An image hiding encryption method based on bidirectional asymmetric nano-printed super surface

Abstract

The application discloses an image hiding encryption method based on a bidirectional asymmetric nano-printing super surface, and comprises the following specific steps: 1) constructing a nano structure unit; 2) optimizing the structure parameters of two nano bricks through an electromagnetic simulation software according to a selected working wavelength λ; 3) constructing a nano structure array, setting linearly polarized light with a polarization direction α1=π / 2 to be incident on the nano structure array from the upper surface and the lower surface respectively and then passing through a polarizer with a transparent axis direction α2=π / 2, the image information displayed under the two incident conditions is consistent, and the first image can be encoded in the nano structure array; linearly polarized light with an unchanged polarization direction is incident on the nano structure array and then passes through a polarizer with a transparent axis direction α2=0, the image information displayed under the two incident conditions is different, and the second image can be encoded in the nano structure array by adding the two images pixel by pixel, thereby providing a method for image hiding encryption and improving the concealment of the image.

Description

Technical Field

[0001] This invention belongs to the field of micro-nano optics and polarization optics, and specifically relates to an image hiding and encryption method based on a bidirectional asymmetric nanoprinted metasurface. Background Technology

[0002] Metasurface materials, composed of subwavelength structural arrays, possess the ability to precisely control various optical parameters such as the amplitude, phase, polarization, and orbital angular momentum of light. Therefore, information storage and optical encryption technologies based on metasurface materials have been extensively studied. Currently, a common approach is to construct multiple information multiplexing channels by changing the wavelength, polarization, and incident direction of the incident light, and then directly or indirectly decode the information based on these observation channels.

[0003] In methods for constructing multiplexed channels through asymmetric forward and reverse transmission, polarization selection is usually required. Currently, most methods only utilize one of the common-polarization or cross-polarization channels, or do not restrict the polarization state of the transmitted light, thus failing to fully utilize polarization. This invention, based on a double-layer variable-angle nanostructure, constructs a nanostructure array with the same bidirectional transmission of common-polarized light and different bidirectional transmission of cross-polarized light, and proposes an image hiding and encryption method based on the aforementioned bidirectional asymmetric transmission. This method has a certain degree of deception, making it difficult to detect the true information without cracking experience, greatly improving the image concealment. Summary of the Invention

[0004] In view of the problems existing in the prior art, the purpose of this invention is to provide an image hiding and encryption method based on a bidirectional asymmetric nanoprinted metasurface, which increases the concealment of information.

[0005] The technical solution of the present invention is as follows:

[0006] An image hiding and encryption method based on a bidirectional asymmetric nanoprinted metasurface includes the following specific steps:

[0007] 1) Constructing nanostructure units: The nanostructure unit includes a transparent substrate, on which nanobricks are deposited respectively on the upper and lower surfaces. An xoy coordinate system is established with the right-angled sides of the structure unit as the x-axis and y-axis. The long sides of the two types of nanobricks are the major axes and the short sides are the minor axes. The angles between the minor axes of the two types of nanobricks and the x-axis are the turning angles θ1 and θ2 of the nanobricks.

[0008] 2) Based on the selected working wavelength λ, the structural parameters of the two nanobricks are optimized using electromagnetic simulation software. The structural parameters include: the working surface side length C of the structural unit, the major axes L1 and L2, the minor axes W1 and W2, and the heights H1 and H2 of the two nanobricks.

[0009] 3) Construct a nanostructure array, which contains multiple nanostructure units; set linearly polarized light with polarization direction α1 = π / 2 to be incident on the nanostructure array from the upper and lower surfaces respectively, and then pass through an analyzer with transmission axis direction α2 = π / 2. The image information displayed under the two incident conditions is consistent, and the first image can be encoded in the nanostructure array. The image information can be directly observed as disguised false information; linearly polarized light with unchanged polarization direction is incident on the nanostructure array and then passes through an analyzer with transmission axis direction α2 = 0. The image information displayed under the two incident conditions is different. Adding the two images pixel by pixel can encode the second image in the nanostructure array. The image information is the hidden real information and cannot be directly observed.

[0010] Furthermore, the transparent substrate mentioned in step 1) is fused silica glass material, and the nano-brick material is silicon material.

[0011] Furthermore, in step 3), the nanobricks on the upper surface of the nanostructure array are all equivalent to a quarter-wave plate, and the nanobricks on the lower surface are all equivalent to a polarizer. When linearly polarized light is incident on the nanostructure array from the upper surface, that is, the linearly polarized light first passes through the quarter-wave plate and then through the polarizer, the Jones vector of the outgoing light can be expressed as:

[0012]

[0013] In the formula: This represents the electric field component along the x-axis. θ1 represents the electric field component in the y-axis direction, θ2 is the turning angle of the upper surface nanobrick, θ2 is the turning angle of the lower surface nanobrick, and α1 is the polarization direction of the incident ray-polarized light.

[0014] When linearly polarized light is incident on the nanostructure array from the lower surface—that is, the linearly polarized light first passes through the polarizer and then through the quarter-wave plate—the Jones vector of the outgoing light can be expressed as:

[0015]

[0016] Since light intensity is proportional to the square of the electric field intensity modulus, i.e., I∝|E| 2 Then there are two possible scenarios:

[0017] a. The polarization direction of the incident polarized light is parallel to the transmission axis of the analyzer (α1=α2=π / 2). The corresponding outgoing light is the same when incident from the upper and lower surfaces, which can be expressed as:

[0018]

[0019] In the formula: I0 is the intensity of the incident polarized light;

[0020] b. The polarization direction of the incident polarized light is perpendicular to the transmission axis of the analyzer (α1 = π / 2, α2 = 0). The corresponding outgoing light is different when the light is incident from the upper surface and the lower surface, which can be expressed as follows:

[0021]

[0022]

[0023] Furthermore, the first false information image is controlled by the emitted light intensity function I1, while the second real information image is controlled by the sum of the emitted light intensity functions I2 and I3 (I4 = I2 + I3).

[0024] Furthermore, the modulation relationship between the four emitted light intensities and the turning angles θ1 and θ2 of the two nanobricks is as follows:

[0025] a. When θ1 = 0 and θ2 = 0, the corresponding intensities are I1 = 0, I2 = 0, I3 = 0, and I4 = 0.

[0026] b. When θ1 = 0 and θ2 = π / 2, the corresponding intensities are I1 = 1, I2 = 0, I3 = 0, and I4 = 0.

[0027] c. When θ1 = π / 4 and θ2 = 0, the corresponding intensities are I1 = 0, I2 = 0.5, I3 = 0, and I4 = 0.5.

[0028] d. When θ1 = π / 4 and θ2 = π / 2, the corresponding intensities are I1 = 0.5, I2 = 0, I3 = 0.5, and I4 = 0.5.

[0029] By rationally arranging the turning angles of the two types of nanobricks, it is possible to simultaneously encode two images in a nanostructure array.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] 1) Using the technical solution of the present invention, when the same linearly polarized light irradiates the metasurface from the front and back sides respectively, the outgoing light with the same polarization in the forward and backward directions displays the same near-field image containing false information, while the outgoing light with cross-polarization can display two different near-field images. Only by superimposing and combining the forward and backward cross-polarized images can the true and complete image information be obtained.

[0032] 2) The double-layer variable-angle nanostructure array has the same co-polarized polarization channel response when incident on the upper and lower surfaces, but different cross-polarized polarization channel response. The difference in response between the two polarization channels allows the image of each channel to be designed independently, making full use of cross-polarized polarization and co-polarized polarization.

[0033] 3) The co-polarized polarization channel generated in this invention is a false image. The display effect is consistent in both forward and reverse incident directions and can be directly observed. The decoding conditions are simple. In contrast, the cross-polarized polarization channel generated is real image information. The image information displayed is different in forward and reverse incident directions. The two must be added together to display the complete information. The decoding conditions are complex and it is not easy to be observed. Therefore, the real image information is more concealed.

[0034] 4) The method of the present invention uses a simple structure. The two-layer nanobrick structure used has a fixed size. The image encoding requirements can be achieved by only changing the rotation angle. The metasurface designed in the present invention is small in size, light in weight, and highly integrated, and is suitable for future miniaturization and micro-miniaturization development. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the front structure of the nano-brick structure unit in this invention;

[0036] Figure 2 This is a schematic diagram of the back structure of the nano-brick structure unit in this invention;

[0037] Figure 3 This is a schematic diagram of the transmittance of the upper and middle nanobrick structures in an embodiment of the present invention.

[0038] Figure 4 This is a phase diagram of the upper and middle nanobrick structures in an embodiment of the present invention;

[0039] Figure 5 This is a schematic diagram of the transmittance and reflectance of the lower nanobrick structure in an embodiment of the present invention;

[0040] Figure 6 This is a schematic diagram of the light intensity response of the structural unit under different combinations of nanobrick rotation angles in the embodiments of the present invention. Figure 1 ;

[0041] Figure 7 This is a schematic diagram of the light intensity response of the structural unit under different combinations of nanobrick rotation angles in the embodiments of the present invention. Figure 2 ;

[0042] Figure 8 This is a schematic diagram of the light intensity response of the structural unit under different combinations of nanobrick rotation angles in the embodiments of the present invention. Figure 3 ;

[0043] Figure 9 This is a schematic diagram of the light intensity response of the structural unit under different combinations of nanobrick rotation angles in the embodiments of the present invention. Figure 4 ;

[0044] Figure 10 This is a binary image in the co-polarization polarization channel in an embodiment of the present invention;

[0045] Figure 11This is a binary image in the cross-polarization channel of an embodiment of the present invention. Detailed Implementation

[0046] 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.

[0047] Example 1

[0048] An image hiding and encryption method based on a bidirectional asymmetric nanoprinted metasurface includes the following steps:

[0049] First, construct nanostructure units, such as Figure 1-2 As shown, the nanounit structure consists of a transparent silica substrate, with silicon nanobricks deposited on the upper and lower surfaces of the silica substrate. An xoy coordinate system is established with the right-angled sides of the structural unit as the x-axis and y-axis, respectively. The long sides of the two types of nanobricks are the major axes, and the short sides are the minor axes. The angles between the major axes of the two types of nanobricks and the x-axis are the turning angles θ1 and θ2 of the nanobricks. The polarization direction of the incident light and the angles between the transmission axis of the analyzer and the x-axis are defined as α1 and α2, respectively. A wavelength of λ = 633 nm is selected. For this wavelength, the nanobrick unit structure is optimized using the electromagnetic simulation software CST. The optimized dimensions of the two types of nanobricks are as follows: the first type has a length of L1 = 120 nm, a width of W1 = 66 nm, and a height of H1 = 500 nm; the second type has a length of L2 = 143 nm, a width of W2 = 80 nm, and a height of H2 = 220 nm. The side length of the unit structure substrate is C = 300 nm; and the thickness of the silica dielectric layer is 300 nm.

[0050] Under the aforementioned structural parameters, separate simulations were performed on the two types of nanobricks. The transmittance and phase of the first type of nanobrick for linearly polarized light incident along its long and short axes are as follows: Figure 3-4 As shown, where T l1 T s1 P represents the transmission efficiency along the long and short axes of the nanobrick, respectively. l1 P s1 These represent the phases of the transmitted light transmitted along the long and short axes of the nanobrick, respectively. The transmission and reflection efficiencies of the second type of nanobrick for linearly polarized light incident along the long and short axes of the nanobrick are as follows: Figure 5 As shown, where R l2 R s2 T represents the reflected light efficiency along the long and short axes of the nanobrick, respectively. l2 T s2 These represent the transmission efficiency along the long and short axes of the nanobrick, respectively.

[0051] Depend on Figure 3-4 It can be seen that at a working wavelength of 633nm, T l1 and Ts1 The efficiency is consistent, both around 89%, while P l1 and P s1 The phase difference between them reaches 90°, indicating that the optimized nanobrick can achieve the function of a quarter-wave plate; by Figure 5 It can be seen that at the operating wavelength of 633nm, R l2 Up to 98%, R s2 When suppressed to below 4%, T s2 Up to 96%, T s2 The light intensity was suppressed to below 0.1%, indicating that almost all light incident along the long axis of the nanobrick was reflected and almost all light incident along the short axis of the nanobrick was transmitted. Therefore, the optimized nanobrick can achieve polarization beam splitting function, that is, it has polarizer function in both transmission and reflection directions.

[0052] Finally, a nanostructure array is constructed, comprising multiple nanostructure units. When linearly polarized light with fixed polarization passes through the nanostructure array from the upper and lower surfaces, respectively, and then through an analyzer whose transmission axis is parallel (or perpendicular) to the polarization direction of the incident light, the modulation functions of the forward and reverse transmitted light vary with the turning angles of the two nanobricks (the modulation functions are the same or different). Assuming the incident light polarization direction is parallel to the analyzer's transmission axis (α1 = α2 = π / 2), the intensity of both the forward and reverse transmitted light is I1. When the incident light polarization direction is perpendicular to the analyzer's transmission axis (α1 = π / 2, α2 = 0), the intensity of the forward transmitted light is I2, and the intensity of the reverse transmitted light is I3. The sum of the two transmitted light intensities is I4 = I2 + I3. The spectral responses of each light intensity under different combinations of the two nanobrick turning angles θ1 and θ2 are as follows: Figure 6-9 As shown, at a wavelength λ = 633 nm, when θ1 = 0 and θ2 = 0, I1 = 0, I2 = 0, I3 = 0, and I4 = 0; when θ1 = 0 and θ2 = π / 2, I1 = 0.9, I2 = 0, I3 = 0, and I4 = 0; when θ1 = π / 4 and θ2 = 0, I1 = 0, I2 = 0.52, I3 = 0, and I4 = 0.52; and when θ1 = π / 4 and θ2 = π / 2, I1 = 0.45, I2 = 0, I3 = 0.51, and I4 = 0.51. Therefore, based on this principle, a false image can be encoded in a co-polarized polarization channel with forward and reverse incident polarization, while a real image is encoded in a cross-polarized polarization channel with forward and reverse incident polarization, and the two images are distributed independently, as shown below. Figure 10 The image shown is a binary image in the co-polarization polarization channel, as follows: Figure 11 The images shown are binary images of the forward and reverse incident cross-polarization channels, and the binary image formed by combining the two.

Claims

1. An image hiding and encryption method based on a bidirectional asymmetric nanoprinted metasurface, characterized in that... The specific steps include the following: 1) Construction of nanostructure units: The nanostructure unit includes a transparent substrate, on which nanobricks are deposited respectively. An xoy coordinate system is established with the right-angled sides of the structure unit as the x-axis and y-axis, respectively. The long sides of the two types of nanobricks are the major axes, and the short sides are the minor axes. The angle between the minor axes of the two types of nanobricks and the x-axis is the orientation of the nanobrick. and ; 2) Based on the selected operating wavelength The structural parameters of the two nanobricks were optimized using electromagnetic simulation software. The structural parameters include: the side length C of the working surface of the structural unit, the major axes L1 and L2, the minor axes W1 and W2, and the heights H1 and H2 of the two nanobricks. 3) Construct a nanostructure array, wherein the nanostructure array comprises multiple nanostructure units; set the polarization direction Linearly polarized light is incident on the nanostructure array from the upper and lower surfaces, respectively, and then passes through the light-transmitting axis. The analyzer displays consistent image information under both incident conditions, enabling the encoding of the first image within the nanostructure array. This image information, acting as decoy information, can be directly observed. Linearly polarized light with its polarization direction unchanged is incident on the nanostructure array and then passes through a path along the transmission axis. The analyzer displays different image information under two incident conditions. By adding the two images pixel by pixel, a second image can be encoded in the nanostructure array. The image information is the hidden real information and cannot be directly observed. In step 3), the nanobricks on the upper surface of the nanostructure array are all equivalent to a quarter-wave plate, and the nanobricks on the lower surface are all equivalent to a polarizer. When linearly polarized light is incident on the nanostructure array from the upper surface, that is, the linearly polarized light first passes through the quarter-wave plate and then the polarizer, the Jones vector of the outgoing light can be expressed as: ； In the formula: This represents the electric field component along the x-axis. This represents the electric field component along the y-axis. The orientation angle of the nano-bricks on the upper surface. The orientation angle of the nano-bricks on the lower surface. The polarization direction of the incident ray-polarized light; When linearly polarized light is incident on the nanostructure array from the lower surface—that is, the linearly polarized light first passes through the polarizer and then through the quarter-wave plate—the Jones vector of the outgoing light can be expressed as: ； According to the principle that light intensity is proportional to the square of the electric field intensity modulus, that is... Then there are two possible scenarios: a. The polarization direction of the incident polarized light is parallel to the transmission axis of the analyzer. When incident from the upper and lower surfaces, the corresponding outgoing light is the same, which can be represented as: ； In the formula: The intensity of the incident polarized light; b. The polarization direction of the incident polarized light is perpendicular to the transmission axis of the analyzer. , The emitted light differs depending on whether it is incident on the upper or lower surface, and can be represented as follows: ； ； The first false information image is obtained through the emitted light intensity function. To regulate, while the second real information image is determined by the outgoing light intensity function. and The sum of combinations ( Regulation.

2. The image hiding and encryption method based on a bidirectional asymmetric nanoprinted metasurface according to claim 1, characterized in that... The transparent substrate mentioned in step 1) is fused silica glass, and the nanobrick material is silicon.

3. The image hiding and encryption method based on a bidirectional asymmetric nanoprinted metasurface according to claim 1, characterized in that... Four emitted light intensities and two nanobricks' turning angles and The modulation relationship is as follows: a. When , When, the corresponding intensity , , , ; b. When , When, the corresponding intensity , , , ; c. When , When, the corresponding intensity , , , ; d. When , When, the corresponding intensity , , , ; By rationally arranging the turning angles of the two types of nanobricks, it is possible to simultaneously encode two images in a nanostructure array.

Images