A multi-channel bidirectional holographic metasurface for orbital angular momentum encryption communication

Abstract

The application provides a multi-channel bidirectional holographic metasurface for orbital angular momentum encryption communication, and belongs to the field of wireless communication and information security, and comprises a feed antenna for generating linearly polarized incident electromagnetic waves, a vortex wave generating metasurface arranged in the radiation direction of the feed antenna and formed by a plurality of polarization selection metasurface units arranged according to an orbital angular momentum phase distribution diagram, and used for converting the linearly polarized electromagnetic waves into vortex electromagnetic waves carrying preset orbital angular momentum modes, and a holographic metasurface arranged in parallel with the vortex wave generating metasurface and formed by a plurality of polarization selection metasurface units arranged according to a holographic amplitude and phase distribution diagram, and used for receiving the vortex electromagnetic waves and reconstructing a holographic image; the holographic metasurface also has bidirectional working capability, and is switched to a forward or reverse working state by being flipped by 180 DEG around a fixed axis, so that the holographic image can be reconstructed under bidirectional incidence. The application realizes safe and high-capacity multi-channel bidirectional holographic encryption imaging in the microwave frequency band.

Description

Technical Field

[0001] This invention belongs to the field of wireless communication and information security, and specifically relates to a multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication. Background Technology

[0002] With the development of microwave communication and electromagnetic imaging technologies towards high capacity and high security, metasurface-based holographic manipulation and information encryption technologies have attracted widespread attention. Metasurface holography can accurately encode the phase and amplitude of electromagnetic waves through subwavelength units, enabling the reconstruction of target images and showing significant application prospects in covert imaging, secure communication, and information storage. Currently, to enhance the security of metasurface holography, a "physical key channel" is typically introduced as an encryption dimension, such as traditional electromagnetic properties like incident polarization, operating frequency, beam direction, and incident angle. However, the degrees of freedom relied upon by these encryption methods are still mainly concentrated in limited dimensions such as polarization, frequency, and propagation direction, resulting in a relatively limited key space and vulnerability to exhaustive attacks or traditional analysis methods. Therefore, there is an urgent need to explore new, higher-dimensional physical encryption channels.

[0003] Orbital angular momentum (OAM) vortex waves, due to their helical phase structure and modal orthogonality, are considered a potential high-dimensional encryption carrier, capable of significantly expanding information capacity and key space. However, in the field of microwave metasurface holography, research on OAM encryption is still in its early stages, and related implementation methods are immature, especially lacking a system design framework for independent encryption mechanisms for OAM modes. Although existing research has proposed OAM encryption holographic schemes for the microwave band, such as: the first combining OAM modes with polarization selection to achieve "polarization + OAM" combined encryption holographic imaging; and the second superimposing OAM with multi-dimensional attributes such as frequency and polarization to achieve encryption holographic reuse of multi-dimensional joint keys, most of these schemes are applications combining OAM with traditional encryption degrees of freedom. OAM is usually only used as one of the multi-dimensional keys and does not form an independent orbital angular momentum encryption channel. At the same time, existing schemes generally lack independent control capabilities under forward and reverse propagation conditions, making it difficult to achieve decoupled loading and independent recovery of bidirectional multi-modal OAM encryption information.

[0004] Therefore, there is an urgent need for a novel metasurface structure that can achieve bidirectional independent control in the microwave band, uses the OAM mode as the core physical key, and supports multi-channel holographic imaging multiplexing, so as to further improve the information capacity and security level of the metasurface holographic encryption system. Summary of the Invention

[0005] To address the aforementioned technical issues, this invention proposes a multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication. It constructs a dual-layer encryption key using orbital angular momentum modes and propagation direction, and achieves bidirectional independent control through alternating arrangement of first and second type units, thereby realizing high-security, high-capacity multi-channel bidirectional holographic encrypted imaging in the microwave frequency band.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention proposes a multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication, comprising: Feed antenna, used to generate linearly polarized incident electromagnetic waves; A vortex wave generating metasurface is set in the radiation direction of the feed antenna. It is composed of multiple polarization-selective metasurface units arranged according to the orbital angular momentum phase distribution diagram. It is used to convert the incident linearly polarized electromagnetic wave into a vortex electromagnetic wave carrying a preset orbital angular momentum mode. A holographic metasurface, arranged parallel to the vortex wave generating metasurface, is composed of multiple polarization-selective metasurface units arranged according to the holographic amplitude and phase distribution map. It is used to receive the vortex electromagnetic waves and reconstruct the holographic image. The holographic metasurface also has bidirectional working capability. It can switch to the forward working state or the reverse working state by rotating 180° around a fixed axis, thereby realizing the reconstruction of the holographic image under bidirectional incident conditions.

[0007] Furthermore, in order to reconstruct the holographic image under bidirectional incident conditions, the polarization-selective metasurface unit of the holographic metasurface is composed of alternating first-type units and second-type units, wherein the first-type units respond to the transmitted vortex electromagnetic waves in the forward operating state, and the second-type units respond to the transmitted vortex electromagnetic waves in the reverse operating state.

[0008] Furthermore, under the bidirectional incident condition, the holographic metasurface is also used to establish multiple holographic channels. Each holographic channel is uniquely determined by the direction of the vortex electromagnetic wave incident on the holographic metasurface and the orbital angular momentum mode, thereby independently generating a preset holographic image under each holographic channel. The holographic metasurface distinguishes the incident direction by its alternating first and second class units and distinguishes the orbital angular momentum modes by its phase distribution; the phase distribution is a distribution encoded using an improved phase retrieval algorithm.

[0009] Furthermore, in the design of the alternating first and second type of units, the effective region set is utilized. The field distribution of the first and second type of units is spatially constrained; specifically, for each working direction... It belongs to the area of ​​action in this direction. The elements retain their field distribution, while the elements that do not belong to the action area have their field distribution set to zero.

[0010] Furthermore, the structure of the polarization-selective metasurface unit is as follows: The stacked structure consists of three metal layers and two dielectric substrates, wherein the dielectric substrates are located between adjacent metal layers; the top metal layer and the bottom metal layer are both open metal ring structures, and the open metal rings of the top metal layer and the bottom metal layer are orthogonally arranged; the middle metal layer is a combination of an open circular ring and a cross structure. By adjusting the arc angle of the intermediate metal layer structure and the angle with the axis The amplitude and phase of the transmitted vortex electromagnetic wave are controlled.

[0011] Furthermore, the orbital angular momentum mode value of the vortex electromagnetic wave and the propagation direction of the vortex electromagnetic wave incident on the holographic metasurface together constitute the first layer of encryption key, and the holographic reconstruction distance of the holographic metasurface constitutes the second layer of encryption key; when both orbital angular momentum mode matching and holographic reconstruction distance matching are satisfied, the preset image information is correctly recovered.

[0012] Furthermore, the field distribution of the holographic metasurface is calculated using an improved phase retrieval algorithm, which includes the following steps: Determine the set of reused holographic channels Each channel is defined by the incident direction and the orbital angular momentum mode, and the holographic metasurface is initialized with a random field distribution; Based on the electromagnetic wave diffraction propagation theory, the field distribution of each channel on the imaging plane is calculated, and an amplitude constraint is applied to the field distribution, replacing the amplitude with the amplitude of the target holographic image; The metasurface emission field corresponding to each channel is deduced, and the expected metasurface field distribution of each channel is further calculated. Perform pixel-by-pixel least-squares fusion and spatial region limitation; for each direction, weighted fusion of the field distribution of each channel in that direction, and based on the effective area of ​​the cell type. The field distribution is filtered to obtain the effective field distributions in both the forward and reverse directions. Then, the forward and reverse field distributions are merged to update the holographic metasurface field distribution. The system evaluates the imaging quality and determines whether it has converged. If it has not converged, it returns to iterating the calculation of the field distribution of each channel on the imaging plane. If it has converged, it outputs the final metasurface field distribution.

[0013] Furthermore, the metasurface emission field corresponding to each channel is calculated in reverse. The process is as follows: ; in, λ is the imaginary unit, and λ is the wavelength of the incident electromagnetic wave. Indicates the imaging distance; Represents the source point Arrival Point The distance.

[0014] Furthermore, the expected metasurface field distribution for each channel is calculated. Specifically: ; in, Represents a very small non-zero number; Indicates the metasurface emission field; This represents the field distribution of the incident vortex wave; Desired metasurface field distribution in each direction Represented as: ; in, Indicates channel The weighting coefficients.

[0015] Furthermore, the method for evaluating image quality is to use correlation coefficients to evaluate the image quality of each channel; ; in, Indicates channel Image quality assessment value; Indicates channel The intensity distribution of the reconstructed holographic image; Indicates channel The mean of the intensity distribution of the reconstructed holographic image; Indicates channel Intensity distribution of the target holographic image; Indicates channel The mean of the intensity distribution of the target holographic image; The overall image quality is evaluated using the average image quality of each channel. ; in, This represents the overall image quality assessment value; This indicates the total number of holographic channels.

[0016] The effects described in the invention are merely those of the embodiments, and not all the effects of the invention. One of the above technical solutions has the following advantages or beneficial effects: This invention proposes a multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication, belonging to the field of wireless communication and information security. It includes: a feed antenna for generating linearly polarized incident electromagnetic waves; a vortex wave generating metasurface, positioned along the radiation direction of the feed antenna, composed of multiple polarization-selective metasurface units arranged according to an orbital angular momentum phase distribution diagram, used to convert the incident linearly polarized electromagnetic waves into vortex electromagnetic waves carrying a preset orbital angular momentum mode; and a holographic metasurface, parallel to the vortex wave generating metasurface, composed of multiple polarization-selective metasurface units arranged according to a holographic amplitude and phase distribution diagram, used to receive the vortex electromagnetic waves and reconstruct a holographic image. The holographic metasurface also possesses bidirectional operating capability, switching to a forward or reverse operating state by rotating 180° around a fixed axis, thereby achieving holographic image reconstruction under bidirectional incident conditions. This invention constructs a dual-layer encryption key using orbital angular momentum mode and propagation direction, and achieves bidirectional independent control through alternating arrangement of first and second type units, thereby realizing high-security, high-capacity multi-channel bidirectional holographic encrypted imaging in the microwave band.

[0017] To address the limitations of existing holographic encryption schemes, where the physical key channel is primarily limited to traditional electromagnetic properties such as polarization, frequency, beam direction, and incident angle, resulting in a limited key space dimension, this invention constructs a two-layer high-dimensional encryption key system. The first-layer encryption key is composed of the orbital angular momentum mode value of the vortex electromagnetic wave and its propagation direction upon incident on the holographic metasurface. The second-layer encryption key is composed of the holographic reconstruction distance of the holographic metasurface. The orbital angular momentum mode itself possesses infinite-dimensional orthogonality, supporting arbitrary high-order modes, resulting in a key space far larger than the traditional single-dimensional polarization and frequency. The propagation direction (forward / backward) doubles the key space. The holographic reconstruction distance, as a continuously variable second-layer key, further increases the difficulty of brute-force attacks. Compared to the 2-3 dimensional key dimensions of traditional schemes, this invention achieves tens or even hundreds of dimensions, significantly improving the system's security level.

[0018] To address the shortcomings of existing microwave band OAM holographic encryption research, which is still immature and often relies on the combined superposition of OAM with traditional keys such as polarization and frequency, without forming an independent encryption channel centered on the OAM mode, this invention establishes an independent encryption channel mechanism centered on the orbital angular momentum mode. Under bidirectional incident conditions, a holographic metasurface establishes multiple holographic channels. Each holographic channel is uniquely determined by the direction of the vortex electromagnetic wave incident on the holographic metasurface and the orbital angular momentum mode, thereby independently generating a preset holographic image under each holographic channel. In this invention, the orbital angular momentum mode is no longer an appendage of traditional keys such as polarization and frequency, but is directly defined as an independent encryption dimension for the holographic channel. The orthogonality between different OAM modes ensures that the images do not interfere with each other when multiple channels are multiplexed, achieving a purer and more efficient independent OAM encryption.

[0019] To address the common problem of existing OAM-encrypted holographic metasurfaces lacking independent modulation capabilities under forward and reverse propagation conditions, and struggling to achieve decoupled loading and independent recovery of bidirectional multimodal OAM encrypted information, this invention achieves independent modulation under forward and reverse propagation conditions for the first time through alternating columns of first and second type units. Specifically, to reconstruct the holographic image under bidirectional incident conditions, the polarization-selective metasurface units of the holographic metasurface are constructed by alternating columns of first and second type units. The first type units respond to transmitted vortex electromagnetic waves in the forward operating state, while the second type units respond to transmitted vortex electromagnetic waves in the reverse operating state. Simultaneously, the field distribution of the first and second type units is spatially constrained using a set of active regions. For each operating direction, units belonging to the active region in that direction retain their field distribution, while units not belonging to that active region have zero field distribution. In the forward operating state, only the first type units participate in modulation, independently generating the corresponding forward holographic image; in the reverse operating state (the holographic metasurface is rotated 180° around the y-axis), only the second type units participate in modulation, independently generating the corresponding reverse holographic image. The forward and reverse holographic information is completely decoupled, can be loaded and recovered independently without affecting each other, breaking through the limitation of the symmetry of forward and reverse responses of traditional metasurfaces.

[0020] Therefore, this invention uses the orbital angular momentum mode as the core physical key channel, independent of mixing and superimposing with traditional dimensions such as polarization and frequency, enabling a purer and higher-dimensional independent OAM encryption mechanism. It breaks through the limitation of symmetrical forward and reverse propagation responses of traditional metasurfaces, achieving independent loading of different OAM modes and different holographic information under forward and reverse propagation conditions. By jointly constructing a multi-channel encryption mapping relationship through OAM modes and imaging distance, it achieves multi-image multiplexing and enhanced security with dual-layer keys. This invention operates in the 12GHz microwave band and employs a transmissive metasurface structure, facilitating integration with communication links, imaging systems, and reconfigurable intelligent metasurface platforms. This invention enables bidirectional orbital angular momentum encrypted multi-channel holographic imaging, providing a new metasurface implementation approach for high-security physical layer information encryption and space-based multiplexing communication. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the overall structure of a multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication proposed in Embodiment 1 of the present invention; Figure 2 This is a phase distribution diagram of the OAM-generated metasurface proposed in Embodiment 1 of the present invention; Figure 3 This is a schematic diagram of the imaging target for each channel proposed in Embodiment 1 of the present invention; Figure 4 This is a schematic diagram of the holographic metasurface unit arrangement proposed in Embodiment 1 of the present invention; Figure 5 This is a flowchart of the improved GS algorithm in an embodiment of the present invention; Figure 6 This is a diagram showing the amplitude and phase distribution of the holographic metasurface in an embodiment of the present invention. Figure 7 This is a schematic diagram of the metasurface unit structure in an embodiment of the present invention; Figure 8 The transmission characteristic curve of the metasurface unit in Embodiment 1 of the present invention; Figure 9 This describes the near-field testing configuration and environment for Embodiment 1 of the present invention. Figure 10 The results are from the near-field test of Embodiment 1 of the present invention; Figure 11 This is a schematic diagram of a multi-channel bidirectional holographic encrypted communication system proposed in Embodiment 2 of the present invention; Legend: 1-Feed antenna; 2-Vortex wave generating metasurface; 3-Holographic metasurface. Detailed Implementation

[0022] To clearly illustrate the technical features of this solution, the invention will be described in detail below through specific embodiments and in conjunction with the accompanying drawings. The following disclosure provides many different embodiments or examples for implementing different structures of the invention. To simplify the disclosure of the invention, components and arrangements of specific examples are described below. Furthermore, reference numerals and / or letters may be repeated in different examples. This repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed. It should be noted that the components illustrated in the drawings are not necessarily drawn to scale. Descriptions of well-known components, processing techniques, and processes are omitted in this invention to avoid unnecessarily limiting the invention.

[0023] Example 1 Embodiment 1 of this invention proposes a multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication, aiming to solve the problems of small key space, lack of independent OAM encryption channel, and inability to achieve bidirectional independent control in existing microwave metasurface holographic encryption technology.

[0024] I. Overall Structure Figure 1 This is a schematic diagram of the overall structure of a multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication proposed in Embodiment 1 of the present invention; Feed antenna 1 is used to generate linearly polarized incident electromagnetic waves; A vortex wave generating metasurface 2 is set in the radiation direction of the feed antenna 1. It is composed of multiple polarization-selective metasurface units arranged according to the orbital angular momentum phase distribution diagram. It is used to convert the incident linearly polarized electromagnetic wave into a vortex electromagnetic wave carrying a preset orbital angular momentum mode. Holographic metasurface 3, arranged parallel to the vortex wave generating metasurface, is composed of multiple polarization-selective metasurface units arranged according to the holographic amplitude and phase distribution map. It is used to receive the vortex electromagnetic waves and reconstruct the holographic image. The holographic metasurface also has bidirectional working capability. It can switch to the forward working state or the reverse working state by rotating 180° around a fixed axis, thereby realizing the reconstruction of the holographic image under bidirectional incident conditions.

[0025] In this invention, the feed antenna 1 uses a WR90 standard waveguide antenna feed to generate along the [wavelength] path. Axial polarization of linear polarization ( (Polarized) incident electromagnetic waves.

[0026] Vortex wave generating metasurface 2 (OAM generated metasurface), located 213 mm from the waveguide antenna feed, is used to divert incident waves. Polarized electromagnetic waves are converted into Polarized OAM vortex electromagnetic waves. Vortex wave generation metasurfaces are replaceable to produce OAM vortex electromagnetic waves of different modes.

[0027] Holographic metasurface 3, placed parallel to the vortex wave generating metasurface at a distance of 500 mm, is used to generate holographic images. Different holographic images are generated for different input vortex wave modes of OAM. The holographic metasurface can be arranged around... The axis is flipped 180° to achieve the reconstruction of bidirectional (including forward and reverse) holograms.

[0028] Both the vortex wave generating metasurface 2 and the holographic metasurface 3 are 500×500mm in size and are composed of 50×50 polarization-selective metasurface units.

[0029] The scope of protection of this invention is not limited to the specific values ​​listed in Example 1, and those skilled in the art can make reasonable selections based on the actual situation.

[0030] II. Phase Distribution of the Vortex Wave Generating Metasurface Setting the center of the metasurface as the origin, the OAM mode is generated as follows: The vortex beam, OAM generates phase distribution at various locations on the metasurface. The following conditions must be met: .

[0031] This represents the orbital angular momentum mode value (topological charge), which determines the helical phase structure of the vortex wave. In this embodiment, the value is taken as +1. 1,+2, Second class; This represents the azimuth angle of the position (x, y) on the hypersurface relative to the origin.

[0032] Considering that the actual feed (waveguide antenna) radiates spherical waves, phase compensation is required to collimate the spherical waves into plane waves. Therefore, the phase distribution at each location on the loaded metasurface should ultimately satisfy: ; in, The wavelength of the incident electromagnetic wave. This represents the axial distance from the phase center of the feed antenna to the vortex wave generating metasurface (f=213mm in this embodiment). This indicates the reference phase compensation distance.

[0033] III. Definition and Encryption Mechanism of Holographic Channel This embodiment 1 adopts This represents a set of multiplexed holographic channels. Each holographic channel is defined as an independent incident condition characterized by the direction of the incident wave and the OAM mode. The mathematical definition of is:

[0034] in, Represents the set of incident directions ( and (representing forward and backward respectively), the corresponding OAM mode values ​​are respectively and The target value for holographic imaging corresponding to each channel is... .

[0035] The holographic metasurface (3) distinguishes the incident direction by its alternating first and second class units and distinguishes the orbital angular momentum modes by its phase distribution. The phase distribution is encoded by an improved Gershberg-Saxton optimization algorithm.

[0036] In terms of encryption mechanism, the orbital angular momentum mode value of the vortex electromagnetic wave and its propagation direction incident on the holographic metasurface together constitute the first layer of encryption key, and the holographic reconstruction distance of the holographic metasurface constitutes the second layer of encryption key. Only when both orbital angular momentum mode matching and holographic reconstruction distance matching are satisfied can the preset image information be correctly recovered.

[0037] based on Figure 1The structure of a multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication and the above formulas are proposed. The working mode is described below. Figure 2 This is a phase distribution diagram of the OAM-generated metasurface proposed in Embodiment 1 of the present invention; Figure 3 This is a schematic diagram of the imaging target for each channel proposed in Embodiment 1 of the present invention; At the waveguide antenna feed input Under the condition of incident electromagnetic wave: When OAM generates metasurface settings Figure 2 The pattern shown in Figure (a) is The phase distribution of OAM vortex waves, with the holographic metasurface set to positive, along... Axial distance holographic metasurface The place will produce Figure 3 The holographic image of the letter "A" shown in Figure (a).

[0038] When OAM generates metasurface settings Figure 2 The pattern shown in Figure (b) is The phase distribution of OAM vortex waves, with the holographic metasurface set to positive, along... Axial distance holographic metasurface The place will produce Figure 3 The holographic image of the letter "B" shown in Figure (b).

[0039] When OAM generates metasurface settings Figure 2 The pattern shown in Figure (c) is The phase distribution of OAM vortex waves, holographic metasurface through surrounding axis flip Set to reverse, along Axial distance holographic metasurface The place will produce Figure 3 The holographic image of the number "1" shown in Figure (c); When OAM generates metasurfaces, it is set to produce Figure 2 As shown in Figure (d) The OAM vortex wave pattern, with the holographic metasurface set in reverse, along... Axial distance holographic metasurface The place will produce Figure 3 The holographic image of the number "2" shown in Figure (d).

[0040] IV. Unit Arrangement and Bidirectional Control of Holographic Metasurfaces To reconstruct the holographic image under bidirectional incident conditions, the polarization-selective metasurface unit of the holographic metasurface 3 is composed of alternating first-type and second-type units arranged in columns. The first-type unit responds to the transmitted vortex electromagnetic wave in the forward operating state, and the second-type unit responds to the transmitted vortex electromagnetic wave in the reverse operating state.

[0041] Figure 4 This is a schematic diagram of the holographic metasurface unit arrangement proposed in Embodiment 1 of the present invention; for the metasurface units with only half facing forward, i.e., the first type of unit, transmission is achieved. The effect of polarized OAM vortex electromagnetic waves, the other half of the unit, namely the second type of unit, only occurs when the holographic metasurface surrounds... Axial flip (i.e., the reverse side of the metasurface is exposed to the incident electromagnetic wave) is required to neutralize the incident electromagnetic wave. Polarized OAM vortex electromagnetic wave response. The metasurface element acting in the positive direction is denoted as "A element," i.e., the first type of element, and the element acting in the negative direction is denoted as "B element," i.e., the second type of element; the region of influence in each direction is denoted as a set. .

[0042] In the design of the first and second types of units arranged in alternating columns, the set of action regions is utilized. The field distribution of the first and second type of units is spatially constrained; specifically, for each working direction... It belongs to the area of ​​action in this direction. The elements retain their field distribution, while the elements that do not belong to the action area have their field distribution set to zero.

[0043] V. Metasurface Unit Structure Figure 7 This is a schematic diagram of the metasurface unit structure in an embodiment of the present invention; the metasurface unit is a basic component of the vortex wave generating metasurface 2 and the holographic metasurface 3.

[0044] Figure 7 Figure (a) shows a schematic diagram of the three-dimensional stacked structure of the metasurface unit; it consists of three metal layers and two dielectric substrates, with the dielectric substrates located between adjacent metal layers.

[0045] Figure 7 Figure (b) shows a schematic diagram of the top metal layer structure of the metasurface unit; the top metal layer is an open metal ring structure with the opening direction along the x-axis.

[0046] Figure 7 Figure (c) shows a schematic diagram of the intermediate metal layer structure of the metasurface unit; the intermediate metal layer is a combination of an open circular ring and a cross structure, wherein the open circular ring has an arc angle. cross structure and Angle between axes .

[0047] Figure 7 Figure (d) shows a schematic diagram of the bottom metal layer structure of the metasurface unit; the bottom metal layer is also an open metal ring structure, but the opening direction is along the y-axis direction, that is, the open metal rings of the top metal layer and the bottom metal layer are orthogonally arranged.

[0048] Figure 7 Figure (e) is a side view cross-sectional diagram of the metasurface unit, clearly showing the stacking order from top to bottom: top metal layer, first dielectric substrate, middle metal layer, second dielectric substrate, and bottom metal layer.

[0049] The period length of the metasurface unit is It consists of two layers with a thickness of F4B (dielectric constant is The loss tangent is ) dielectric substrate, and three layers with a thickness of It consists of layers of copper. The top and bottom layers are a pair of orthogonally arranged open metal rings, and the middle layer is an open circular ring, forming a cross-shaped structure. The arc angle of the middle layer is denoted as... ,and The included angle of the axis is denoted as Other structural parameters and their values ​​are as follows: outer radius of the open metal ring. Inner radius of the open metal ring The line width of the metal ring The opening angle of the open metal ring .

[0050] By adjusting the arc angle of the intermediate metal layer structure and the angle with the axis The amplitude and phase of the transmitted vortex electromagnetic wave can be independently controlled. The second type of unit (B unit) is composed of the first type of unit (A unit) rotated 180° around the y-axis.

[0051] For along A linearly polarized electromagnetic wave incident along its axial direction will have its polarization direction parallel to the line connecting the openings in the top metal layer of the metasurface. In this case, the metasurface unit will transmit the incident wave, achieving orthogonal polarization conversion. However, if the polarization direction is perpendicular to the line connecting the openings, the metasurface unit will reflect the incident wave. The transmission along... Axial propagation The metasurface structure of the polarized incident wave is designated as the "A-unit," i.e., the first-type unit. The "A-unit" is then surrounded... Axis rotation This constitutes "Unit B," the second type of unit. It employs a winding... Rotate the axis clockwise The A-units constitute the OAM-generated metasurface. Both A and B units are used to construct the holographic metasurface.

[0052] The cross-polarization transmission performance of metasurface units was obtained through simulation using the commercial electromagnetic simulation software CST2020. Figure 8 This is the transmission characteristic curve of the metasurface unit in Embodiment 1 of the present invention. Figure 8 Figure (a) shows the transmission amplitude as a function of... and Changes; Figure 8 Figure (b) shows the transmission phase following the curve. and Changes; by adjusting the structural parameters of the metasurface The amplitude and phase of the transmitted electromagnetic wave can be independently controlled by θ. The amplitude control range is 0~1, and the phase control range is -180°~180°. Based on the calculated amplitude and phase distribution of the metasurface, metasurface units at each location are designed. and This constitutes the OAM-generated metasurface and the holographic metasurface.

[0053] 6. Improved GS optimization algorithm To achieve reusable holography, the field distribution of the holographic metasurface is calculated using an improved Gerchberg-Saxton (GS) optimization algorithm, making... Get as close as possible to the set target value . Figure 5 This is a flowchart of the improved GS algorithm in an embodiment of the present invention; Step 1: Determine the set of reused holographic channels Each channel is defined by the incident direction and the orbital angular momentum mode; Step 2: Initialize the holographic metasurface as a random field distribution ; Step 3: Based on the electromagnetic wave diffraction propagation theory, calculate the field distribution of each channel on the imaging plane. ; Step 4: Assess the field distribution Apply a GS amplitude constraint, replacing the amplitude with the amplitude of the target holographic image; obtain ; Step 5: Using the inverse operation of the Rayleigh-Sommerfeld formula, deduce the metasurface field of each channel.

[0054] ; in, λ is the imaginary unit, and λ is the wavelength of the incident electromagnetic wave. Indicates the imaging distance; Represents the source point Arrival Point The distance.

[0055] Step 6: Inversely deduce the expected metasurface field distribution for each channel:

[0056] in It is a very small non-zero value (in this embodiment, it is taken as ). (to prevent the denominator from being zero); Indicates the metasurface emission field; This represents the field distribution of the incident vortex wave.

[0057] Step 7: For each direction Using a weighted average, pixel-wise least-squares fusion is performed to obtain the expected metasurface field distribution in each direction: ; in, Indicates channel Weighting coefficients Limit the scope of effect:

[0058] By merging the field distributions in the two directions, we obtain the updated result. ; Step 8. Use correlation coefficients to evaluate the imaging quality of each channel:

[0059] in, , ; Indicates channel Image quality assessment value; Indicates channel The intensity distribution of the reconstructed holographic image; Indicates channel The mean of the intensity distribution of the reconstructed holographic image; Indicates channel Intensity distribution of the target holographic image; Indicates channel The mean of the intensity distribution of the target holographic image; The overall image quality is evaluated using the average image quality of each channel.

[0060] in, This represents the overall image quality assessment value; Indicates the total number of holographic channels Step 9: Determine if convergence has occurred. The convergence condition is either executing the first preset number of iterations, or continuing the loop for a second preset number of iterations while maintaining image quality. If convergence fails, continue to Step 3; if convergence occurs, the process is complete. This represents the distribution of the metasurface field.

[0061] Figure 6 The amplitude and phase distributions at various locations of the holographic metasurface are obtained through iterative optimization using the improved GS algorithm.

[0062] Figure 6 Figure (a) shows the amplitude distribution of the holographic metasurface, illustrating the amplitude modulation values ​​of transmitted electromagnetic waves at each unit location. The horizontal axis represents the x-direction position on the metasurface (unit: mm), and the vertical axis represents the y-direction position (unit: mm). The grayscale value (or color depth) represents the transmission amplitude value of the metasurface unit at that location. As can be seen from the figure, the amplitude distribution exhibits spatial variations related to the target holographic image, with the amplitude modulation range covering 0~1. Brighter areas represent high transmission amplitude, and darker areas represent low transmission amplitude. This amplitude distribution corresponds to the intensity distribution of the target holographic image and is one of the keys to achieving holographic reconstruction.

[0063] Figure 6 Figure (b) shows the phase distribution of the holographic metasurface, illustrating the phase modulation values ​​of the transmitted electromagnetic waves at each unit location. The horizontal axis represents the x-axis position, and the vertical axis represents the y-axis position. The grayscale value (or color depth) represents the transmitted phase value of the metasurface unit at that location (ranging from -180° to 180°). As can be seen from the figure, the phase distribution exhibits a complex spatial variation pattern, obtained through iterative optimization of the target image from multiple holographic channels using an improved GS algorithm. This phase distribution simultaneously encodes the holographic information corresponding to different OAM modes under both forward and reverse conditions, enabling the holographic metasurface to independently reconstruct the corresponding holographic image under different incident conditions. Figure 6 (a) and Figure 6 (b) Together they constitute the complex amplitude distribution of the holographic metasurface (i.e., the product of the amplitude distribution and the phase distribution), which is the core encoded information for realizing multi-channel bidirectional holographic imaging.

[0064] 7. Near-field test results The proposed metasurface was tested in a microwave near-field testing environment. Figure 9 This describes the near-field test configuration and environment for Embodiment 1 of the present invention. The waveguide antenna feed and scanning probe are connected to the vector network analyzer via an SMA cable, and the scanning area is 500×500mm.

[0065] Figure 10The results are from the near-field test of Embodiment 1 of the present invention. Consistent with the design goals: Test results verified the effectiveness and feasibility of the invention: When the vortex wave generating metasurface is set to generate OAM vortices in +1 mode and the holographic metasurface is placed upright, a holographic image of the letter "A" is generated at a distance of 300 mm from the holographic metasurface along the -z axis; when the vortex wave generating metasurface is set to generate OAM vortex waves in -1 mode and the holographic metasurface is placed upright, a holographic image of the letter "B" is generated at a distance of 400 mm from the holographic metasurface along the -z axis; when the vortex wave generating metasurface is set to generate OAM vortex waves in +2 mode and the holographic metasurface is placed in reverse by rotating 180° around the y-axis, a holographic image of the number "1" is generated at a distance of 300 mm from the holographic metasurface along the -z axis; when the vortex wave generating metasurface is set to generate OAM vortex waves in -2 mode and the holographic metasurface is placed in reverse, a holographic image of the number "2" is generated at a distance of 400 mm from the holographic metasurface along the -z axis.

[0066] The present invention, in embodiment 1, proposes a multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication. It constructs a two-layer encryption key by using orbital angular momentum modes and propagation direction. It achieves bidirectional independent control through alternating arrangement of first and second type units, thereby realizing high-security, high-capacity multi-channel bidirectional holographic encrypted imaging in the microwave band.

[0067] The multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication proposed in Embodiment 1 of this invention can achieve high-security, high-capacity holographic encrypted communication in the microwave band. Its transmissive structure facilitates integration with existing communication links, imaging systems, and reconfigurable intelligent metasurface platforms, demonstrating good industrial practicality and promising prospects for widespread application.

[0068] Example 2 Based on the multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication proposed in Embodiment 1 of this invention, Embodiment 2 of this invention also proposes a multi-channel bidirectional holographic encrypted communication system. Figure 11 This is a schematic diagram of a multi-channel bidirectional holographic encrypted communication system proposed in Embodiment 2 of the present invention. The system includes: I. Transmitter The transmitting end includes a feed antenna 1, a vortex wave generating metasurface 2, and a holographic metasurface 3.

[0069] Feed antenna 1 is a WR90 standard waveguide antenna used to generate linearly polarized (y-polarized) incident electromagnetic waves polarized along the y-axis. The feed antenna is fixed at the center position in front of the vortex wave generating metasurface, at a distance of 213 mm from the vortex wave generating metasurface.

[0070] The vortex wave generating metasurface 2 is set in the radiation direction of the feed antenna. It is composed of multiple polarization-selective metasurface units arranged according to the orbital angular momentum phase distribution diagram. It is used to convert the incident y-polarized electromagnetic wave into an x-polarized vortex electromagnetic wave carrying a preset orbital angular momentum mode.

[0071] The metasurface for generating vortex waves is replaceable to produce OAM vortex electromagnetic waves of different modes. In this embodiment, the preset orbital angular momentum mode values ​​include... =±1,±2, which correspond to different encryption channels.

[0072] The position and phase distribution of each polarization-selective metasurface unit on the vortex wave generating metasurface satisfies: ; in, The wavelength of the incident electromagnetic wave. This represents the axial distance from the phase center of the feed antenna to the vortex wave generating metasurface (f=213mm in this embodiment). This indicates the reference phase compensation distance.

[0073] Holographic metasurface 3 and vortex wave generating metasurface 2 are set parallel to each other at a distance of 500 mm. They are composed of multiple polarization-selective metasurface units arranged according to the holographic amplitude and phase distribution map, and are used to receive vortex electromagnetic waves and reconstruct holographic images.

[0074] The holographic metasurface also possesses bidirectional operating capability, switching between forward and reverse operating states by rotating 180° around the y-axis, thereby enabling the reconstruction of holographic images under bidirectional incident conditions. To achieve holographic image reconstruction under bidirectional incident conditions, the polarization-selective metasurface unit cells of the holographic metasurface are constructed by alternating first and second type unit cells in a column-alternating arrangement. The first type unit cells respond to transmitted vortex electromagnetic waves in the forward operating state, while the second type unit cells respond to transmitted vortex electromagnetic waves in the reverse operating state.

[0075] The field distribution of the holographic metasurface is calculated using an improved Gerschberg-Sachston optimization algorithm, which allows different combinations of incident directions and different OAM modes to correspond to different preset holographic images.

[0076] The receiving end includes a holographic image acquisition device and a decryption module.

[0077] The holographic image acquisition device is used to acquire reconstructed holographic images at a preset reconstruction distance. In this embodiment, the holographic image acquisition device includes a scanning probe and a vector network analyzer, with a scanning surface size of 500×500mm, capable of acquiring the intensity distribution of the holographic image.

[0078] The decryption module is used to decrypt the acquired holographic image based on the preset orbital angular momentum mode value and propagation direction.

[0079] In this embodiment, the decryption module works as follows: First-level decryption: The decryption module first verifies whether the orbital angular momentum mode value of the received vortex electromagnetic wave matches the preset mode, and simultaneously verifies whether the propagation direction (forward or reverse) matches the preset direction. Only when both match can the second-level decryption proceed.

[0080] The second layer of decryption: The decryption module verifies whether the reconstruction distance of the holographic image acquisition device matches the preset distance. Only when the distance matches can the content of the holographic image be correctly identified.

[0081] If the OAM mode, propagation direction, or reconstruction distance input by the receiver does not match the preset key, the holographic image information cannot be correctly recovered, thus achieving the effect of physical layer encryption.

[0082] The workflow of the multi-channel bidirectional holographic encrypted communication system proposed in Embodiment 2 of this invention is as follows: Information encoding: The transmitting end encodes the information to be transmitted into preset holographic targets.

[0083] Metasurface configuration: Based on the target holographic images of multiple holographic channels, the field distribution of the holographic metasurface is calculated using an improved GS optimization algorithm, so that different combinations of incident directions and different OAM modes correspond to different holographic images.

[0084] Signal transmission: The feed antenna generates linearly polarized incident electromagnetic waves, which are converted into vortex electromagnetic waves carrying preset OAM modes by the vortex wave generating metasurface. These vortex electromagnetic waves are incident on the holographic metasurface to reconstruct the corresponding holographic image.

[0085] Signal reception: The receiver uses a holographic image acquisition device to acquire the reconstructed holographic image at a preset reconstruction distance.

[0086] Decryption: The decryption module decrypts the acquired hologram based on the preset OAM mode value, propagation direction, and reconstruction distance to restore the original information.

[0087] The description of the relevant parts of the multi-channel bidirectional holographic encrypted communication system provided in Embodiment 2 of this application can be found in the detailed description of the corresponding parts in the multi-channel bidirectional holographic metasurface of orbital angular momentum encrypted communication provided in Embodiment 1 of this application, and will not be repeated here.

[0088] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that the elements inherent in a process, method, article, or apparatus that includes a list of elements are included. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. Additionally, portions of the technical solutions provided in the embodiments of this application that are consistent with the implementation principles of corresponding technical solutions in the prior art have not been described in detail to avoid excessive elaboration.

[0089] While specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art can make other modifications or variations based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication, characterized in that, include: Feed antenna (1) is used to generate linearly polarized incident electromagnetic waves; A vortex wave generating metasurface (2) is set in the radiation direction of the feed antenna (1). It is composed of multiple polarization-selective metasurface units arranged according to the orbital angular momentum phase distribution diagram. It is used to convert the incident linearly polarized electromagnetic wave into a vortex electromagnetic wave carrying a preset orbital angular momentum mode. The holographic metasurface (3) is arranged parallel to the vortex wave generating metasurface (2) and is composed of multiple polarization-selective metasurface units arranged according to the holographic amplitude and phase distribution diagram. It is used to receive the vortex electromagnetic wave and reconstruct the holographic image. The holographic metasurface (3) also has bidirectional working capability. It can switch to the forward working state or the reverse working state by rotating 180° around a fixed axis, thereby realizing the reconstruction of the holographic image under bidirectional incident conditions.

2. The multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication according to claim 1, characterized in that, To achieve the reconstruction of a holographic image under bidirectional incident conditions, the polarization-selective metasurface unit of the holographic metasurface (3) is composed of alternating first and second type units, wherein the first type unit responds to the transmitted vortex electromagnetic wave in the forward working state, and the second type unit responds to the transmitted vortex electromagnetic wave in the reverse working state.

3. The multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication according to claim 1, characterized in that, Under the bidirectional incident conditions, the holographic metasurface (3) is also used to establish multiple holographic channels. Each holographic channel is uniquely determined by the direction of the vortex electromagnetic wave incident on the holographic metasurface and the orbital angular momentum mode, thereby generating a preset holographic image independently under each holographic channel. The holographic metasurface (3) distinguishes the incident direction by its alternating first and second class units and distinguishes the orbital angular momentum modes by its phase distribution; the phase distribution is a distribution encoded by an improved phase recovery algorithm.

4. The multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication according to claim 1, characterized in that, In the design of the first and second types of units arranged in alternating columns, the set of action regions is utilized. The field distribution of the first and second type of units is spatially constrained; specifically, for each working direction... It belongs to the area of ​​action in this direction. The elements retain their field distribution, while the elements that do not belong to the action area have their field distribution set to zero.

5. The multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication according to claim 2, characterized in that, The structure of the polarization-selective metasurface unit is as follows: A stacked structure consisting of three metal layers and two dielectric substrates, wherein the dielectric substrates are located between adjacent metal layers; both the top and bottom metal layers are open metal ring structures, and the open metal rings of the top and bottom metal layers are orthogonally arranged. The intermediate metal layer is a combination of an open circular ring and a cross structure; By adjusting the arc angle of the intermediate metal layer structure and the angle with the axis The amplitude and phase of the transmitted vortex electromagnetic wave are controlled.

6. The multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication according to claim 1, characterized in that, The orbital angular momentum mode value of the vortex electromagnetic wave and the propagation direction of the vortex electromagnetic wave incident on the holographic metasurface together constitute the first layer of encryption key, and the holographic reconstruction distance of the holographic metasurface constitutes the second layer of encryption key; when the orbital angular momentum mode matching and the holographic reconstruction distance matching are satisfied, the preset image information is correctly recovered.

7. The multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication according to claim 1, characterized in that, The field distribution of the holographic metasurface (3) is calculated using an improved phase retrieval algorithm, which includes the following steps: Determine the set of reused holographic channels Each channel is defined by the incident direction and the orbital angular momentum mode, and the holographic metasurface is initialized with a random field distribution; Based on the electromagnetic wave diffraction propagation theory, the field distribution of each channel on the imaging plane is calculated, and an amplitude constraint is applied to the field distribution, replacing the amplitude with the amplitude of the target holographic image; The metasurface emission field corresponding to each channel is deduced, and the expected metasurface field distribution of each channel is further calculated. Perform pixel-by-pixel least-squares fusion and spatial region limitation; for each direction, weighted fusion of the field distribution of each channel in that direction, and based on the effective area of ​​the cell type. The field distribution is filtered to obtain the effective field distributions in both the forward and reverse directions. Then, the forward and reverse field distributions are merged to update the holographic metasurface field distribution. The system evaluates the imaging quality and determines whether it has converged. If it has not converged, it returns to iterating the calculation of the field distribution of each channel on the imaging plane. If it has converged, it outputs the final metasurface field distribution.

8. The multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication according to claim 7, characterized in that, Reverse calculation of the metasurface emission field corresponding to each channel The process is as follows: ； in, λ is the imaginary unit, and λ is the wavelength of the incident electromagnetic wave. Indicates the imaging distance; Represents the source point Arrival Point The distance.

9. The multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication according to claim 8, characterized in that, Further calculations were performed on the expected metasurface field distribution for each channel. Specifically: ； in, Represents a very small non-zero number; Indicates the metasurface emission field; This represents the field distribution of the incident vortex wave; Desired metasurface field distribution in each direction Represented as: ； in, Indicates channel The weighting coefficients.

10. The multi-channel bidirectional holographic metasurface for orbital angular momentum encrypted communication according to claim 9, characterized in that, The method for evaluating image quality is as follows: use the correlation coefficient to evaluate the image quality of each channel; ； in, Indicates channel Image quality assessment value; Indicates channel The intensity distribution of the reconstructed holographic image; Indicates channel The mean of the intensity distribution of the reconstructed holographic image; Indicates channel Intensity distribution of the target holographic image; Indicates channel The mean of the intensity distribution of the target holographic image; The overall image quality is evaluated using the average image quality of each channel. ； in, This represents the overall image quality assessment value; This indicates the total number of holographic channels.

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