Polarization multiplexing four-channel reconfigurable terahertz holographic encryption metasurface based on vanadium dioxide

Abstract

The invention provides a polarization multiplexing four-channel reconfigurable terahertz holographic encryption metasurface based on vanadium dioxide. A unit structure comprises three layers, wherein the bottom layer is a metal copper reflection substrate, the middle layer is a polyimide dielectric layer, and the top layer is a periodic pattern structure composed of a metal copper and vanadium dioxide composite material. According to the invention, the metal-insulation state conversion of vanadium dioxide is controlled through temperature, and polarization multiplexing function reconstruction is realized in combination with a transmission phase regulation and control principle. When vanadium dioxide is in an insulated state, x-polarization incident electromagnetic waves can generate a holographic pattern A, and y-polarization incident electromagnetic waves can generate a holographic pattern B; when vanadium dioxide is converted into a metal state, x polarization incidence corresponds to a holographic pattern C, and y polarization incidence corresponds to a holographic pattern D. According to the metasurface, independent regulation and control and four-state holographic function switching of a dual-polarization channel are achieved through a single physical excitation source, the function singleness limitation of a traditional metasurface is broken through through the composite structure design, and the integration level and scene adaptability of a reconfigurable device are remarkably improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metamaterial electromagnetic wave regulation, and relates to a reconfigurable reflective metasurface, in particular to a polarization multiplexing four-channel reconfigurable terahertz holographic encryption metasurface based on vanadium dioxide, which is suitable for dynamic holographic encryption communication, multi-user independent decryption and anti-interception terahertz information system. BACKGROUND

[0002] A metasurface is a two-dimensional artificial electromagnetic material composed of periodically arranged subwavelength unit structures. By designing the geometric parameters and material properties of the unit structure, the phase, amplitude and polarization state of electromagnetic waves can be efficiently regulated. Reflective metasurfaces have important applications in beamforming, holographic imaging and other fields due to their high regulation efficiency and compact structure. Traditional reflective metasurfaces mostly use fixed metal structures, which are single-function and lack dynamic reconfiguration capability. In recent years, the introduction of phase change materials (such as vanadium dioxide) has provided a new way for dynamic regulation of metasurfaces, but the functional reconfiguration dimension is limited, making it difficult to achieve coordinated control of polarization multiplexing and multi-modal switching.

[0003] In the prior art, a polarization control coherent perfect absorber based on multi-layer metamaterial is disclosed in patent CN114267959A, which realizes independent wave absorption characteristics of X / Y polarized waves through asymmetric metal split ring structure, but its function is limited to static wave absorption and cannot dynamically switch modes. In addition, existing reconfigurable metasurfaces mostly rely on single regulation mechanism (such as electrical control or temperature control), resulting in serious polarization channel coupling. For example, when using vanadium dioxide to realize function switching, the asymmetry of X / Y polarization response is insufficient, making it difficult to independently generate different holographic patterns; while using electrical control materials such as graphene, the high-frequency loss is large, limiting its application in the terahertz frequency band. The above defects make it difficult for existing technology to meet the needs of multi-scene dynamic holographic display and multi-channel encryption communication. SUMMARY

[0004] The purpose of the present application is to overcome the problems of single function and low polarization regulation freedom of existing reconfigurable metasurfaces, and to propose a polarization multiplexing four-channel reconfigurable terahertz holographic encryption metasurface based on vanadium dioxide. This scheme combines phase encoding encryption and polarization-state double-key mechanism reconfigurable reflective metasurface technology. The metasurface has a linear polarization multifunctional reconfigurable reflective function based on vanadium dioxide phase change regulation, and can realize four-state holographic function switching through composite structure design and double-polarization independent regulation mechanism.

[0005] To achieve the above object, the technical scheme adopted by the present application is as follows: A kind of polarization multiplexing four-channel reconfigurable terahertz holographic encryption super surface based on vanadium dioxide, including multiple super surface units periodically arranged in three-dimensional coordinate system OXY plane;The super surface unit includes three-layer structure: bottom layer is copper reflection base, middle layer is polyimide dielectric layer, top layer is the resonant layer composed of copper and vanadium dioxide composite pattern;The pattern of the resonant layer includes copper cross structure located in the center of unit, vanadium dioxide connecting part that copper cross structure end and four edges of unit contact and the square area of vanadium dioxide in four corners of unit;By temperature control metal-insulating state conversion of vanadium dioxide, polarization multiplexing function switching is realized in combination with transmission phase control principle: When vanadium dioxide is in insulating state: x polarized incident electromagnetic wave generates 0°, 90°, 180°, 270° phase gradient by adjusting the length of horizontal bar of copper cross structure, to form first holographic pattern; y polarized incident electromagnetic wave generates 0°, 90°, 180°, 270° phase gradient by adjusting the length of vertical bar of copper cross structure, to form second holographic pattern; When vanadium dioxide is in metal state: x polarized incident electromagnetic wave generates 0°, 90°, 180°, 270° phase gradient by adjusting the spacing between two vanadium dioxide patches in square area of vanadium dioxide in four corners, to form third holographic pattern; y polarized incident electromagnetic wave generates 0°, 90°, 180°, 270° phase gradient by adjusting the spacing between two vanadium dioxide patches in square area of vanadium dioxide in four corners, to form fourth holographic pattern.

[0006] Further, the first holographic pattern is letter A, and the second holographic pattern is letter B;The third holographic pattern is letter C, and the fourth holographic pattern is letter D.

[0007] Further, when forming first holographic pattern A, the length of horizontal bar of copper cross structure corresponding to 0°, 90°, 180°, 270° phase is 72 μm, 30 μm, 95 μm, 82 μm respectively; When forming second holographic pattern B, the length of vertical bar of copper cross structure corresponding to 0°, 90°, 180°, 270° phase is 72 μm, 30 μm, 95 μm, 82 μm respectively; When forming third holographic pattern C, the spacing between two vanadium dioxide patches in square area of vanadium dioxide in four corners corresponding to 0°, 90°, 180°, 270° phase is 70 μm, 30 μm, 78 μm, 74 μm respectively; When the fourth holographic pattern D is formed, the parameters of the interval between the two vanadium dioxide patches in the four-corner vanadium dioxide square region corresponding to the phases of 0°, 90°, 180° and 270° are 70 μm, 30 μm, 78 μm and 74 μm respectively.

[0008] Further, the thickness of the metal copper reflective substrate is 2 μm, and the surface roughness is less than 50 nm; the thickness of the polyimide dielectric layer is 30 μm, and the dielectric constant is 3.5; the side length of the metasurface unit is 100 μm.

[0009] Further, the line width of the metal copper cross-shaped structure is 15 μm, and the end is directly connected with the vanadium dioxide connection part without seamless connection; the vanadium dioxide connection part is a rectangular region with a width of 15 μm, covering the center segment of the four edges of the unit; the vanadium dioxide square region in the four corners of the unit has a side length of 25 μm to 33 μm.

[0010] The phase distribution of the holographic patterns A, B, C and D is jointly optimized by the following parameters: The mapping relationship between the length of the horizontal / vertical bar of the metal copper cross-shaped structure and the phase gradient (insulating state) is as follows: 0° (72 μm), 90° (30 μm), 180° (95 μm) and 270° (82 μm). The mapping relationship between the interval between the two vanadium dioxide patches in the four-corner vanadium dioxide region and the phase gradient (metallic state) is as follows: 0° (70 μm), 90° (30 μm), 180° (78 μm) and 270° (74 μm).

[0011] Further, the contact interface between the vanadium dioxide connection part and the metal copper cross-shaped structure is a rectangular transition structure for suppressing the edge scattering effect in the 0.8 terahertz frequency band.

[0012] Further, the conductivity of the vanadium dioxide connection part and the vanadium dioxide square region in the corner of the unit is 200 S / m in the insulating state, and the conductivity is 2×10 5 S / m in the metallic state, and the phase change response time is less than 1 μs.

[0013] Further, the vanadium dioxide connection part and the vanadium dioxide square region in the corner of the unit are co-planarly integrated with the metal copper cross-shaped structure through a nanoimprint process, and the etching precision error is less than 0.5 μm.

[0014] Further, the working frequency band of the metasurface is 0.8 terahertz, and the ratio relationship between the unit size and the wavelength is λ / 3 to λ / 4, where λ=375 μm.

[0015] Compared with the prior art, the present application has the following advantages: Four-state holographic function dynamic switching: Through the design of vanadium dioxide phase change and copper / vanadium dioxide composite structure, independent regulation of the dual-polarization channel in two states is realized, supporting X / Y polarized waves to generate four holographic patterns, which breaks through the limitation of single function of traditional super surface.

[0016] High-precision phase control: Based on the geometric parameter-phase gradient mapping relationship of copper cross and vanadium dioxide area (such as 72 mu m copper horizontal bar corresponding to 0 degree phase), the accuracy and consistency of holographic imaging in the terahertz band are ensured.

[0017] High frequency compatibility: The unit size (100 mu m) and working wavelength (lambda=375 mu m) are strictly matched, and the medium layer thickness (30 mu m) and trapezoidal transition structure design effectively suppress high frequency loss and edge scattering, which is suitable for 0.8 terahertz communication system.

[0018] Process scalability: Through nanoimprint process, submicron etching accuracy (error<0.5 mu m) is realized, which ensures the uniformity of large-scale array preparation, and is suitable for dynamic display and encryption label scenes.

[0019] The super surface of the application realizes independent regulation of the dual-polarization channel and four-state holographic function switching through a single physical excitation source, significantly improves the integration and scene adaptability of the reconfigurable device, and has important application value in the fields of dynamic holographic display, multi-channel information encryption, etc. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 It is a schematic diagram of the dynamic switchable super surface hologram of the application; Figure 2 It is an electric field diagram of the super surface resonant layer VO2 in metal state and insulating state respectively; Figure 3 It is a three-dimensional structure decomposition of the super surface unit and a surface schematic diagram of the whole super surface unit; 1-metal copper reflection base, 2-polyimide dielectric layer, 3-resonant layer, 31-metal copper cross structure, 32-vanadium dioxide connecting part, 33-vanadium dioxide square area; Figure 4 It is a phase distribution of the super surface after selecting letters "A", "B", "C" and "D" as objective images of super surface hologram and GS algorithm iteration; Figure 5 It is a four-corner vanadium dioxide width-phase gradient mapping relationship diagram under X and Y polarization incidence when vanadium dioxide is in metal state and insulating state respectively; Figure 6 It is the parameter value corresponding to the four phase gradients of vanadium dioxide in insulating state and metal state in 0.8 terahertz band; Figure 7A comparison chart of simulated 3D far-field results of holographic images and theoretical calculation results of far field; Figure 8 An effect chart of far-field holographic imaging at different frequencies; Figure 9 A strategy verification of holographic encryption and decryption. DETAILED DESCRIPTION

[0021] In order to more clearly understand the above-mentioned purposes, features and advantages of the present application, the schemes of the present application will be further described below. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0022] In the description, it should be noted that the terms "first", "second" are only for the purpose of description, and cannot be understood as indicating or implying relative importance. It should be noted that, unless otherwise explicitly specified and limited, the terms "mounting", "connection", "connection" should be understood in a broad sense, for example, it can be fixedly connected, or it can be detachably connected, or integrally connected; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium, and it can be the communication inside two elements. For those skilled in the art, the specific meanings of the above terms can be understood according to the specific circumstances.

[0023] In the following description, many specific details are set forth in order to provide a thorough understanding of the present application, but the present application can also be implemented in other ways different from those described herein; obviously, the examples in the specification are only some of the embodiments of the present application, not all the embodiments.

[0024] The present application provides a method for realizing terahertz encryption metasurface based on vanadium dioxide (VO2) phase change material. The essence of the method is to use a special "smart surface" to control temperature and the polarization direction of terahertz waves as two independent "keys", thereby dynamically generating four different holographic images on a single device to achieve high-security information encryption.

[0025] The implementation process first designs the exquisite structure of the metasurface unit. The bottom layer is a metal reflection layer, the middle layer is a dielectric substrate, and the top layer integrates vanadium dioxide material and metal microstructure. The characteristic of vanadium dioxide is that its state changes with temperature: at low temperature, it is like an insulator, and at high temperature, it changes into a conductor. This phase change characteristic makes it a natural "temperature key". By accurately designing the geometric shape of the top layer structure, the surface can produce unique electromagnetic responses to terahertz waves of different polarization directions (such as vertical or horizontal direction vibration) in each state (insulation or metal), thereby laying the foundation for the "polarization key".

[0026] Next, the target images (e.g. four different letters) are phase-encoded. By a special iterative algorithm (Gerchberg-Saxton algorithm), the bright-dark information of each target image is calculated and converted into a specific "phase distribution map" that can reconstruct the image. Then, the calculated continuous phase distribution is accurately mapped to the physical structure size of the metasurface unit under different "temperature-polarization" combinations. This means that for the "low temperature + vertical polarization" key, the corresponding metasurface structure will be carved into a specific pattern, ensuring that only when illuminated with this key, the preset "A" image can be correctly displayed. Similarly, the other three keys (low temperature + horizontal polarization, high temperature + vertical polarization, high temperature + horizontal polarization) also uniquely correspond to the structure parameters of "B", "C", "D" images respectively.

[0027] Finally, the entire design is functionally verified by electromagnetic simulation. A model of an array composed of 50x50 units is built in a computer using matlab and cst joint modeling, and the far-field imaging effect under different temperature states and different polarized terahertz wave illumination is simulated. The simulation results confirm that by switching the four combinations of the two keys, four preset high-quality target images can be clearly reproduced in the far field, and each channel does not interfere with each other, thus realizing the four-channel dynamic encryption function.

[0028] In summary, the technical scheme successfully realizes the dynamic switching and reading of four encrypted information on a single planar device by deeply integrating the global thermal phase change characteristics of vanadium dioxide with the polarization multiplexing principle of electromagnetic waves, providing a novel and efficient hardware solution for high-security communication and information encryption in the terahertz band.

[0029] The specific embodiments of the present application will be described in detail below with reference to the accompanying drawings. Figures 1 to 8 The specific embodiments of the present application will be described in detail below with reference to the accompanying drawings.

[0030] Figure 1 In the above, (a) is the hologram of letters "A" and "B" respectively displayed as 90 °C linearly polarized reflected waves under X-polarized wave and Y-polarized wave incidence at a temperature of 25 °C. (b) is the hologram of letters "C" and "D" respectively displayed as 90 °C linearly polarized reflected waves under X-polarized wave and Y-polarized wave incidence at a temperature of 68 °C.

[0031] Figure 2 (a) is the electric field diagram of the metasurface when VO2 is in the metallic state, and (b) is the electric field diagram of the metasurface when VO2 is in the insulating state.

[0032] Figure 3(a) Showing the copper cross-shaped structure, vanadium dioxide connector, and vanadium dioxide square region on the resonant layer; (b) Showing the size adjustment of each region or structure on the resonant layer; (c) Showing the stacking relationship of the copper reflective substrate, polyimide dielectric layer, and top resonant layer; (d) Showing the surface structure of the metasurface unit; Based on the "phase distribution map" that can reconstruct the image obtained through a specialized iterative algorithm, the calculated continuous phase distribution is precisely mapped to the physical structure size of the metasurface unit under different "temperature-polarization" combinations, thereby adjusting the corresponding size of the spacing between the copper cross-shaped structure and the vanadium dioxide square region to ensure that the metasurface unit can reflect a specific pattern when electromagnetic waves are incident; By combining electromagnetic waves and temperature, multiple patterns can be obtained respectively. In Figure (d), it can be seen that the size of the spacing between the copper cross-shaped structure and the vanadium dioxide square region at different positions is different, which is the result of the precise mapping of the physical structure size under different "temperature-polarization" combinations.

[0033] Combination Figure 3 (c) Table 1 shows the parameter values ​​for the metasurface unit structure:

[0034] Depend on Figure 4 It can be seen that the metasurface four-dimensional far-field holographic image calculated by GB simulation is very close to the target pattern.

[0035] Figure 5 This is a "key-phase-structure parameter" mapping table. The horizontal columns are key combinations (insulating state-x, insulating state-y, metallic state-x, metallic state-y), and the vertical columns are four-step phases. The table content is the corresponding feature size, labeled with the unit (μm).

[0036] Figure 6 The parameter values ​​corresponding to the four phase gradients in the vanadium dioxide insulating and metallic states are shown respectively. A phase difference of 90° was achieved at 0.8 THz, thus the unit cell array on the metasurface can cover a phase of 360°.

[0037] Figure 7 In the middle, the simulated 3D far-field results of (ad) holographic images have Figure 4 The phase difference distribution in the image. The electric field intensity of the holographic image obtained at z = 50 mm. The theoretical calculation results of the 3D far field of the holographic image (il) show that the actual simulation results are very close to the theoretical calculation results.

[0038] Figure 8 (a) shows the simulation results at different frequencies in the metallic state, and (b) shows the simulation results at different frequencies in the insulating state.

[0039] Figure 9 The message from the reconnaissance aircraft is encrypted on a hyper surface of fighters (AC), ships (BD), camps (AD), etc. The same hyper surface is sent to the information recipient. Each recipient has its own custom key, which includes temperature and polarization information. The recipients obtain different hologram combinations according to the custom key, and then continue to decrypt the respective message. The fighters interpret the information as "initiate attack", the ships interpret the information as "stealth", and the camps interpret the information as "specific enemy location".

[0040] The above examples are to be understood only as illustrative of the present application and not restrictive of the scope of protection of the present application. After reading the description of the present application, those skilled in the art can make various modifications or changes to the present application, and these equivalent changes and modifications also fall within the scope defined by the claims of the present application.

[0041] The above description is merely a specific implementation of the present application, enabling those skilled in the art to understand or implement the present application. Although the foregoing embodiments are described in detail, those skilled in the art should understand that modifications or changes can be made to the technical solutions described in the foregoing embodiments, or some or all of the technical features can be replaced by equivalents; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments, and should be covered in the protection scope of the claims.

Claims

1. A vanadium dioxide-based polarization-multiplexed four-channel reconfigurable terahertz holographic encryption metasurface, comprising multiple metasurface units periodically arranged along a three-dimensional coordinate system OXY plane; characterized in that, The metasurface unit comprises a three-layer structure: a bottom layer is a copper reflective substrate (1), a middle layer is a polyimide dielectric layer (2), and a top layer is a resonant layer (3) composed of a composite pattern of copper and vanadium dioxide. The pattern of the resonant layer (3) includes a copper cross-shaped structure (31) at the center of the unit, vanadium dioxide connecting parts (32) at the ends of the copper cross-shaped structure (31) that contact the four sides of the unit, and vanadium dioxide square areas (33) at the four corners of the unit. The polarization multiplexing function is switched by controlling the metal-insulating state transition of vanadium dioxide through temperature control and combining the transmission phase modulation principle. When vanadium dioxide is in an insulating state: x-polarized incident electromagnetic waves generate 0°, 90°, 180°, and 270° phase gradients by adjusting the length of the horizontal bars of the copper cross-shaped structure (31), forming the first holographic pattern; The y-polarized incident electromagnetic wave generates phase gradients of 0°, 90°, 180°, and 270° by adjusting the length of the vertical bars of the copper cross-shaped structure (31), thus forming a second holographic pattern; When vanadium dioxide is in the metallic state: The x-polarized incident electromagnetic wave generates phase gradients of 0°, 90°, 180°, and 270° respectively by adjusting the spacing between two vanadium dioxide patches in the square vanadium dioxide region (33) at the four corners, thus forming a third holographic pattern; The y-polarized incident electromagnetic wave generates phase gradients of 0°, 90°, 180°, and 270° respectively by adjusting the spacing between two vanadium dioxide patches in the four-corner vanadium dioxide square region (33), thus forming the fourth holographic pattern.

2. The vanadium dioxide-based polarization multiplexed four-channel reconfigurable terahertz holographic encryption metasurface as described in claim 1, characterized in that, The first holographic pattern is the letter A, the second holographic pattern is the letter B, the third holographic pattern is the letter C, and the fourth holographic pattern is the letter D.

3. The vanadium dioxide-based polarization multiplexed four-channel reconfigurable terahertz holographic encryption metasurface as described in claim 2, characterized in that, When the first holographic pattern A is formed, the lengths of the horizontal bars of the copper cross-shaped structure (31) corresponding to the 0°, 90°, 180° and 270° phases are 72μm, 30μm, 95μm and 82μm, respectively; When the second holographic pattern B is formed, the lengths of the vertical bars of the copper cross-shaped structure (31) corresponding to the 0°, 90°, 180° and 270° phases are 72μm, 30μm, 95μm and 82μm, respectively; When the third holographic pattern C is formed, the parameters of the spacing between the two vanadium dioxide patches in the square vanadium dioxide region (33) at the four corners, corresponding to the phases of 0°, 90°, 180° and 270°, are 70μm, 30μm, 78μm and 74μm, respectively. When the fourth holographic pattern D is formed, the parameters of the spacing between the two vanadium dioxide patches in the four corner vanadium dioxide square region (33) corresponding to the phases of 0°, 90°, 180° and 270° are 70μm, 30μm, 78μm and 74μm, respectively.

4. The vanadium dioxide-based polarization multiplexed four-channel reconfigurable terahertz holographic encryption metasurface according to any one of claims 1-3, characterized in that: The thickness of the copper reflective substrate (1) is 2 μm and the surface roughness is less than 50 nm; the thickness of the polyimide dielectric layer (2) is 30 μm and the dielectric constant is 3.5; the side length of the metasurface unit is 100 μm.

5. The vanadium dioxide-based polarization multiplexed four-channel reconfigurable terahertz holographic encryption metasurface according to claim 4, characterized in that: The line width of the copper cross-shaped structure (31) is 15 μm, and its end is directly and seamlessly connected to the vanadium dioxide connection part (32); the vanadium dioxide connection part (32) is a rectangular area with a width of 15 μm, covering the central section of the four sides of the unit; the vanadium dioxide square area (33) at the four corners of the unit has a side length of 25 μm to 33 μm.

6. The vanadium dioxide-based polarization multiplexed four-channel reconfigurable terahertz holographic encryption metasurface according to any one of claims 1-3, characterized in that: The contact interface between the vanadium dioxide connector (32) and the copper cross-shaped structure (31) is a rectangular transition structure, which is used to suppress the edge scattering effect in the 0.8 terahertz band.

7. The vanadium dioxide-based polarization multiplexed four-channel reconfigurable terahertz holographic encryption metasurface according to any one of claims 1-3, characterized in that: The vanadium dioxide connector (32) and the vanadium dioxide square region (33) at the corner of the unit have an electrical conductivity of 200 S / m in the insulating state and 2 × 10⁻⁶ S / m in the metallic state. 5 S / m, phase change response time less than 1μs.

8. The vanadium dioxide-based polarization multiplexed four-channel reconfigurable terahertz holographic encryption metasurface according to any one of claims 1-3, characterized in that: The vanadium dioxide connector (32) and the vanadium dioxide square area (33) at the corner of the unit are integrated with the copper cross-shaped structure (31) through nanoimprinting process, with an etching accuracy error of less than 0.5 μm.

9. The vanadium dioxide-based polarization multiplexed four-channel reconfigurable terahertz holographic encryption metasurface according to any one of claims 1-3, characterized in that: The metasurface operates at a frequency of 0.8 terahertz, and the ratio of unit size to wavelength is λ / 3 to λ / 4, where λ = 375 μm.

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