A dual-wave encryption design method based on minimalist metasurface and metasurface device
Through the dual-wave encryption design of the minimalist metasurface nanobrick array, the combination of specific wavelength and polarization state is used to solve the problem of insufficient security of existing metasurface information encryption schemes, and a highly secure and easy-to-process miniaturized encryption device is realized, which is suitable for the fields of information encryption and anti-counterfeiting.
Patent Information
- Application Number
- CN202411275865.9
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2024-09-12
- Publication Date
- 2025-09-12
- Estimated Expiration
- 2044-09-12
AI Technical Summary
Existing metasurface-based information encryption schemes have insufficient security issues, especially the ability to perform brute-force decryption by scanning the wavelength and polarization state of the incident light.
A dual-wave encryption design method based on a minimalist metasurface is adopted. By constructing a base unit structure of a nanobrick array, two nanobricks of different sizes and circularly polarized light of a specific rotation direction are used to realize the superposition of encrypted images at different wavelengths. The encrypted images can only be decrypted at specific wavelengths and polarization states.
It improves the security of information encryption, reduces the difficulty of processing and designing metasurface devices, and can achieve miniaturization and high integration, making it suitable for the fields of information encryption and anti-counterfeiting.
Smart Images
Figure CN119337572B_ABST
Abstract
Description
Technical Field
[0001] The present invention belongs to the field of micro-nano optical technology, and more specifically, relates to a dual-wave encryption design method based on a minimalist metasurface and a metasurface device. Background Art
[0002] As a new type of artificial subwavelength structure, metasurfaces can be designed to realize a variety of functions, such as high-numerical-aperture lenses, nanoprinting, vortex light generators, and achromatic lenses. Compared with traditional optical components, metasurfaces have more controllable degrees of freedom and can therefore be applied to the field of information encryption.
[0003] Current metasurface-based information encryption schemes mostly use metasurface encoding to encrypt information under specific incident conditions. However, this scheme can achieve brute-force decryption by scanning the wavelength and polarization state of the incident light, resulting in insufficient security. Summary of the Invention
[0004] The present invention solves the problem that the security of information encryption schemes based on metasurfaces in the prior art needs to be further improved by providing a dual-wave encryption design method based on a minimalist metasurface and a metasurface device.
[0005] The present invention provides a dual-wave encryption design method based on a minimalist metasurface, comprising the following steps:
[0006] Constructing the basic structure of a metasurface device, wherein the metasurface device is a minimalist metasurface and comprises a substrate and a nanobrick array located on a working surface of the substrate; the nanobrick array comprises nanobricks of two sizes; the substrate is divided into a plurality of substrate unit structures of uniform size, each of which is provided with a nanobrick;
[0007] Selecting a first operating wavelength, a second operating wavelength, and an encrypted image; determining the geometric dimensions of the metasurface device, as well as the size arrangement and steering angle arrangement of a plurality of nanobricks in the nanobrick array based on the two selected operating wavelengths and the encrypted image, to obtain the desired metasurface device;
[0008] After obtaining the required metasurface device, circularly polarized light is incident on the first working wavelength and the second working wavelength respectively. After reflection from the metasurface device, a meaningless binary holographic image is obtained in the far field. Only when circularly polarized light with a first rotation direction at the first working wavelength and circularly polarized light with a second rotation direction at the second working wavelength are simultaneously incident on the metasurface and reflected from the metasurface device, the encrypted image is obtained by superposition in the far field.
[0009] Preferably, an xoy coordinate system is established with the directions of the two sides parallel to the working surface of the substrate set as the x-axis and the y-axis respectively, the nanobrick is a rectangular structure, the major axis and the minor axis of the nanobrick are parallel to the working surface of the substrate, and the lengths of the major axis and the minor axis are different; the steering angle of the nanobrick is the angle between the major axis of the nanobrick and the x-axis; the working surface of the substrate unit structure is square; the two sizes of nanobricks have the same height, different lengths and different widths.
[0010] Preferably, when determining the geometric dimensions of the metasurface device, the height of the nanobrick and the side length of the working surface of the substrate unit structure are first determined, and then the length and width dimension parameters of the nanobrick are scanned at the first working wavelength and the second working wavelength respectively through electromagnetic simulation to obtain the transmission phase scanning results at the two working wavelengths; finally, two groups of dimensions are selected as the determined dimension parameters of the nanobrick based on the transmission phase scanning results at the two working wavelengths, and the transmission phase difference between the two sizes of nanobricks selected at the first working wavelength is π / 2, and the transmission phase difference between the two sizes of nanobricks selected at the second working wavelength is also π / 2.
[0011] Preferably, when determining the geometric dimensions of the metasurface device, it also includes: scanning the length and width dimension parameters of the nanobrick at the first working wavelength and the second working wavelength respectively through electromagnetic simulation to obtain the polarization conversion efficiency scanning results at the two working wavelengths; based on the transmission phase scanning results and the polarization conversion efficiency scanning results at the two working wavelengths, two sets of dimensions that meet the transmission phase requirements and have the highest polarization conversion efficiency are selected as the determined dimension parameters of the nanobrick.
[0012] Preferably, after determining the geometric dimensions of the metasurface device, one of the substrate unit structures and a nanobrick located on its working surface is taken as a pixel point, and the phase of the reflected light passing through the metasurface device is modulated using the steering angle of the nanobrick and the transmission phase difference between two sizes of nanobricks, and the size and steering angle of the nanobrick on each substrate unit structure are determined based on the encrypted image.
[0013] Preferably, the Jones matrix J of the nanobrick with a steering angle of θ is expressed as: The Jones vector of circularly polarized light is expressed as: The circularly polarized light reflected by the nanobrick with a steering angle of θ is expressed as: where A and B are coefficients related to the reflectivity in the major and minor axis directions of the nanobrick, respectively.
[0014] Preferably, when the nanobrick is designed as an equivalent half-wave plate, A=-B is satisfied, and the reflected light only contains the reverse circular polarization component.
[0015] Preferably, the sizes of the base unit structure and the nanobricks are both sub-wavelength level.
[0016] Preferably, the nanobricks are made of single crystal silicon material, and the substrate is made of single crystal silicon material as a lower layer and fused quartz glass material as an upper layer.
[0017] On the other hand, the present invention provides a metasurface device, which is obtained based on the above-mentioned dual-wave encryption design method based on a minimalist metasurface; the metasurface device includes a substrate and a nanobrick array located on the working surface of the substrate, and the nanobrick array is composed of nanobricks of two sizes; circularly polarized light is incident at a first working wavelength and a second working wavelength respectively, and after reflection by the metasurface device, a meaningless binary holographic image is obtained in the far field; only when circularly polarized light with a first rotation direction at the first working wavelength and circularly polarized light with a second rotation direction at the second working wavelength are simultaneously incident on the metasurface, after reflection by the metasurface device, an encrypted image is obtained by superposition in the far field.
[0018] One or more technical solutions provided in the present invention have at least the following technical effects or advantages:
[0019] The present invention performs dual-wave encryption design based on a minimalist metasurface. The basic structure of the metasurface device (i.e., minimalist metasurface) constructed by the present invention includes a substrate and a nanobrick array located on the working surface of the substrate. The nanobrick array is composed of nanobricks of two sizes. The substrate is divided into several substrate unit structures of the same size. Each substrate unit structure is provided with a nanobrick. The present invention determines the geometric dimensions of the metasurface device, as well as the size arrangement and steering angle arrangement of several nanobricks in the nanobrick array based on the two selected working wavelengths and the encrypted image, to obtain the required metasurface device; after obtaining the required metasurface device, circularly polarized light is incident at a first working wavelength. After reflection from the metasurface device, a meaningless binary holographic image will be obtained in the far field. Circularly polarized light is incident at a second working wavelength. After reflection from the metasurface device, a meaningless binary holographic image will also be obtained in the far field; only when circularly polarized light with a first rotation direction at the first working wavelength and circularly polarized light with a second rotation direction at the second working wavelength are simultaneously incident on the metasurface and reflected by the metasurface device, an encrypted image is obtained by superposition in the far field. That is, the present invention can only obtain an encrypted image in the far field under the dual-wave condition of a specific wavelength plus a specific polarization state. On the one hand, the present invention introduces the wavelength dimension to encrypt information to improve the security of information encryption. On the other hand, it further increases the variables by limiting the specific polarization state (rotation direction), thereby increasing the difficulty of cracking. Moreover, the present invention constructs a minimalist metasurface based on only two sizes of nanobricks. Encryption can be achieved using this minimalist metasurface, which can greatly reduce the difficulty of processing and designing metasurface devices and has the advantages of simple design and easy processing. The sizes of the substrate unit structure and the nanobricks are both subwavelength-level, so the metasurface device designed by the present invention is small in size, light in weight, and can be highly integrated, adapting to the development of miniaturization and micro-miniaturization. The encryption scheme proposed by the present invention can increase the security of information encryption, and can also provide a new technical solution for fields such as information anti-counterfeiting. The dual-wave encryption holographic scheme proposed by the present invention has a good application development prospect and can be applied to fields such as information encryption and anti-counterfeiting. BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Figure 1 A schematic diagram of a substrate unit structure and nanobricks located on its working surface in a dual-wave encryption design method based on a minimalist metasurface provided in Example 1 of the present invention;
[0021] Figure 2 The nanobrick transmission phase scan diagram at dual wavelengths in a dual-wavelength encryption design method based on a minimalist metasurface provided in Example 1 of the present invention; wherein, Figure 2 (a) is the transmission phase scan of the nanobrick at a wavelength of 550nm. Figure 2 (b) is the transmission phase scan of the nanobrick at a wavelength of 633 nm;
[0022] Figure 3This is a scan diagram of the polarization conversion efficiency of nanobricks at dual wavelengths in a dual-wavelength encryption design method based on a minimalist metasurface provided in Example 1 of the present invention; wherein, Figure 3 (a) is a scanning diagram of the polarization conversion efficiency of nanobricks at a wavelength of 550nm. Figure 3 (b) is a scanning diagram of the polarization conversion efficiency of the nanobrick at a wavelength of 633 nm;
[0023] Figure 4 Schematic diagram of the effect of achieving dual-wave encryption using metasurface devices. DETAILED DESCRIPTION
[0024] In order to better understand the above technical solution, the above technical solution will be described in detail below with reference to the accompanying drawings and specific implementation methods.
[0025] Example 1:
[0026] Example 1 provides a dual-wave encryption design method based on a minimalist metasurface, comprising the following steps:
[0027] Constructing the basic structure of a metasurface device, wherein the metasurface device is a minimalist metasurface and comprises a substrate and a nanobrick array located on a working surface of the substrate; the nanobrick array comprises nanobricks of two sizes; the substrate is divided into a plurality of substrate unit structures of uniform size, each of which is provided with a nanobrick;
[0028] Selecting a first operating wavelength, a second operating wavelength, and an encrypted image; determining the geometric dimensions of the metasurface device, as well as the size arrangement and steering angle arrangement of a plurality of nanobricks in the nanobrick array based on the two selected operating wavelengths and the encrypted image, to obtain the desired metasurface device;
[0029] After obtaining the required metasurface device, circularly polarized light is incident on the first working wavelength and the second working wavelength respectively. After reflection from the metasurface device, a meaningless binary holographic image is obtained in the far field. Only when circularly polarized light with a first rotation direction at the first working wavelength and circularly polarized light with a second rotation direction at the second working wavelength are simultaneously incident on the metasurface and reflected from the metasurface device, the encrypted image is obtained by superposition in the far field.
[0030] Among them, see Figure 1, an xoy coordinate system is established with the directions of the two sides parallel to the working surface of the substrate set as the x-axis and y-axis respectively, the nanobrick is a rectangular parallelepiped structure, the major axis and minor axis of the nanobrick are both parallel to the working surface of the substrate, and the lengths of the major axis and the minor axis are different; the steering angle θ of the nanobrick is the angle between the major axis of the nanobrick and the x-axis; the working surface of the substrate unit structure is a square with a side length of C; the two sizes of nanobricks have the same height H, different lengths L, and different widths W.
[0031] The two sizes of nanobricks are designated as first and second nanobricks, respectively. The first nanobrick's structural dimensions include a first length L1, a first width W1, and a height H. The second nanobrick's structural dimensions include a second length L2, a second width W2, and a height H. These dimensional parameters were optimized through electromagnetic simulation based on the two selected operating wavelengths.
[0032] Specifically, when determining the geometric dimensions of the metasurface device, the height H of the nanobrick and the side length C of the working surface of the substrate unit structure are first determined, and then the length and width dimension parameters of the nanobrick are scanned at the first working wavelength and the second working wavelength respectively through electromagnetic simulation to obtain the transmission phase scanning results at the two working wavelengths; finally, two groups of dimensions are selected as the determined dimension parameters of the nanobrick based on the transmission phase scanning results at the two working wavelengths, and the transmission phase difference of the two selected nanobrick sizes at the first working wavelength is π / 2, and the transmission phase difference of the two nanobrick sizes at the second working wavelength is also π / 2, that is, the transmission phase difference of the two nanobricks at a single working wavelength is π / 2, and the phase shift caused by the change in wavelength to the nanobricks of the two sizes is equal; in addition, the nanobricks of the two sizes have different transmission phase characteristics, which can be seen in Figure 2 .
[0033] In a preferred solution, when determining the geometric dimensions of the metasurface device, the length and width dimension parameters of the nanobrick are scanned respectively at the first working wavelength and the second working wavelength through electromagnetic simulation to obtain the polarization conversion efficiency scanning results at the two working wavelengths; based on the transmission phase scanning results and the polarization conversion efficiency scanning results at the two working wavelengths, two sets of dimensions that meet the transmission phase requirements and have the highest polarization conversion efficiency are selected as the determined dimensional parameters of the nanobrick.
[0034] After determining the geometric dimensions of the metasurface device, a base unit structure and a nanobrick located on its working surface are used as a pixel. The phase of the reflected light from the metasurface device is modulated using the nanobrick's steering angle and the difference in transmission phase between two sizes of nanobricks. The size and steering angle of the nanobrick on each base unit structure are determined based on the encrypted image. Specifically, the size of each nanobrick in the nanobrick array is determined, along with the steering angle of each nanobrick.
[0035] The Jones matrix J of the nanobrick with a steering angle of θ is expressed as: The Jones vector of circularly polarized light is expressed as: The circularly polarized light reflected by the nanobrick with a steering angle of θ is expressed as: where A and B are coefficients related to the reflectivity in the major and minor axis directions of the nanobrick, respectively.
[0036] It can be seen that the reflected light consists of a co-circularly polarized component and an inversely polarized component. The phase of the inversely polarized component can be controlled by the nanobrick's steering angle θ. Furthermore, when the nanobrick is designed as an equivalent half-wave plate, A=-B is satisfied. In this case, the reflected light contains only the inversely polarized component, resulting in the highest polarization conversion efficiency.
[0037] The sizes of the base unit structure and the nano bricks are both sub-wavelength.
[0038] In terms of materials, the nanobricks can be made of single-crystal silicon, and the substrate can be made of a single-crystal silicon lower layer and a fused silica upper layer. Because the metasurface device designed in this invention operates in a reflective mode, and the single-crystal silicon + fused silica + single-crystal silicon material exhibits high efficiency in this mode, the above-mentioned preparation method is preferred.
[0039] The dual-wave encryption design method based on a minimalist metasurface provided in Example 1 can realize holographic encryption display at dual wavelengths by using only a minimalist metasurface (i.e., a metasurface device) containing two nanobricks of different sizes.
[0040] The present invention will be further described below.
[0041] When light of any polarization state is incident and passes through the minimalist metasurface, the phase of the reflected light is modulated. By optimizing the arrangement of two sizes of nanobricks and the steering angle θ of the minimalist metasurface, at the first operating wavelength λ1, when left-handed circularly polarized light passes through the minimalist metasurface, the phase of the reflected light is modulated, resulting in a meaningless binary image in the far field. At the second operating wavelength λ2, when right-handed circularly polarized light passes through the minimalist metasurface, the phase of the reflected light is modulated, resulting in another meaningless binary image in the far field. Only when left-handed circularly polarized light at the first operating wavelength λ1 and right-handed circularly polarized light at the second operating wavelength λ2 are simultaneously incident on the minimalist metasurface, an encrypted image is superimposed in the far field. That is, the design method of the present invention can achieve dual-wavelength encrypted display.
[0042] It should be noted that the above description is given by taking left-handed circularly polarized light incident at the first operating wavelength and right-handed circularly polarized light incident at the second operating wavelength as examples, but is not limited to the above options.
[0043] Taking the first working wavelength λ1 = 550nm and the second working wavelength λ2 = 633nm as an example, electromagnetic simulation software is used for modeling and simulation. With left-handed circularly polarized light incident vertically, the structural parameters of the nano-unit are scanned at the working wavelength, including the structural dimensions of the first nano-brick: L1, W1, H, the structural dimensions of the second nano-brick: L2, W2, H, and the working surface dimension C of the base unit structure. The scanning results of the transmission phase are shown in Figure 2. Figure 2 As shown, Figure 2 (a) is the transmission phase scan of the nanobrick at a wavelength of 550nm. It represents the transmission phase result of scanning at 550nm wavelength. Figure 2 (b) is the transmission phase scan of the nanobrick at a wavelength of 633nm. It shows the transmission phase results of scanning at 633nm wavelength.
[0044] Under the premise of ensuring that the transmission phase difference of the two sizes of nanobricks at a single working wavelength is π / 2, the left-handed circularly polarized light is incident vertically, and the proportion of the right-handed circularly polarized light component in the reflected light field is optimized by the nanobricks to maximize the polarization conversion efficiency (that is, in the preferred solution, several groups of structures are selected under the premise of ensuring the phase difference, and the two structures with the highest polarization conversion efficiency are selected as the final selected structures). The scanning results are as follows Figure 3 As shown, Figure 3 (a) is the polarization conversion efficiency scan of the nanobrick at a wavelength of 550nm, E f1 It represents the polarization conversion efficiency result of scanning at 550nm wavelength. Figure 3 (b) is a scanning diagram of the polarization conversion efficiency of nanobricks at a wavelength of 633 nm, E f2It shows the polarization conversion efficiency results scanned at 633nm wavelength.
[0045] In this example, the final structural parameters include: L1 = 250 nm, W1 = 105 nm, H = 220 nm; L2 = 295 nm, W2 = 125 nm, C = 400 nm.
[0046] It should be noted that the calculation results are the same for simulations using left-handed or right-handed circularly polarized light. For left-handed circularly polarized light, the right-handed circularly polarized light component is extracted, while for right-handed circularly polarized light, the left-handed circularly polarized light component is extracted. The simulation examples above use left-handed circularly polarized light as an example.
[0047] When circularly polarized light is incident on the minimalist metasurface, the phase of the reflected light is modulated. When left-handed circularly polarized light at 633nm is incident on the minimalist metasurface, a meaningless binary image is obtained in the far field; when right-handed circularly polarized light at 550nm is incident on the minimalist metasurface, another meaningless binary image is obtained in the far field. When two light beams are incident at the same time, the two binary images in the far field are superimposed to obtain an encrypted image. That is, the design method of the present invention can realize dual-wave encryption based on the minimalist metasurface, and the effect is as follows: Figure 4 shown.
[0048] Example 2:
[0049] Example 2 provides a metasurface device, which is obtained based on the dual-wave encryption design method based on the minimalist metasurface as described in Example 1. The metasurface device provided in Example 2 includes a substrate and a nanobrick array located on a working surface of the substrate, wherein the nanobrick array is composed of nanobricks of two sizes; circularly polarized light is incident at a first working wavelength and a second working wavelength, respectively, and after reflection from the metasurface device, a meaningless binary holographic image is obtained in the far field; only when circularly polarized light with a first rotation direction at the first working wavelength and circularly polarized light with a second rotation direction at the second working wavelength are simultaneously incident on the metasurface, after reflection from the metasurface device, an encrypted image is obtained by superposition in the far field.
[0050] Since the device provided in Example 2 corresponds to the design method provided in Example 1, Example 2 can be understood by referring to the description of Example 1, and will not be described in detail here.
[0051] Finally, it should be noted that the above specific implementation methods are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.
Claims
1. A dual-wave encryption design method based on a minimalist metasurface, characterized in that: The following steps are involved: Constructing the basic structure of a metasurface device, wherein the metasurface device is a minimalist metasurface and comprises a substrate and a nanobrick array located on a working surface of the substrate; the nanobrick array comprises nanobricks of two sizes; the substrate is divided into a plurality of substrate unit structures of uniform size, each of which is provided with a nanobrick; Selecting a first operating wavelength, a second operating wavelength, and an encrypted image; determining the geometric dimensions of the metasurface device, as well as the size arrangement and steering angle arrangement of a plurality of nanobricks in the nanobrick array based on the two selected operating wavelengths and the encrypted image, to obtain the desired metasurface device; After obtaining the desired metasurface device, circularly polarized light is incident on the metasurface at the first operating wavelength and the second operating wavelength, respectively. After reflection from the metasurface device, a meaningless binary holographic image is obtained in the far field. Only when circularly polarized light with a first handedness at the first operating wavelength and circularly polarized light with a second handedness at the second operating wavelength are simultaneously incident on the metasurface, after reflection from the metasurface device, the encrypted image is obtained by superposition in the far field. An xoy coordinate system is established with the directions of two sides parallel to the working surface of the substrate as the x-axis and the y-axis respectively. The nanobrick is a rectangular parallelepiped structure, with the major axis and minor axis of the nanobrick both parallel to the working surface of the substrate, and the major axis and the minor axis are of different lengths. The turning angle of the nanobrick is the angle between the major axis of the nanobrick and the x-axis. The working surface of the substrate unit structure is square. The two sizes of nanobricks have the same height, different lengths, and different widths. When determining the geometric dimensions of the metasurface device, the height of the nanobrick and the side length of the working surface of the base unit structure are first determined, and then the length and width dimension parameters of the nanobrick are scanned at the first working wavelength and the second working wavelength respectively through electromagnetic simulation to obtain the transmission phase scanning results at the two working wavelengths; finally, two groups of sizes are selected as the determined size parameters of the nanobrick based on the transmission phase scanning results at the two working wavelengths, and the transmission phase difference of the two selected nanobrick sizes at the first working wavelength is The transmission phase difference of the two sizes of nanobricks at the second working wavelength is also .
2. The dual-wave encryption design method based on a minimalist metasurface according to claim 1 is characterized in that: When determining the geometric dimensions of the metasurface device, it also includes: scanning the length and width dimension parameters of the nanobrick at the first working wavelength and the second working wavelength respectively through electromagnetic simulation to obtain the polarization conversion efficiency scanning results at the two working wavelengths; based on the transmission phase scanning results and the polarization conversion efficiency scanning results at the two working wavelengths, selecting two sets of dimensions that meet the transmission phase requirements and have the highest polarization conversion efficiency as the determined dimensional parameters of the nanobrick.
3. The dual-wave encryption design method based on a minimalist metasurface according to claim 1 is characterized in that: After determining the geometric dimensions of the metasurface device, one of the substrate unit structures and a nanobrick located on its working surface is taken as a pixel point. The phase of the reflected light passing through the metasurface device is modulated using the steering angle of the nanobrick and the transmission phase difference between two sizes of nanobricks. The size and steering angle of the nanobrick on each substrate unit structure are determined based on the encrypted image.
4. The dual-wave encryption design method based on a minimalist metasurface according to claim 3 is characterized in that: The steering angle is θ Jones matrix of nanobricks J Expressed as: , the Jones vector of circularly polarized light is expressed as: , the circularly polarized light is turned at an angle of θ The nanobricks are expressed after reflection as: ,in, A and B are the coefficients related to the reflectivity in the major and minor axis directions of the nanobricks, respectively.
5. The dual-wave encryption design method based on a minimalist metasurface according to claim 4 is characterized in that: When the nanobrick is designed as an equivalent half-wave plate, , the reflected light only contains the reverse circular polarization component.
6. The dual-wave encryption design method based on a minimalist metasurface according to claim 1 is characterized in that: The sizes of the base unit structure and the nano bricks are both sub-wavelength.
7. The dual-wave encryption design method based on a minimalist metasurface according to claim 1 is characterized in that: The nano bricks are made of single crystal silicon material, and the substrate is made of single crystal silicon material as a lower layer and fused quartz glass material as an upper layer.
8. A metasurface device, characterized in that: The metasurface device is obtained based on the dual-wave encryption design method based on the minimalist metasurface as described in any one of claims 1 to 7; the metasurface device includes a substrate and a nanobrick array located on the working surface of the substrate, and the nanobrick array is composed of nanobricks of two sizes; circularly polarized light is incident at a first working wavelength and a second working wavelength respectively, and after reflection by the metasurface device, a meaningless binary holographic image is obtained in the far field; only when circularly polarized light with a first rotation direction at the first working wavelength and circularly polarized light with a second rotation direction at the second working wavelength are simultaneously incident on the metasurface, after reflection by the metasurface device, an encrypted image is obtained by superposition in the far field.