is different. In the present invention, the equivalent refractive index is changed by changing the length and width of the nanobrick.

[0057] Based on the above formula, it can be seen that the modulation quantity of the transmission phase is closely related to the size of the nanobrick and the wavelength. By changing the size of the nanobrick and the working wavelength, the transmission phase of the nanobrick at different wavelengths can be made different. Based on the above theory, the phase modulation quantities of two sizes of nanobricks at three wavelengths can be designed as:

[0058]

[0059]

[0060] Since the transmission phase differences of the two structures at three wavelengths need to be different, in the present invention, it is set that LCP light is incident on the first two wavelength channels. In order to expand the difference of the three wavelength channels and thus reduce crosstalk, it is set that RCP light is incident on the third wavelength channel. It should be noted that other polarization state setting methods can also be adopted in the present invention, and the above setting is only for illustration.

[0061] According to the three target images, the simulated annealing algorithm can be used to optimize the normalized intensities of the three wavelength channels, so as to obtain the geometric phase distribution and transmission phase distribution of the nanobrick.

[0062] (4) Determine the size selection and arrangement of several nanobricks in the metasurface encryption device according to three phase modulation quantities.

[0063] Arrange the nanobricks according to the geometric phase distribution and transmission phase distribution of the nanobricks obtained in the previous steps. Based on the formula of the geometric phase modulation quantity and the optimized geometric phase, the rotation angle distribution of the nanobricks can be obtained. The transmission phase of each wavelength is selectively composed of the transmission phases of two sizes of nanobricks at the corresponding wavelength. By corresponding the transmission phases of the two sizes of nanobricks at the corresponding wavelength to the transmission phase distribution at a certain wavelength, a selective arrangement array composed of two sizes of nanobricks can be obtained.

[0064] The encryption process and decryption process corresponding to the metasurface encryption device obtained based on the above design method are as follows.

[0065] Encryption: First, perform Arnold scrambling on the encrypted image, and then use the Arnold transformation parameters a, b, and the iteration number K as the first key, and encode the encrypted image after Arnold transformation and the first key into the metasurface for non-directly observable Fresnel near-field holographic display. Encode the observation conditions of the encrypted image and the first key into the metasurface for directly observable Fourier far-field holographic display. In this way, through layer-by-layer encryption, the security and reliability of the three-wavelength multi-key encryption device are greatly improved.

[0066] Decryption: The decryption process is the reverse process of the encryption process. After obtaining the wavelength information, the observation method of the first key and the encrypted image can be directly observed, that is, the second key can be directly observed. According to the second key and the wavelength information, the first key and the encrypted information after Arnold transformation can be decoded. Then, according to the first key, an inverse Arnold transformation can be performed, and finally the encrypted image can be obtained.

[0067] For example, the material of the substrate is silicon dioxide ( ), and the material of the nanobricks is phosphosilicide ( ).

[0068] Select two nanobricks with different sizes that meet the conditions. The polarization conversion efficiencies of the two sizes of nanobricks at the same wavelength are as consistent as possible, and the transmission phase differences at three wavelengths need to be different.

[0069] As Figure 1 shown, establish a right-handed xyz coordinate system. The long side direction of the nanorot is the long axis direction, parallel to the x-axis; the short side direction is the short axis direction, parallel to the y-axis; θ is the angle with the long axis direction of the nanobrick (the range of the angle is 0 to 180°). Initially determine that the period CS is 400 nm, and the height H of the nanobrick is 380 nm.

[0070] Optimize the length and width of the nanobricks using electromagnetic simulation software, scan the nanobricks with a step size of 10 nm in the length range from 150 nm to 300 nm and the width range from 50 nm to 150 nm. Finally, select three wavelengths of 470 nm, 560 nm, and 650 nm. The length L of the first-size nanobricks is 170 nm and the width is 80 nm; the length L of the second-size nanobricks is 230 nm and the width is 120 nm.

[0071] The transmission phases of the two sizes of nanobricks at the three corresponding wavelengths are , , , , .

[0072] That is, the transmission phase differences of the two sizes of nanobricks (i.e., Structure 1 and Structure 2 in Figure 2 ) at the three wavelengths are 0, , π respectively, and the polarization conversion efficiencies are close. As shown in Figure 2 , where Figure 2 (a) is the graph of the polarization conversion efficiency of the two sizes of nanobricks varying with wavelength, Figure 2 (b) is the graph of the relative phase of the two sizes of nanobricks varying with wavelength.

[0073] When selectively arranging the two sizes of nanobricks, one nanobrick corresponds to one pixel. Select one of the sizes of nanobricks according to the transmission phase corresponding to a certain pixel obtained by the simulated annealing algorithm and arrange it at the position corresponding to the pixel. For example, if the transmission phase at a certain wavelength is , then the first-size nanobricks are selected at this position.

[0074] The parameters for the Arnold transform of the encrypted image are a = b = 20 and the number of iterations is 1. The conditions for Fresnel holographic observation are that LCP light is incident and the diffraction distance z is 100 μm.

[0075] Select the Figure 3 (a) with a pixel size of 100×100 as the encrypted image. The image after the Arnold transform is as shown in Figure 3 (b), the transformation parameters are a = b = 20 and the number of iterations is 1. Figure 4 (a) is used as the first key, Figure 4 (b) is used as the second key. That is Figure 4 (a) is the second target image containing the first key information, Figure 4 (b) is the third target image containing the second key.

[0076] Figure 3 (b) inFigure 4 in (a) of Figure 4 in (b) of correspond to the three wavelength channels of 470nm, 560nm, and 650nm respectively. The phase modulation amounts for the images of the three wavelength channels are:

[0077]

[0078]

[0079] Taking Figure 3 in (b) of Figure 4 in (a) of and Figure 4 in (b) of as the three target images, the simulated annealing algorithm is used to optimize the phase at three wavelengths. For Figure 3 in (b) of and Figure 4 in (a) of , approximate numerical calculations of Fresnel diffraction are performed. For Figure 4 in (b) of , numerical simulations of Fourier transform are performed, that is:

[0080]

[0081]

[0082] In the formula, is the light intensity of the outgoing light corresponding to the i th wavelength channel , is the phase corresponding to the i th wavelength channel , i = 1, 2, 3; represents the Fresnel transform, represents the fast Fourier transform.

[0083] Finally, three-channel simulation images are obtained, that is, the simulation images after the three target images are optimized by the simulated annealing algorithm, as shown in Figure 5 . Among them, Figure 5 in (a) of is the simulation image obtained after optimizing the wavelength channel λ3, that is, the simulation image corresponding to the third target image, which contains the second key information; Figure 5 in (b) of is the simulation image obtained after optimizing the wavelength channel λ2, that is, the simulation image corresponding to the second target image, which contains the first key information; Figure 5 in (c) of is the simulation image obtained after optimizing the wavelength channel λ1, that is, the simulation image corresponding to the first target image, which is also the ciphertext image after Arnold transformation. Performing the Arnold inverse transformation on Figure 5 in (c) of can obtain the encrypted information, as shown inFigure 6 As shown, namely Figure 6 is the encrypted image obtained after decoding.

[0084] Embodiment 2: Embodiment 2 provides a metasurface encryption device, which is obtained based on the design method of the metasurface encryption device described in Embodiment 1.

[0085] The schematic diagram of information encoding and decoding provided by Embodiment 2 of the metasurface encryption device is as Figure 7 shown. At the third wavelength, the third target image is observed in the far field of the metasurface encryption device; according to the second wavelength and the second key information contained in the third target image, the second target image and the first target image are observed in the near field of the metasurface encryption device; the first target image is inversely transformed according to the first key information contained in the second target image to obtain the encrypted image.

[0086] For example, first, the encrypted image after Arnold transformation and the first key are phase-encoded into the metasurface according to the optimized phase for Fresnel near-field holographic display, and then the second key is phase-encoded into the metasurface according to the optimized phase formula for Fourier far-field holographic display. The decoding of information is the inverse process of encoding, and finally the encrypted image is obtained.

[0087] Since the metasurface encryption device provided in Embodiment 2 is obtained based on the design method of the metasurface encryption device provided in Embodiment 1, the description of Embodiment 1 can be referred to for understanding Embodiment 2, and details will not be repeated here.

[0088] In summary, the present invention provides a three-wavelength multi-key encryption scheme based on a minimalist metasurface. The constructed metasurface encryption device is composed of only two types of nanostructures with different sizes, and can achieve three-wavelength decoupling with flexible design, simple structure, and low processing complexity. The multi-key encryption device with layer-by-layer encryption provided by the present invention can greatly improve the reliability and security of information encryption. The present invention also provides a multi-channel multiplexed metasurface, which is beneficial to improving the information storage capacity and density of the metasurface, and provides a new idea for the multi-channel multiplexing of the metasurface.

[0089] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention and are not restrictive. 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 design method for a metasurface encryption device, characterized in that, It includes the following steps: Construct the basic structure of the metasurface encryption device, where the metasurface encryption device includes a substrate and a nano-brick array located on the substrate, and the nano-brick array is composed of nano-bricks of two sizes; Select an encrypted image, scramble the encrypted image using a scrambling algorithm, and use the obtained scrambled image as the first target image; Take the transformation parameters and the number of iterations in the scrambling algorithm as the first key, and use the image containing the first key information as the second target image; take the observation conditions of the first target image and the second target image as the second key, and use the image containing the second key information as the third target image; Obtain the first phase modulation amount corresponding to the first target image at the first wavelength, the second phase modulation amount corresponding to the second target image at the second wavelength, and the third phase modulation amount corresponding to the third target image at the third wavelength; Determine the size selection and arrangement of several nano-bricks in the metasurface encryption device according to the three phase modulation amounts.

2. The design method of the metasurface encryption device according to claim 1, characterized in that Encode the first target image and the second target image into the metasurface encryption device for near-field holographic display; encode the third target image into the metasurface encryption device for far-field holographic display.

3. The design method of the metasurface encryption device according to claim 1, characterized in that, The observation conditions include the diffraction distance and the polarization state of the incident light.

4. The design method of the metasurface encryption device according to claim 1, characterized in that, The substrate is divided into a plurality of unit structures with the same size. The working surface of the unit structure is square. One unit structure and one nano-brick located on the working surface of the unit structure form a nano-unit structure, and the size of the nano-unit structure is sub-wavelength level; the nano-bricks of the two sizes have the same height, different lengths and different widths.

5. The design method of the metasurface encryption device according to claim 4, characterized in that, The phase modulation amount includes the modulation amount of the transmission phase and the modulation amount of the geometric phase; The modulation amount of the transmission phase is expressed as: The modulation amount of the geometric phase is expressed as: In the formula, is the regulation amount of the transmission phase, is the equivalent refractive index of the nanobrick, is the wavelength, is the height of the nanobrick, is the regulation amount of the geometric phase, is the rotation angle of the nanobrick.

6. The design method of the metasurface encryption device according to claim 5, characterized in that, Incident with LCP light at both the first wavelength and the second wavelength, and incident with RCP light at the third wavelength; the phase modulation amounts at the three wavelengths are expressed as: Wherein, is the phase modulation amount at the first wavelength, is the phase modulation amount at the second wavelength, is the phase modulation amount at the third wavelength, is the modulation amount of the transmission phase of the nanobrick of the first size at the first wavelength, is the modulation amount of the transmission phase of the nanobrick of the second size at the first wavelength, is the modulation amount of the transmission phase of the nanobrick of the first size at the second wavelength, is the modulation amount of the transmission phase of the nanobrick of the second size at the second wavelength, is the modulation amount of the transmission phase of the nanobrick of the first size at the third wavelength, is the modulation amount of the transmission phase of the nanobrick of the second size at the third wavelength.

7. The design method of the metasurface encryption device according to claim 1, characterized in that Use the simulated annealing algorithm to obtain the phase modulation amounts at the three wavelengths.

8. The design method of the metasurface encryption device according to claim 1, characterized in that The scrambling algorithm uses the Arnold scrambling algorithm.

9. A metasurface encryption device, characterized in that, The metasurface encryption device is obtained based on the design method of the metasurface encryption device according to any one of claims 1-8.

10. The metasurface encryption device according to claim 9, wherein At the third wavelength, the third target image is observed in the far field of the metasurface encryption device; according to the second wavelength and the second key information contained in the third target image, the second target image and the first target image are observed in the near field of the metasurface encryption device; the first target image is inversely transformed according to the first key information contained in the second target image to obtain the encrypted image.

Images