Method for realizing near-far field multichannel image encryption based on chiral metasurface

Abstract

The invention discloses a method for realizing near-far field multichannel image encryption based on a chiral metasurface. The method comprises the following steps: 1) constructing a chiral nanometer unit structure; 2) determining incident light wavelengths lambda1 and lambda2 through scanning unit structure parameters; and 3) constructing a nano-structure array, wherein the nano-structure array comprises a plurality of nano-structure units. According to the invention, for incident light with different wavelengths and polarization states, multi-channel near-far field geographic imaging can be realized, and the device has the characteristics of simple structure and easy integration, and has potential application value in multifunctional metasurface image encryption.

Description

Technical Field

[0001] The present invention relates to the technical field of micro-nano photonics, 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] Metasurface is a special two-dimensional metamaterial that can flexibly and effectively control the phase, polarization and amplitude of electromagnetic waves at a sub-wavelength scale, thus creating unique electromagnetic responses that do not exist in nature, and is therefore widely used in various fields. Therefore, the sudden change in the phase of the structure surface can be used to achieve wavefront manipulation of the reflected or transmitted wave, and to control the phase, amplitude and polarization state of the electromagnetic wave, thereby achieving the purpose of focusing, beam deflection and generating special beams.

[0003] Chiral structures refer to those structures that cannot overlap with their mirror images, that is, these structures cannot be superimposed with their mirror images under symmetry transformation. Its main working principle is based on the different regulatory responses of the structure to left and right circularly polarized light (LCP / RCP) at the singular point. For example, when the structure is incident with LCP at a specific wavelength, the cross-polarization efficiency will be very high, that is, LCP is converted to RCP; and when RCP is incident at the same wavelength, the cross-polarization efficiency is 0, that is, it will not be converted into LCP. Based on the polarization selection characteristics of the chiral structure, more degrees of freedom are provided for image encryption.

[0004] Near-field nanoprinting and far-field holographic imaging technologies each have their own advantages in metasurface image encryption. Near-field nanoprinting technology can produce high-resolution nanopatterns, achieve high-density information storage, invisible encryption, and multi-level encryption; while far-field holographic imaging technology can record and reconstruct complex three-dimensional holograms, achieve efficient holographic anti-counterfeiting, dynamic holographic encryption, and multi-spectral encryption. The application of these technologies has greatly improved the security and complexity of information encryption and anti-counterfeiting, and is suitable for various fields with high security requirements. Summary of the invention

[0005] The present invention makes full use of the wavelength, polarization and structural singularity characteristics of light and other degrees of freedom and combines Malus's law to comprehensively realize multi-channel imaging. The proposed metasurface can only obtain correct and clear near-field grayscale, binary images and far-field holographic images under specific wavelength, polarization state and emission condition settings.

[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 nano structure on the upper layer, a silicon dioxide dielectric layer in the middle layer, and a metal aluminum nano substrate in the lower layer. The metal aluminum nano structure on the upper layer is arranged on the top platform of the middle layer, including a first nano brick and a second nano brick spliced ​​into an L shape, and another independent third nano brick, the long axis of the second nano brick is parallel to the long axis of the third nano brick, and the long axis of the first nano brick is arranged on the side of the long axis of the second nano brick away from the third nano brick. The three nano bricks are all rectangular cubes, with long sides being the long axes L1, L2, and L3, short sides being the short axes W, and heights 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) Optimize the nano-unit structure: Optimize the unit structure parameters through electromagnetic simulation to determine the wavelength λ1 at which the singular point characteristics can be 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 optimization parameters include: the long axis L1, L3 of the upper nano-brick, but the short axis W, height H1 and distribution spacing G, the period P and thickness H2 of the intermediate dielectric layer and the period P and thickness H3 of the lower nano-base remain unchanged;

[0009] 3) Constructing nanostructure arrays: The nanostructure array is composed of upper structures with different rotation angles and fixed middle and lower structures, in which the rotating upper structure plays a modulating role. When designing the superlens structure, the focus is on determining the angle relationship of the upper structure, and 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.

[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 nanounit structure by 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, the incident left-handed circularly polarized light can achieve far-field holographic imaging, while the 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, the cross-polarization rate conversion can be 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 different wavelengths and different polarization states are incident. 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-linear polarized light is incident on the nanostructure array at λ2, the outgoing light beam will form a grayscale nanoprint image in the near field after passing through the analyzer in the X direction, and when the analyzer direction is rotated to 45°, the outgoing light beam will form another binary nanoprint image in the near field.

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

[0014] First, the near-field grayscale imaging is realized 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 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 value light intensity is set to 1, and the low and threshold value light intensity is positioned to 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 simulated annealing algorithm can be used to arrange the correct phase distribution of the far-field holographic image (the phase size is twice the rotation angle). At this time, the same metasurface can simultaneously meet the imaging effects of the three near-field and far-field images. Design process:

[0015] The design steps are divided into three steps: first, the rotation angle of the unit structure corresponding to the intensity value of each pixel in the grayscale image is determined by combining Malus's law. Considering the characteristics of the curve, the same intensity can correspond to four different angles within one cycle; secondly, if the 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 intensity points on the new intensity curve distribution, and the image is binarized by setting the threshold, that is, the high value intensity is set to 1 and the low value intensity is positioned to 0. Finally, since different angles can be selected at the same intensity, the resulting geometric phase regulation is different, which is very important for holographic imaging phase information. The final unit structure arrangement can be determined by the simulated annealing algorithm. It can be seen in this design process that, first of all, this design only considers the use of geometric phase, avoiding the trouble of size design caused by the transmission phase; secondly, this only needs to adjust the polarization state and polarization direction of the incident light and the outgoing light, that is, the same metasurface can complete the imaging of different channels, reflecting 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 the present invention makes full use of the properties of light in image imaging, such as the setting of wavelength, polarization state and output light conditions, and ensures the security of encrypted information under the multi-degree-of-freedom design;

[0019] 3) Integrate different functions on the same metasurface, reduce volume and weight while ensuring imaging quality, and adapt to the development of miniaturization and micro-miniaturization in the future; 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 is a top view of the unit structure of the present invention;

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

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

[0024] Figure 5a and Figure 5b 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 is further described below in conjunction with embodiments, but the scope of protection of the present invention is not limited to the described scope.

[0026] A method for realizing 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 nano unit structure is composed of metal aluminum Al and silicon dioxide SiO2 materials, with two layers of aluminum nano bricks and silicon dioxide as the intermediate dielectric layer. First, select a suitable chiral structure to avoid large crosstalk in the experiment and ensure the effect; secondly, construct the nano unit structure, and scan the upper structure parameters L1 and L3 through electromagnetic simulation software. As shown in Figure 2, the working wavelength is determined to be λ1=540nm, λ2=580nm, and the size parameters of the optimized unit structure are: length L1=45nm, L2=150nm, L3=95nm, width W=50nm, height H1=35nm, H2=40nm, H3=190nm, and the unit structure base period is P=300nm.

[0028] As shown in Figure 2, for circularly polarized light incident, at the working wavelength λ1 (540nm), the cross-polarization amplitude reaches 0.5 when LCP is incident and is almost 0 when RCP is incident, which indicates that the designed unit structure can have different control capabilities for circularly polarized light of different rotation directions at the same wavelength; for linearly polarized light incident, at the working wavelength of λ2 (580nm), the cross-polarization amplitude reaches a maximum value of 0.35, which indicates that the device achieves cross-polarization maximization at this wavelength. It should be pointed out here that the MIM design of the unit structure leads to a low maximum amplitude value, which is within the normal range and will not have a significant impact on 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 nanoprint 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 nanoprint 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] The present invention can achieve near-field and far-field multi-channel imaging on the same metasurface by changing the wavelength, polarization properties of the incident light and the polarization direction of the outgoing light. It has the characteristics of multiple degrees of freedom, simple structure and easy integration, while ensuring the imaging quality, reducing the volume and weight, and adapting to the development of miniaturization and micro-miniaturization in the future;

[0031] 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 method for realizing near-field and far-field multi-channel image encryption based on chiral metasurface, characterized in that The steps include: 1) Constructing a nano unit structure: The nano unit structure is a metal-medium-metal (MIM) structure, including a metal aluminum nano structure at the upper layer, a silicon dioxide dielectric layer at the middle layer, and a metal aluminum nano substrate at the lower layer; The metal aluminum nanostructure of 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 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 long axis of the second nanobrick away from the third nanobrick; the three nanobricks are all rectangular cubes, the long sides are the long axis L1, L2, L3, 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 nano unit structure: Optimizing the unit structure parameters through electromagnetic simulation, determining the wavelength λ1 at which the singular point characteristics can be achieved when circularly polarized light is incident and the wavelength λ2 at which the cross polarization rate conversion is higher when linearly polarized light is incident, and the optimization parameters include: the long axis L1, L3 of the upper nano brick, but the short axis W, height H1 and distribution spacing G, 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 nanostructure arrays: The nanostructure array is composed of upper structures with different rotation angles and fixed middle and lower structures, in which the rotating upper structure plays a modulating role. When designing the superlens structure, the focus is on determining the angle relationship of the upper structure, and 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 materials.

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, that is, incident light with 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 the wavelength of λ2, a higher cross-polarization conversion rate is sufficient, and combined with Malus's law and the degeneracy of the rotation angle of the unit structure, different imaging effects can be achieved for different polarization states of the outgoing light.

4. The method for implementing near-field and far-field multi-channel image encryption based on a 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 at λ1, the outgoing light beam will form a holographic image in the far field; if Y-linear polarized light is incident on the nanostructure array at λ2, the outgoing light beam will form a grayscale nanoprinting image in the near field after passing through the analyzer in the X direction, and when the analyzer direction is rotated to 45°, the outgoing light beam will form another binary nanoprinting image 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 curves of the analyzer along the x-axis direction and at 45° should satisfy in turn: I=sin 2 (2θ) (1) The geometric phase (PB) control should meet the following requirements: ψ=2θ (3) Among them, 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 chiral metasurface according to claim 4, characterized in that: 3) The step of designing the super lens structure in step 1 includes: 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 the 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 to 0; finally, since different angles can be selected at the same light intensity, the resulting geometric phase regulation is different, which is very important for holographic imaging phase information, and the final arrangement of the unit structure can be determined by a simulated annealing algorithm.

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