A method for realizing near-far field multi-channel image encryption based on chiral metasurface

Abstract

The application discloses a method for realizing near-far field multi-channel image encryption based on a chiral super surface, and comprises the following steps: 1) constructing a chiral nano unit structure; 2) determining incident light wavelengths λ1 and λ2 by scanning unit structure parameters; and 3) constructing a nano structure array, wherein the nano structure array comprises a plurality of nano structure units. The application can realize multi-channel near-far field geographic imaging for incident light of different wavelengths and polarization states, and has the characteristics of simple structure and easy integration, and has potential application value in multi-functional super surface image encryption.

Description

Technical Field

[0001] The present invention relates to the field of micro-nano photonics technology, and in particular to a method for implementing near-field and far-field multi-channel image encryption based on a chiral metasurface. Background Art

[0002] Metasurfaces are a special type of two-dimensional metamaterial that can flexibly and effectively manipulate the phase, polarization, and amplitude of electromagnetic waves at subwavelength scales. This allows for the creation of unique electromagnetic responses not found in nature, leading to their widespread application in various fields. The abrupt phase shifts on the surface of these structures can be exploited to manipulate the wavefront of reflected or transmitted waves, controlling the phase, amplitude, and polarization of electromagnetic waves for purposes such as focusing, beam deflection, and the generation of specialized beams.

[0003] Chiral structures are those that cannot be superimposed on their mirror images, meaning they cannot be superimposed on their mirror images under symmetry transformations. Their primary operating principle is based on the structure's differential response to the manipulation of left and right circularly polarized light (LCP / RCP) at singular points. For example, when LCP is incident on the structure at a specific wavelength, the cross-polarization efficiency is very high, converting LCP to RCP. However, when RCP is incident on the same wavelength, the cross-polarization efficiency is zero, meaning it does not convert to LCP. The polarization-selective nature of chiral structures provides additional degrees of freedom for image encryption.

[0004] Near-field nanoprinting and far-field holographic imaging technologies each offer advantages in metasurface image encryption. Near-field nanoprinting can create high-resolution nanopatterns, enabling high-density information storage, invisible encryption, and multi-level encryption. Far-field holographic imaging, on the other hand, can record and reconstruct complex three-dimensional holograms, enabling efficient holographic anti-counterfeiting, dynamic holographic encryption, and multispectral encryption. The application of these technologies significantly improves the security and sophistication of information encryption and anti-counterfeiting, making them suitable for a variety of high-security applications. Summary of the Invention

[0005] This invention achieves multi-channel imaging by leveraging multiple degrees of freedom (DOF) such as wavelength, polarization, and structural singularity characteristics of light, combined with Malus's law. The proposed metasurface can only capture accurate and clear near-field grayscale and binary images and far-field holographic images under specific wavelength, polarization, and emission conditions.

[0006] The present invention provides a method for implementing near-field and far-field multi-channel image encryption based on a chiral metasurface, comprising the following steps:

[0007] 1) Constructing a nano-unit structure: The nano-unit structure is a metal-dielectric-metal (MIM) structure, including a metal aluminum nanostructure on the upper layer, a silicon dioxide dielectric layer in the middle layer, and a metal aluminum nanosubstrate on the lower layer. The metal aluminum nanostructure on the upper layer is arranged on the top platform of the middle layer, including a first nanobrick and a second nanobrick spliced ​​into an L shape, and another independent third nanobrick, the long axis of the second nanobrick is parallel to the long axis of the third nanobrick, and the long axis of the first nanobrick is arranged on the side of the long axis of the second nanobrick away from the third nanobrick. The three nanobricks are all rectangular cubes, with the long sides being the long axes L1, L2, and L3 respectively, the short sides being the short axes W, and the height being H1; the thickness of the middle silicon dioxide structure is H2, and the period is P; the thickness of the lower aluminum substrate is H3, and the period is P;

[0008] 2) Optimizing the nanostructure: Electromagnetic simulation was used to optimize the structural parameters of the nanostructure to determine the wavelength λ1 at which the singularity characteristic is achieved when circularly polarized light is incident, and the wavelength λ2 at which the cross-polarization conversion rate is high and the co-polarization conversion rate is low when linearly polarized light is incident. The optimized parameters include the major axes L1 and L3 of the upper nanobricks, while the minor axis W, height H1, and distribution spacing G, as well as the period P and thickness H2 of the intermediate dielectric layer and the period P and thickness H3 of the lower nanobase layer remain unchanged.

[0009] 3) Constructing a Nanostructure Array: The nanostructure array is composed of upper structures with different rotation angles and fixed middle and lower structures. The rotating upper structure plays a modulating role. When designing the metalens structure, the key is to determine the angular relationship of the upper structure. The rotation angle must ensure that it satisfies the light intensity relationship for near-field grayscale and binary imaging and the phase distribution relationship for far-field holographic imaging.

[0010] Furthermore, the intermediate dielectric layer in step 1) is made of SiO2 material, and the upper nanostructure and the lower substrate are both made of metal Al material.

[0011] Furthermore, the wavelength λ of the incident working light described in step 2) is determined by scanning the parameters L1 and L3 of the nano-unit structure through electromagnetic simulation software. For far-field holographic imaging, polarization conversion can be achieved for right-handed circularly polarized light at a wavelength of λ1, while there is no polarization conversion response for left-handed circularly polarized light. That is, incident left-handed circularly polarized light can achieve far-field holographic imaging, while incident right-handed circularly polarized light cannot achieve it, that is, incident light of different polarization states at the same wavelength has a selective holographic imaging response. For near-field grayscale and binary image imaging, when linearly polarized light is incident at a wavelength of λ2, its cross-polarization rate conversion is higher. Combined with Malus's law and the degeneracy of the unit structure rotation angle, different imaging effects can be achieved for different polarization states of the outgoing light.

[0012] Furthermore, in step 3), when designing the metalens structure, different imaging effects should be achieved for the outgoing light in the near and far fields when incident at different wavelengths and polarization states. Specifically, if right-handed circularly polarized light is incident on the nanostructure array at λ1, the outgoing light beam will form a holographic image in the far field; if Y-linearly polarized light is incident on the nanostructure array at λ2, the outgoing light beam will form a grayscale nanoprint in the near field after passing through the X-direction analyzer. When the analyzer direction is rotated to 45°, the outgoing light beam will form another binary nanoprint in the near field.

[0013] The working principle of the present invention is:

[0014] First, the near-field grayscale imaging is achieved by applying Malus's law and the functional relationship between the unit structure rotation angle and the transmitted light intensity. At this time, the polarizer and the analyzer are perpendicular to each other; secondly, within one cycle, the same light intensity can correspond to four different angles. When the analyzer is turned from the vertical direction to 45°, the light intensity curve distribution moves by π / 8 relative to the original distribution; the above four angles will form high and low light intensities on the new light intensity curve distribution. By setting the threshold, the high and threshold values ​​of light intensity are set to 1, and the low and threshold values ​​of light intensity are positioned at 0, thereby realizing binary image imaging; finally, considering that there can be multiple angles to choose from under the same light intensity, and different angles represent different geometric phases, on this basis, the correct far-field holographic image phase distribution can be arranged through the simulated annealing algorithm (the phase size is twice the rotation angle). At this time, the same metasurface can simultaneously meet the imaging effects of the three images in the near and far fields. Design process:

[0015] The design process consists of three steps: First, Malus's law is used to determine the rotation angle of the unit structure corresponding to the intensity value of each pixel in the grayscale image. Considering the curve characteristics, within one cycle, the same intensity can correspond to four different angles. Second, if this intensity curve is shifted to the right by π / 8, corresponding to a 45° rotation of the outgoing polarization light, the four angles will form a high and low intensity distribution in the new intensity curve. The image is binarized by setting a threshold, setting the high intensity value to 1 and the low intensity value to 0. Finally, because different angles can be selected for the same intensity, the resulting geometric phase control is different. Phase information is crucial for holographic imaging, and the final unit structure arrangement can be determined using a simulated annealing algorithm. This design process shows that, first, the design only considers the geometric phase, avoiding the dimensional design difficulties caused by the transmission phase. Second, the design only needs to adjust the polarization state and polarization direction of the incident and outgoing light, meaning that the same metasurface can complete imaging of different channels, embodying the integration of different functions.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] 1) The technical solution of the present invention only considers the use of geometric phase, and the design method is ingenious and the processing difficulty is low;

[0018] 2) The metasurface proposed in this invention fully utilizes the properties of light in imaging, such as wavelength, polarization state, and the setting of output light conditions. Through a multi-degree-of-freedom design, it ensures the security of encrypted information.

[0019] 3) Integrate different functions on the same metasurface, reducing volume and weight while ensuring imaging quality, adapting to future miniaturization and micro-microscopy development; BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1a and Figure 1b is a schematic diagram of the unit structure of the present invention, wherein Figure 1a is a perspective view of the unit structure of the present invention, Figure 1b It is a top view of the unit structure of the present invention;

[0021] Figure 2a and Figure 2b is a cross polarization rate diagram of the unit structure of the present invention under different polarized light incidence, where Figure 2a is the cross polarization ratio diagram under circularly polarized light incidence, Figure 2b is the cross-polarization ratio diagram under polarized light incidence;

[0022] Figure 3a and Figure 3b is the effect diagram of the near-field grayscale image of the present invention, wherein Figure 3a It's the original picture. Figure 3b It is a simulation diagram;

[0023] Figure 4a and Figure 4b This is the effect diagram of the near-field binary image of the present invention, where Figure 4a It's the original picture. Figure 4b It is a simulation diagram;

[0024] Figure 5a and Figure 5b This is the effect diagram of the far-field holographic image of the present invention, wherein Figure 5a It's the original picture. Figure 5b It is a simulation picture. DETAILED DESCRIPTION

[0025] The present invention will be further described below with reference to the embodiments, but the scope of protection of the present invention is not limited to the described scope.

[0026] A method for achieving near-field and far-field multi-channel imaging based on a chiral metasurface, the specific steps are as follows:

[0027] As shown in Figure 1, the nanostructure is composed of aluminum (Al) and silicon dioxide (SiO2), with two layers of aluminum nanobricks and silicon dioxide serving as the intermediate dielectric layer. First, a suitable chiral structure was selected to avoid significant crosstalk during the experiment and ensure effective results. Second, the nanostructure was constructed, and electromagnetic simulation software was used to scan the upper structural parameters, L1 and L3. As shown in Figure 2, the operating wavelengths were determined to be λ1 = 540nm and λ2 = 580nm. The optimized unit structure dimensions are: length L1 = 45nm, L2 = 150nm, L3 = 95nm; width W = 50nm; height H1 = 35nm, H2 = 40nm, H3 = 190nm; and the unit structure base period P = 300nm.

[0028] As shown in Figure 2, for circularly polarized light, at the operating wavelength λ1 (540nm), the cross-polarization amplitude reaches 0.5 for LCP light and is almost 0 for RCP light, indicating that the designed unit structure can control circularly polarized light of different rotation directions at the same wavelength. For linearly polarized light, at the operating wavelength λ2 (580nm), the cross-polarization amplitude reaches a maximum value of 0.35, indicating that the device maximizes cross-polarization at this wavelength. It should be noted that the MIM design of the unit structure results in a low maximum amplitude value, which is within the normal range and will not significantly affect the final imaging effect.

[0029] Construct a nanostructure array, wherein the nanostructure array comprises a plurality of nanostructure units. As shown in FIG3 , if Y linear polarized light is incident on the nanostructure array at λ2, the outgoing light beam will form a grayscale nanoprinting image (image smiley face) in the near field after passing through the analyzer in the X direction. As shown in FIG4 , if Y linear polarized light is incident on the nanostructure array at λ2, when the analyzer direction is rotated counterclockwise from the horizontal direction to 45°, the outgoing light beam will form another binary nanoprinting image (letter Z) in the near field. As shown in FIG5 , if right-handed circularly polarized light is incident on the nanostructure array at λ1, the outgoing light beam will form a holographic image (letter D) in the far field.

[0030] This invention achieves near- and far-field multi-channel imaging on the same metasurface by changing the wavelength and polarization properties of the incident light and the polarization direction of the outgoing light. It has multiple degrees of freedom, a simple structure, and is easy to integrate. It reduces size and weight while ensuring imaging quality, adapting to future miniaturization and micro-processing developments.

[0031] The contents described in the embodiments of this specification are merely an enumeration of the implementation forms of the inventive concept. The scope of protection of the present invention should not be regarded as limited to the specific forms described in the embodiments. The scope of protection 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 method for near-field and far-field multi-channel image encryption based on chiral metasurfaces, characterized in that The steps include: 1) Constructing a nanostructure: The nanostructure is a metal-dielectric-metal (MIM) structure comprising an upper aluminum nanostructure, a middle silicon dioxide dielectric layer, and a lower aluminum nanostructure substrate. The upper layer of the metal aluminum nanostructure is arranged on the top platform of the middle layer, including a first nanobrick and a second nanobrick spliced ​​into an L shape and an independent third nanobrick, the long side of the second nanobrick is parallel to the long side of the third nanobrick, and the long side of the first nanobrick is arranged on the side of the second nanobrick away from the long axis of the third nanobrick; the three nanobricks are rectangular cubes, the long sides are the long axis L1, L2, and L3 respectively, the short sides are the short axis W, and the height is H1; the thickness of the silicon dioxide structure of the middle layer is H2, and the period is P; the thickness of the lower aluminum substrate is H3, and the period is P; 2) Optimizing the nanostructure: Optimizing the structural parameters of the nanostructure through electromagnetic simulation to determine the wavelength λ1 at which the singularity characteristic can be achieved when circularly polarized light is incident, and the wavelength λ2 at which the cross-polarization conversion rate is higher when linearly polarized light is incident. The optimized parameters include: the major axis L1, L3, minor axis W, height H1, and distribution spacing G of the upper nanobricks; the period P and thickness H2 of the intermediate dielectric layer and the period P and thickness H3 of the lower nano-substrate remain unchanged; 3) Constructing a nanostructure array: The nanostructure array is composed of upper structures with different rotation angles and fixed middle and lower structures. The rotating upper structure plays a modulating role. When designing the superlens structure, the focus is on determining the angular relationship of the upper structure. It is necessary to ensure that the rotation angle simultaneously satisfies the light intensity relationship of near-field grayscale and binary image imaging and the phase distribution relationship of far-field holographic imaging.

2. The method for realizing near-field and far-field multi-channel image encryption based on chiral metasurface according to claim 1, characterized in that 1) The intermediate dielectric layer described in the step is SiO2 material, and the upper nanostructure and the lower substrate are both metal Al material.

3. The method for realizing near-field and far-field multi-channel image encryption based on chiral metasurface according to claim 1, characterized in that 2) The incident light wavelengths λ1 and λ2 described in the step are obtained by simulation using electromagnetic simulation software: for far-field holographic imaging, polarization conversion can be achieved for right-handed circularly polarized light at the wavelength of λ1, while there is no polarization conversion response for left-handed circularly polarized light; that is, incident left-handed circularly polarized light can achieve far-field holographic imaging, while incident right-handed circularly polarized light cannot achieve it, that is, incident light with different polarization states at the same wavelength has selective holographic imaging response; for near-field grayscale and binary image imaging, when linearly polarized light is incident at the wavelength of λ2, its cross-polarization rate conversion is higher, and combined with Malus's law and the degeneracy of the unit structure rotation angle, different imaging effects can be achieved for different polarization states of the output light.

4. The method for realizing near-field and far-field multi-channel image encryption based on chiral metasurface according to claim 1, characterized in that: When designing the metalens structure in step 3), different imaging effects should be achieved for the outgoing light in the near and far fields when different wavelengths and different polarization states are incident: if right-handed circularly polarized light is incident on the nanostructure array with λ1, the outgoing light beam will form a holographic image in the far field; if Y-linearly polarized light is incident on the nanostructure array with λ2, the outgoing light beam will form a grayscale nanoprinting pattern in the near field after passing through the analyzer in the X direction. When the analyzer direction is rotated to 45°, the outgoing light beam will form another binary nanoprinting pattern in the near field; the intensity distribution of the outgoing light at each point on the metalens surface is obtained according to the following formula, and the light intensity curve of the analyzer along the x-axis direction and 45° should satisfy the following in turn: I=sin 2 (2θ) (1) Geometric phase (PB) control should meet the following requirements: ψ=2θ (3) Where I is the output light intensity, θ is the unit structure rotation angle, and ψ is the geometric phase.

5. The method for implementing near-field and far-field multi-channel image encryption based on a chiral metasurface according to claim 4, characterized in that: 3) The steps of designing the super lens structure in step 3 include: first, combining Malus's law to determine the rotation angle of the unit structure corresponding to the light intensity value of each pixel point in the grayscale image, considering the curve characteristics, within one cycle, the same light intensity can correspond to four different angles; secondly, if this light intensity curve is translated to the right by π / 8, that is, the corresponding outgoing polarized light is rotated by 45°, the above four angles will form high and low light intensities on the new light intensity curve distribution, and the image is binarized by setting the threshold, that is, the high value light intensity is set to 1 and the low value light intensity is positioned at 0; finally, since different angles can be selected at the same light intensity, the resulting geometric phase control is different, which is very important for the phase information of holographic imaging, and the final arrangement of the unit structure can be determined by the simulated annealing algorithm.

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