Near-far field encryption method based on singular point amplitude characteristics

Abstract

A near-far field encryption method based on singular point amplitude characteristics comprises the following steps: 1) constructing a nano structure unit which is composed of a substrate and nano bricks deposited on the substrate; 2) optimizing parameters: optimizing the structural parameters of the nano brick through CST electromagnetic simulation software according to the selected working wavelength; 3) determining the structural parameters of the EP, and determining a singular point EP by using CST electromagnetic simulation software by changing the structural parameters of the nano brick; 4) determining phase distribution of the two holographic images IA and IB; (5) arranging a nanostructure array: according to the phase distribution of the two holographic images IA and IB determined in the step (4), obtaining a PB phase by changing the rotating angles of the two nano bricks to meet the phase distribution; (6) observing the nano array arranged in the step (5), enabling RCP light to enter from the front surface of the metasurface, detecting a first hologram IA on a reflecting surface, and displaying a pattern IC in a near field; lCP light is incident from the front surface of the metasurface, a second hologram IB can be detected on the reflecting surface, and a complementary pattern of the pattern IC is displayed in a near field. The method is ingenious in design, the phase information of the unit structure is manipulated, the near-field pattern is achieved through the amplitude difference, and the method can be used for simultaneous encryption of the near field and the far field.

Description

Technical Field

[0001] The present invention relates to the field of optical metasurfaces, and more specifically to using the amplitude characteristics of singular points in metasurfaces to achieve polarization decoupling of metasurfaces. Metasurfaces achieve regulation of multiple parameters such as light field amplitude, phase, and polarization through sub-wavelength-scale artificial microstructures, and can accurately control the propagation and phase of light beams, bringing innovative solutions to multiple fields such as optical communications, imaging, and sensing. Background Art

[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 provides new ideas and methods for optical encryption. Metasurface is a two-dimensional functional material composed of artificial structural units at the subwavelength scale. Through the precise design of the geometric shape, size, orientation and distribution of the unit structure, the optical properties of the electromagnetic wavefront, such as amplitude, phase, polarization, etc., 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 mostly uses holography. However, these methods have limitations in dynamic switching and flexible adjustability. In recent years, encryption technology based on metasurfaces has made some progress. For example, some studies have used metasurface waveplates to achieve far-field polarization holographic encryption, hiding and decrypting information by controlling the polarization state. Other studies have proposed the use of non-volatile and switchable metasurfaces to achieve synchronous nanoprinting and holography, and achieved dynamic switching between near-field nanoprinting patterns and far-field holographic images by controlling the polarization state of the incident light, the imaging distance, and the crystallization level of the material.

[0004] In addition, as a special physical phenomenon in non-Hermitian systems, singularities have unique optical properties. The introduction of singularities provides new degrees of freedom for the regulation of metasurfaces, which can realize phenomena such as asymmetric transmission and topological phase. However, there are relatively few studies that combine the amplitude characteristics of singularities with near- and far-field encryption. In the prior art, although there are attempts to achieve encryption using amplitude and phase modulation of metasurfaces, most of them are concentrated on a single near-field or far-field application, and the encryption method is relatively single. In view of the fact that the prior art still has deficiencies in the flexibility, dynamics and security of near- and far-field encryption, this paper proposes a new method to give full play to the amplitude characteristics of metasurface singularities and realize near- and far-field integrated encryption. Summary of the invention

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

[0006] The present invention utilizes the amplitude characteristics of the singular points in the metasurface, combines the chiral structure and its mirror structure, realizes two far-field holograms with polarization decoupling, and simultaneously utilizes their amplitude difference to form a near-field pattern, and designs an encryption scheme based on these.

[0007] In order to achieve the above object, a near-far field encryption method based on the amplitude characteristics of a singular point of the present invention is implemented by the following steps:

[0008] 1.) Constructing a nanostructure unit: The nanostructure unit is composed 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 the y-axis, and a z-axis is established along the vertical direction.

[0009] 2.) Optimization parameters: According to the selected working wavelength, the structural parameters of the nanobrick are optimized by CST electromagnetic simulation software, including: 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 nanobrick height h2.

[0010] 3.) Determine the structural parameters of the EP: By changing the structural parameters of the nanobrick, use CST electromagnetic simulation software to determine the singularity point EP.

[0011] 4.) Determine two holographic images I A and I B Phase distribution: Based on the principle of computational holography, the improved GS algorithm is used to analyze the phase distribution of two images with pattern I A and Pattern I B The target image is operated to obtain two phase distribution matrices, where I A The phase distribution is only in the near-field pattern I C Up, I B The phase distribution in the near field pattern I C complementary pattern.

[0012] 5.) Arrange the nanostructure array: According to the two holographic images I determined in step 4) A and I B The phase distribution is achieved by changing the rotation angles of the two nanobricks to obtain the PB phase to satisfy the phase distribution.

[0013] 6.) Observe the nanoarray arranged in step 5) and use the chiral structure and its mirror structure to achieve polarization decoupling. Specifically, when RCP light is incident from the front of the metasurface, the first hologram I can be detected on the reflective surface. A , while displaying pattern I in the near field C ; Using LCP light incident from the front of the metasurface, a second hologram I can be detected on the reflective surface B , while displaying pattern I in the near field CThis polarization decoupling property enables encrypted information to be read selectively by polarization state.

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

[0015] Furthermore, in step 5), the optimization process using the GS algorithm is completed by calculation using matlab software, wherein the target holographic image I A , I B and near-field pattern I C As the optimization target of the algorithm is input, the optimized function is set to the error between the restored holographic information and the target hologram, and the error function is defined as:

[0016]

[0017] Where I is the target image, I′ is the image restored by the algorithm, i and j represent the row and column numbers of the pixel points respectively; the output is two holographic images I A , I B and near-field pattern I C .

[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 of two holographic images. These phase distribution matrices are used to guide the arrangement of the nanostructure array so that the target holographic image can be reconstructed in the far field. Through the amplitude characteristics of the singular point, the amplitude difference of the nanostructure is used to form an encrypted pattern in the near field. This near-field pattern is combined with the holographic image to achieve near- and far-field integrated encryption.

[0019] Through the above design, the target information is encoded into the holographic image and near-field pattern. Due to the characteristics of polarization decoupling and singularity, 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 encrypted pattern and holographic image can be flexibly changed to achieve dynamic encryption and information update. Since the encrypted information depends on the amplitude characteristics of the singularity, polarization decoupling and precise nanostructure design, it is difficult to crack the encrypted information by conventional means. 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] Singular points in non-Hermitian metasurfaces refer to the intersections where the eigenvalues ​​and eigenstates of the Hamiltonian of a system intersect with each other in a non-Hermitian system. At these points, two or more eigenvalues ​​are equal, that is, degeneracy occurs, and the corresponding eigenstates are no longer orthogonal. Singular points in metasurfaces demonstrate unique functions in terms of light field enhancement, energy concentration, polarization control, and topological protection.

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

[0022]

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

[0024] Formula (1) can be transformed into

[0025]

[0026] in When x =ω y =ω0, The eigenvalues ​​and corresponding eigenstates can be obtained directly

[0027]

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

[0029] Next, in order to study EP intuitively, we consider describing the intrinsic transmission of the metasurface based on circular polarization. The transmission matrix can be described as:

[0030]

[0031] where r jk is 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. xx ≠r yy , due to reciprocity r xy =r yx It can be seen that r ++ =r -- , the eigenvalue is The conditions required to achieve EP are:

[0032] r +- r-+ =0

[0033] The present invention provides a near-far field encryption method based on the amplitude characteristics of singular points, which uses the chiral structure and its mirror structure to respectively regulate the far-field holographic combination camouflage information, and simultaneously uses the amplitude difference to form a near-field encryption pattern. The method of the present invention is cleverly designed, which not only manipulates the phase information of the unit structure, but also uses the amplitude difference to realize the near-field pattern, and can be used for near-far field simultaneous encryption schemes.

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

[0035] By utilizing the amplitude characteristics of the singular points of the non-Hermitian metasurface, combined with the PB phase, and utilizing the polarization decoupling characteristics of the singular points and the improved GS algorithm, a scheme for simultaneous encryption of near and far fields is realized. This structure can regulate multiple parameters of the light field, and can accurately control the propagation and phase of the light beam, bringing innovative solutions to multiple fields such as optical communications, imaging, and sensing. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a schematic diagram of the nanobrick unit structure in the present invention;

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

[0038] Figure 3 Schematic diagram of the reflectivity of the nanobrick structure in an embodiment of the present invention.

[0039] Figure 4 Schematic diagram of the relationship between the real part of the eigenvalue and the wavelength of the nanobrick structure in an embodiment of the present invention.

[0040] Figure 5 Schematic diagram of the relationship between the imaginary part of the eigenvalue and the wavelength of the nanobrick structure in an embodiment of the present invention.

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

[0042] Figure 7 Schematic diagram of two cross-polarization reflectivities of nanobricks at different rotation angles in an embodiment of the present invention.

[0043] Figure 8a is a target image for generating a hologram in an embodiment of the present invention, Figure 8b Image I reconstructed after holographic imaging is achieved through the metasurface A .

[0044] Figure 9a The crying face in the figure is a target image for generating a hologram in an embodiment of the present invention. Figure 9bImage I reconstructed after holographic imaging is achieved through the metasurface B .

[0045] Fig.10a is the far-field hologram I when incident with RCP A , Fig.10b The butterfly near field diagram I is caused by the amplitude characteristics C .

[0046] Fig.11a is the far-field hologram I when LCP is incident B , Fig.11b The butterfly near field diagram I is caused by the amplitude characteristics C The complementary graph of .

[0047] Fig.12a For two holograms under the incidence of wrong wavelength or linear polarization light A and I B Camouflage infographic formed by superposition, Figure 12b As well as the amplitude difference reduction, the near field diagram I C Disappearing diagram. DETAILED DESCRIPTION

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

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

[0050] 1.) Constructing a nanostructure unit: The nanostructure unit is composed 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 the y-axis, and a z-axis is established along the vertical direction.

[0051] 2.) Optimization parameters: According to the selected working wavelength, the structural parameters of the nanobrick are optimized by CST electromagnetic simulation software, including: 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 nanobrick height h2.

[0052] 3.) Determine the structural parameters of the EP: By changing the structural parameters of the nanobrick, use CST electromagnetic simulation software to determine the singularity point EP.

[0053] 4.) Determine two holographic images I A and I B Phase distribution: Based on the principle of computational holography, the improved GS algorithm is used to analyze the phase distribution of two images with pattern I A and Pattern I B The target image is operated to obtain two phase distribution matrices, where I AThe phase distribution is only in the near-field pattern I C Up, I B The phase distribution in the near field pattern I C complementary pattern.

[0054] 5.) Arrange the nanostructure array: According to the two holographic images I determined in step 4) A and I B The phase distribution is achieved by changing the rotation angles of the two nanobricks to obtain the PB phase to satisfy the phase distribution.

[0055] 6.) Observe the nanoarray arranged in step 5) by incident RCP light from the front of the metasurface, and the first hologram I can be detected on the reflection surface. A , while displaying pattern I in the near field C ; Using LCP light incident from the front of the metasurface, a second hologram I can be detected on the reflective surface B , while displaying pattern I in the near field C complementary pattern.

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

[0057] Furthermore, in step 5), the optimization process using the GS algorithm is completed by calculation using matlab software, wherein the target holographic image I A , I B and near-field pattern I C As the optimization target of the algorithm is input, the optimized function is set to the error between the restored holographic information and the target hologram, and the error function is defined as:

[0058]

[0059] Where I is the target image, I′ is the image restored by the algorithm, i and j represent the row and column numbers of the pixel points respectively; the output is two holographic images I A , I B and near-field pattern I C .

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

[0061] The nanobrick is simulated under the above structural parameters, and the reflection spectrum of vertically incident circularly polarized light is shown in Figure 3 Shown, where R ij Represents the reflection from j-polarized input light to i-polarized output light. It can be seen that one of the cross polarizations tends to zero, corresponding to the operating wavelength of 487nm. Figure 4 and Figure 5 When the wavelength is 487nm, the real and imaginary parts of the eigenvalue are degenerate, which indicates that the eigenvalue is degenerate and proves the existence of the singular point at 487nm.

[0062] By rotating the nanobrick, the phase and reflectivity change with the rotation angle as shown in Figure 6 and Figure 7 As shown. The cross-polarization component R +- The reflectivity remains basically unchanged, and the phase Continuous phase control can be achieved, which is consistent with the PB phase. The other cross-polarization component R -+ Although not completely 0, its phase The PB phase is not satisfied. Therefore, the crosstalk between the two holograms is relatively small.

[0063] Next, we use FDTD simulation to verify the solution. LCP and RCP are incident vertically on the metasurface, and the holographic images they generate are shown in Figure 2. Figure 8a-9b As shown in the figure. The smiling and crying faces on the left are input images, which are the target images used to generate holograms and represent ideal input images. The corresponding images on the right are reconstructed images after holographic imaging is achieved by the metasurface. These images accurately restore the contours and features of the input images as a whole, demonstrating the superior performance of the metasurface in light field control.

[0064] At the same time, the "butterfly" near-field pattern caused by the amplitude characteristics of the chiral structure is displayed in the near field, such as Figure 10a-Figure 11b When the wrong wavelength or linear polarization is used as the incident light, the two holograms will be superimposed to generate an erroneous image, such as Fig.12a As shown, the near-field diagram will become invisible due to the similar amplitude difference between the two structures, as shown in Figure 12b The simulation results verify the feasibility of the near-field and far-field encryption schemes, providing a new approach for information coding and encryption, and have potential application prospects in the fields of information multiplexing, information hiding and coding.

[0065] The contents described in the embodiments of this specification are merely an enumeration of the implementation forms of the inventive concept. The protection scope of the present invention should not be regarded as limited to the specific forms described in the embodiments. The protection scope of the present 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 its mirror structure are used to regulate the far-field holographic combination camouflage information respectively, and the amplitude difference is used to form a near-field encryption pattern. The following steps are involved: 1) constructing a nanostructure unit: the nanostructure unit is composed 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 the y-axis, and a z-axis is established along the vertical direction; 2) Optimization parameters: According to the selected working wavelength, the structural parameters of the nanobrick are optimized by CST electromagnetic simulation software; including: 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 nanobrick height h2; 3) Determine the structural parameters of the EP: By changing the structural parameters of the nanobrick, use CST electromagnetic simulation software to determine the singular point EP; 4) Determine two holographic images I A and I B Phase distribution: Based on the principle of computational holography, the improved GS algorithm is used to analyze the phase distribution of two images with pattern I A and Pattern I B The target image is operated to obtain two phase distribution matrices, where I A The phase distribution is only in the near-field pattern I C Up, I B The phase distribution in the near field pattern I C on the complementary pattern of 5) Arrange the nanostructure array: According to the two holographic images I determined in step 4) A and I B The phase distribution is achieved by changing the rotation angles of the two nanobricks to obtain the PB phase to meet the phase distribution; 6) Observe the nanoarray arranged in step 5) by incident RCP light from the front of the metasurface, and the first hologram I can be detected on the reflection surface. A , while displaying pattern I in the near field C ; Using LCP light incident from the front of the metasurface, a second hologram I can be detected on the reflective surface B , while displaying pattern I in the near field C complementary pattern.

2. According to claim 1, a near-far field encryption method based on the amplitude characteristics of a singular point is characterized in that: The substrate of the nanostructure unit in step 1) is made of silicon dioxide material, and the nanobrick part is made of silicon.

3. The near-far field encryption method based on the singular point amplitude characteristics according to claim 1 is characterized in that: The structural parameters of the nanobricks described in step 2) are obtained by electromagnetic simulation optimization based on the selected incident light wavelength λ, and the optical performance of the device is improved by optimizing these structural parameters.

4. The near-far field encryption method based on the singular point amplitude characteristic according to claim 1 is characterized in that: The improved GS algorithm described in step 4) is calculated by Matlab software, wherein the target holographic image I A , I B and near-field pattern I C As the optimization target of the algorithm is input, the optimized function is set to the error between the restored holographic information and the target hologram, and the error function is defined as: Where I is the target image, I′ is the image restored by the algorithm, i and j represent the row and column numbers of the pixel points respectively; the output is two holographic images I A , I B and near-field pattern I C .

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