A near-far field encryption method based on singularity point amplitude characteristics

Abstract

A near-field and far-field encryption method based on the amplitude characteristics of singular points, comprising the following steps: 1) constructing a nano-structure unit, the nano-structure unit being composed of a substrate and nano-bricks deposited on the substrate; 2) optimizing parameters: according to a selected working wavelength, optimizing the structural parameters of the nano-bricks by using CST electromagnetic simulation software; 3) determining the structural parameters at which the EP is located, determining the singular point EP by using the CST electromagnetic simulation software by changing the structural parameters of the nano-bricks; 4) determining the phase distributions of two holographic images I A and I B ; 5) arranging a nano-structure array: according to the phase distributions of the two holographic images I A and I B determined in step 4), obtaining the PB phase by changing the rotation angles of the two kinds of nano-bricks to meet the phase distributions; 6) observing the nano array arranged in step 5), with RCP light incident from the front of the metasurface, the first holographic image I A can be detected on the reflection surface, and at the same time, the near-field displays the pattern I C ; with LCP light incident from the front of the metasurface, the second holographic image I B can be detected on the reflection surface, and at the same time, the near-field displays the complementary pattern of the pattern I C . The method of the present application is ingeniously designed, not only manipulates the phase information of the unit structure, but also realizes the near-field pattern by using the amplitude difference, and can be used for near-field and far-field encryption at the same time.

Description

Technical Field

[0001] This invention relates to the field of optical metasurfaces, specifically to achieving polarization decoupling of metasurfaces by utilizing the amplitude characteristics of singular points within them. Metasurfaces, through subwavelength-scale artificial microstructures, enable the manipulation of multiple parameters such as optical field amplitude, phase, and polarization, allowing for precise control of beam propagation and phase. This provides innovative solutions for various fields including optical communication, imaging, and sensing. Background Technology

[0002] With the rapid development of information technology, the importance of information security and encryption technology has become increasingly prominent. Optical encryption, as an efficient, fast, and difficult-to-crack encryption method, has received widespread attention in recent years. The emergence of metasurface technology has provided new ideas and methods for optical encryption. A metasurface is a two-dimensional functional material composed of subwavelength-scale artificial structural units. By precisely designing the geometry, size, orientation, and distribution of the unit structure, the optical properties such as amplitude, phase, and polarization of electromagnetic waves can be flexibly controlled.

[0003] In the field of optical encryption, traditional near-field encryption methods mainly rely on nanoprinting technology, while far-field encryption often employs holography. However, these methods have limitations in terms of dynamic switching and flexible tunability. In recent years, encryption technologies based on metasurfaces have made some progress. For example, some studies have used metasurface zone plates to achieve far-field polarization holographic encryption, hiding and decrypting information by controlling the polarization state. Other studies have proposed using non-volatile and switchable metasurfaces to achieve simultaneous nanoprinting and holography, dynamically switching between near-field nanoprinted patterns and far-field holographic images by controlling the polarization state of the incident light, the imaging distance, and the crystallinity level of the material.

[0004] Furthermore, singularities, as special physical phenomena in non-Hermitian systems, possess unique optical properties. The introduction of singularities provides new degrees of freedom for the manipulation of metasurfaces, enabling phenomena such as asymmetric transmission and topological phase. However, research combining the amplitude characteristics of singularities with near-field and far-field encryption is still relatively limited. While existing techniques have attempted to achieve encryption using the amplitude and phase modulation of metasurfaces, most focus on single near-field or far-field applications, and the encryption methods are relatively simple. Given the shortcomings of existing techniques in terms of flexibility, dynamism, and security in near-field and far-field encryption, this paper proposes a novel method that fully leverages the amplitude characteristics of metasurface singularities to achieve integrated near-field and far-field encryption. Summary of the Invention

[0005] The present invention aims to overcome the above-mentioned problems existing in the prior art and provide a near-field and far-field encryption method based on the amplitude characteristics of singular points.

[0006] This invention utilizes the amplitude characteristics of singular points in metasurfaces, combined with chiral structures and their mirror structures, to achieve polarization-decoupled far-field holograms. Simultaneously, it utilizes the amplitude difference to form near-field patterns and designs encryption schemes based on these.

[0007] To achieve the above objectives, the present invention provides a near-field and far-field encryption method based on singular point amplitude characteristics, implemented through the following steps:

[0008] 1.) Constructing Nanostructure Units: The nanostructure unit consists of a substrate and nanobricks deposited on the substrate. An xoy coordinate system is established with the right-angled sides of the nanostructure unit as the x-axis and y-axis, and a z-axis is established along the vertical direction.

[0009] 2.) Parameter Optimization: Based on the selected operating wavelength, the structural parameters of the nanobricks are optimized using CST electromagnetic simulation software. These include: the period P of the nanostructure unit, the length L1 and width W1 of the long nanobrick, the length L2 and width W2 of the short nanobrick, the substrate thickness h1, and the height h2 of the nanobrick.

[0010] 3.) Determine the structural parameters of EP: By changing the structural parameters of the nanobrick, the singularity point EP is determined using CST electromagnetic simulation software.

[0011] 4.) Determine the two holographic images I A ′ and I B Phase distribution of ': Based on the principle of computational holography, the improved GS algorithm is used to process two target images with patterns A and B respectively, resulting in two phase distribution matrices, where the holographic image I... A The phase distribution matrix of ′ is only in the near-field pattern I C Above, holographic image I B The phase distribution matrix of ′ in the near-field pattern I C On complementary patterns.

[0012] 5.) Arrange the nanostructure array: Based on the two holographic images I determined in step 4). A ′ and I B The phase distribution of ' is obtained by changing the rotation angle of the two nanobricks to satisfy the phase distribution of PB.

[0013] 6.) The nanoarray arranged in step 5) is observed, and polarization decoupling is achieved using the chiral structure and its mirror structure. Specifically, by incident RCP light from the front of the metasurface, the first holographic image I can be detected on the reflecting surface. A Simultaneously displaying pattern I in the near field C By incident LCP light from the front of the metasurface, a second holographic image I can be detected on the reflecting surface. B Simultaneously displaying pattern I in the near field CThe complementary pattern. This polarization decoupling property allows encrypted information to be selectively read through polarization states.

[0014] Furthermore, the substrate mentioned in step 1) is a silicon dioxide material, and the nanobrick material is a silicon material.

[0015] Furthermore, the optimization process using the GS algorithm in step 5) is completed using MATLAB software, where the target image with pattern A and pattern B and the near-field pattern I are used. C The optimization objective is input to the algorithm, and the optimization function is set as the error between the holographic image reconstructed by the algorithm and the target holographic image. The error function is defined as follows:

[0016]

[0017] Where I is the target holographic image, I′ is the holographic image reconstructed by the algorithm, and i and j represent the row and column numbers of the pixels, respectively; the output is the phase distribution matrix of the two holographic images.

[0018] Furthermore, in step 5), based on the principle of computational holography, the target image is optimized using an improved GS algorithm to generate phase distribution matrices for two holographic images. These phase distribution matrices are used to guide the arrangement of the nanostructure array, enabling the reconstruction of the target holographic image in the far field. By utilizing the amplitude characteristics of singular points, an encryption pattern is formed in the near field using the amplitude difference of the nanostructures. This near-field pattern, combined with the holographic image, achieves integrated near-field and far-field encryption.

[0019] Through the above design, target information is encoded into holographic images and near-field patterns. Due to polarization decoupling and the characteristics of singularities, the encrypted information can only be correctly read under specific polarization states and wavelengths. By adjusting the structural parameters and rotation angles of the nanobricks, the encryption pattern and holographic image can be flexibly changed, achieving dynamic encryption and information updates. Because the encrypted information relies on the amplitude characteristics of singularities, polarization decoupling, and precise nanostructure design, it is difficult to crack the encryption information using conventional methods. By irradiating the nanoarray with the correct polarization state and wavelength, the target holographic image can be reconstructed in the far field, and the corresponding pattern or its complementary pattern can be displayed in the near field.

[0020] Singularities in non-Hermitian metasurfaces are points where the eigenvalues ​​and eigenstates of the Hamiltonian of a non-Hermitian system intersect. At these points, two or more eigenvalues ​​are equal, i.e., degeneracy occurs, and the corresponding eigenstates are no longer orthogonal. Singularities in metasurfaces exhibit unique functionalities in areas such as optical field enhancement, energy concentration, polarization manipulation, and topology protection.

[0021] To study EP, a 2×2 Hamiltonian matrix is ​​used to describe this non-Hermitian system.

[0022]

[0023] Where δ x,y It is the frequency of resonant detuning, δ x,y =ω x,y -ω0, where ω0 is the resonant frequency. g represents the coupling strength between the two directions. γ x,y The loss parameter γ represents the loss in the x and y directions. x and γ y It can be controlled by changing the geometry or the material.

[0024] Equation (1) can be transformed into

[0025]

[0026] in When ω x =ω y When ω = 0, Eigenvalues ​​and corresponding eigenstates can be obtained directly.

[0027]

[0028] By changing the geometry and relative position of the two nanobricks, the coupling strength g and the loss parameter γ can be altered. x and γ y When g = Γ, that is, at EP, both the eigenvalues ​​and eigenstates of the system become degenerate.

[0029] To intuitively study EP, we will consider describing the intrinsic transport of the metasurface based on circular polarization. The transport matrix can be described as:

[0030]

[0031] Where r jk This represents the reflection from k-polarized input light to j-polarized output light. The subscripts + and - represent left-hand circular polarization (LCP) and right-hand circular polarization (RCP), respectively. Due to anisotropy r xx ≠r yy Due to reciprocity r xy =r yx It can be seen that r ++ =r -- eigenvalues The conditions required to achieve EP are:

[0032] r +- r -+ =0

[0033] This invention discloses a near-field and far-field encryption method based on the amplitude characteristics of singular points. It utilizes chiral structures and their mirror images to respectively manipulate far-field holographic composite camouflage information, while simultaneously using their amplitude differences to form a near-field encryption pattern. The method of this invention is ingeniously designed, not only manipulating the phase information of the unit structure but also utilizing amplitude differences to realize the near-field pattern, making it applicable to simultaneous near-field and far-field encryption schemes.

[0034] Compared with the prior art, the advantages of the present invention are as follows:

[0035] By utilizing the amplitude characteristics of singular points on non-Hermitian metasurfaces, combined with PB phase analysis, and leveraging the polarization decoupling properties of singular points and an improved GS algorithm, a scheme for simultaneous near-field and far-field encryption was achieved. This structure enables the manipulation of multiple parameters of the optical field, allowing for precise control of beam propagation and phase, and providing innovative solutions for various fields such as optical communication, imaging, and sensing. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the nano-brick unit structure in this invention;

[0037] Figure 2 This is a schematic diagram of the mirror structure of the nanobrick in this invention.

[0038] Figure 3 This is a schematic diagram of the reflectivity of the nanobrick structure in an embodiment of the present invention.

[0039] Figure 4 This is a schematic diagram showing the relationship between the real part of the eigenvalues ​​and the wavelength of the nanobrick structure in an embodiment of the present invention.

[0040] Figure 5 This is a schematic diagram showing the relationship between the imaginary part of the eigenvalues ​​and the wavelength of the nanobrick structure in an embodiment of the present invention.

[0041] Figure 6 This is a schematic diagram of two cross-polarization phases of the nanobrick at different rotation angles in an embodiment of the present invention.

[0042] Figure 7 This is a schematic diagram of the cross-polarization reflectance of the nanobrick at different rotation angles in an embodiment of the present invention.

[0043] Figure 8a The target image used to generate the hologram in this embodiment of the invention. Figure 8b To achieve holographic imaging and subsequent reconstruction of holographic images using metasurfaces A ′.

[0044] Figure 9a The crying face in the image is the target image used to generate the hologram in this embodiment of the invention. Figure 9b To achieve holographic imaging and subsequent reconstruction of holographic images using metasurfaces B ′.

[0045] Figure 10a Far-field holographic image I when incident with RCP A , Figure 10b The near-field pattern of the butterfly caused by amplitude characteristics I C .

[0046] Figure 11a Far-field holographic image I when LCP is incident B , Figure 11b The near-field pattern of the butterfly caused by amplitude characteristics I C Complementary graph.

[0047] Figure 12a Two holographic images I under incident light of the wrong wavelength or linearly polarized light A and I B The camouflaged infographic formed by overlay Figure 12b To and the reduction in amplitude difference leading to near-field pattern I C A diagram illustrating the disappearance. Detailed Implementation

[0048] The present invention will be further described below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited to the scope described.

[0049] A near-field and far-field encryption method based on singular point amplitude characteristics includes the following steps:

[0050] 1.) Constructing Nanostructure Units: The nanostructure unit consists of a substrate and nanobricks deposited on the substrate. An xoy coordinate system is established with the right-angled sides of the nanostructure unit as the x-axis and y-axis, and a z-axis is established along the vertical direction.

[0051] 2.) Parameter Optimization: Based on the selected operating wavelength, the structural parameters of the nanobricks are optimized using CST electromagnetic simulation software. These include: the period P of the nanostructure unit, the length L1 and width W1 of the long nanobrick, the length L2 and width W2 of the short nanobrick, the substrate thickness h1, and the height h2 of the nanobrick.

[0052] 3.) Determine the structural parameters of EP: By changing the structural parameters of the nanobrick, the singularity point EP is determined using CST electromagnetic simulation software.

[0053] 4.) Determine the two holographic images I A ′ and I B Phase distribution of ': Based on the principle of computational holography, the improved GS algorithm is used to process two target images with patterns A and B respectively, resulting in two phase distribution matrices, where the holographic image I... A The phase distribution matrix of ′ is only in the near-field pattern I C Above, holographic image I BThe phase distribution matrix of ′ in the near-field pattern I C On complementary patterns.

[0054] 5.) Arrange the nanostructure array: Based on the two holographic images I determined in step 4). A ′ and I B The phase distribution of ' is obtained by changing the rotation angle of the two nanobricks to satisfy the phase distribution of PB.

[0055] 6.) The nanoarray arranged in step 5) is observed. RCP light is incident from the front of the metasurface, and the first holographic image I can be detected on the reflecting surface. A Simultaneously displaying pattern I in the near field C By incident LCP light from the front of the metasurface, a second holographic image I can be detected on the reflecting surface. B Simultaneously displaying pattern I in the near field C Complementary patterns.

[0056] Furthermore, the substrate mentioned in step 1) is a silicon dioxide material, and the nanobrick material is a silicon material.

[0057] Furthermore, the optimization process using the GS algorithm in step 5) is completed using MATLAB software, where the target image with pattern A and pattern B and the near-field pattern I are used. C The optimization objective is input to the algorithm, and the optimization function is set as the error between the holographic image reconstructed by the algorithm and the target holographic image. The error function is defined as follows:

[0058]

[0059] Where I is the target holographic image, I′ is the holographic image reconstructed by the algorithm, and i and j represent the row and column numbers of the pixels, respectively; the output is the phase distribution matrix of the two holographic images.

[0060] The metasurface described in this embodiment comprises multiple metasurface units arranged in an array. Each metasurface unit includes a silica substrate and silicon nanobricks deposited on the silica substrate. First, nanostructure units are constructed. The period of each nanostructure unit is P. The substrate thickness is h1 = 200 nm. The long nanobricks have a length L1 = 200 nm and a width W1 = 70 nm. The short nanobricks have a length L2 = 70 nm and a width W2 = 60 nm. The thickness of each nanobrick is h2 = 200 nm.

[0061] Simulations were performed on the nanobricks under the aforementioned structural parameters, and the reflection spectrum of vertically incident circularly polarized light is shown below. Figure 3 As shown, where R ij This represents the reflection from j-polarized input light to i-polarized output light. We can see that one of the cross-polarizations approaches zero, corresponding to an operating wavelength of 487 nm. Figure 4 and Figure 5 At a wavelength of 487 nm, the real and imaginary parts of the eigenvalues ​​are degenerate, indicating eigenvalue degeneracy and proving the existence of a singularity at 487 nm.

[0062] The changes in phase and reflectivity with rotation angle by rotating the nanobrick are as follows: Figure 6 and Figure 7 As shown. Cross-polarization component R +- The reflectivity remains basically unchanged, and the phase... Continuous phase modulation can be achieved, conforming to the PB phase. The other cross-polarization component R... -+ Although it is not completely zero, its phase The PB phase is not satisfied. Therefore, the crosstalk between the two holograms is relatively small.

[0063] Next, FDTD simulation was used to verify the scheme. LCP and RCP were incident perpendicularly onto the metasurface, and the resulting holographic images are shown below. Figures 8a-9b As shown, the smiling and crying faces on the left are the input images, which are the target images used to generate the hologram, representing the ideal input images. The images on the right are the reconstructed images after holographic imaging using a metasurface. These images accurately reproduce the contours and features of the input images, demonstrating the superior performance of metasurfaces in light field manipulation.

[0064] Simultaneously, a "butterfly" near-field pattern caused by the amplitude characteristics of the chiral structure is displayed, such as... Figures 10a-11b The binary image shown. When the wrong wavelength or linearly polarized light is incident, the two holograms will superimpose to generate an incorrect image, such as... Figure 12a As shown, and because the amplitude difference between the two structures is similar, the near-field plot will also become invisible, such as... Figure 12b As shown in the figure, the simulation results verify the feasibility of the near-field and far-field encryption scheme, providing a new approach for information encoding and encryption, and showing potential application prospects in information reuse, information hiding and encoding.

[0065] The embodiments described in this specification are merely examples of implementations of the inventive concept. The scope of protection of this invention should not be considered as limited to the specific forms stated in the embodiments. The scope of protection of this invention also extends to equivalent technical means that can be conceived by those skilled in the art based on the inventive concept.

Claims

1. A near-far field encryption method based on the amplitude characteristics of singular points, characterized in that, The chiral structure and the mirror image structure are used for respectively regulating far-field holographic combined camouflage information, and an amplitude difference between the chiral structure and the mirror image structure is used for forming a near-field encryption pattern; The method comprises the following steps: 1) constructing a nano-structure unit: the nano-structure unit is composed of a substrate and nano-bricks deposited on the substrate; taking a right-angle side of the nano-structure unit as an x-axis and a y-axis, an xoy coordinate system is established, and a z-axis is established along a vertical direction; 2) optimizing parameters: according to a selected working wavelength, structure parameters of the nano-bricks are optimized by using CST electromagnetic simulation software; the structure parameters include a period P of the nano-structure unit, a length L1 and a width W1 of a long nano-brick, a length L2 and a width W2 of a short nano-brick, a substrate thickness h1 and a nano-brick height h2; 3) determining structure parameters where an EP is located: by changing the structure parameters of the nano-bricks, the CST electromagnetic simulation software is used to determine the singular point EP; 4) Determining the phase distribution of two holographic images I A and I B Based on the principle of computer holography, the improved GS algorithm is used to operate the two target images with pattern A and pattern B respectively, to obtain two phase distribution matrices, wherein the phase distribution matrix of holographic image I A is only on the near-field pattern I C , and the phase distribution matrix of holographic image I B is on the complementary pattern of near-field pattern I C ; 5) arranging the nanostructure array: the phase distribution of the two holographic images I A and I B ' is determined according to step 4), the PB phase is obtained by changing the rotation angle of the two nano-bricks to meet the phase distribution; 6) The nanoarray arranged in step 5) is observed, with RCP light incident from the front of the metasurface, and a first holographic image I is detected on the reflection plane A , while in the near field a pattern I C is displayed. The second holographic image I B is detected on the reflection plane, while in the near field a complementary pattern I C is displayed, with LCP light incident from the front of the metasurface.

2. The method of claim 1, wherein the amplitude of the singularity point is used to encrypt the near-field and far-field.

3. In step 1), the substrate of the nano-structure unit is composed of a silicon dioxide material, and the nano-brick part is composed of silicon.

3. The method of claim 1, wherein the amplitude of the singularity point is used to encrypt the near-field and far-field.

3. In step 2), the structure parameters of the nano-bricks are obtained by electromagnetic simulation optimization according to a selected working wavelength λ, and the optical performance of the device is improved by optimizing the structure parameters.

4. The method of claim 1, wherein the amplitude of the singularity point is used to encrypt the near-field and far-field. 5 Step 4) The improved GS algorithm is calculated by matlab software, wherein the target image with pattern A and pattern B and the near-field pattern I C As the optimization target of the algorithm, the function of optimization is set as the error between the holographic image restored by the algorithm and the target holographic image, and the error function is defined as: Wherein I is a target holographic image, I' is a holographic image restored by the algorithm, i and j respectively represent a row number and a column number of a pixel point; and the output is a phase distribution matrix of two holographic images.

Images