Nonlinear optical metasurface and design and preparation method thereof, image encryption system

Abstract

The application discloses a nonlinear optical superstructure surface and a design and preparation method and an image encryption system thereof. The nonlinear optical superstructure surface comprises a C3 metal layer, a dielectric layer and an ENZ layer; the dielectric layer comprises a dielectric film with multiple step thicknesses; the C3 metal layer comprises a plurality of metal plasmonic superstructure units with three-fold rotational symmetry; the dielectric layer is located on the surface of the ENZ layer, and the C3 metal layer is located on the surface of the dielectric layer. The intensity of nonlinear emergent light is regulated through the dielectric film with multiple step thicknesses, so that an encrypted gray-scale image is read based on the nonlinear emergent light field distribution, and the image encryption application is realized. Moreover, the nonlinear optical superstructure surface provided by the application presents a uniform light field distribution under linear optical measurement, does not leak the encrypted information, and improves the confidentiality and decryption difficulty of the nonlinear optical superstructure surface image encryption.

Description

Technical Field

[0001] This application belongs to the field of metasurface technology, and more specifically, relates to a nonlinear optical metasurface and its design and preparation method, as well as an image encryption system. Background Technology

[0002] Optical metamaterials are novel optical materials constructed from artificially designed metacellular units, enabling light to propagate in ways difficult to achieve with natural materials. Unlike three-dimensional optical metamaterials, the spatial distribution of metacellular units in optical metasurfaces is reduced from three-dimensional to two-dimensional, significantly reducing fabrication difficulty and optical losses. Based on the advantages of thinness and integration, optical metasurfaces are widely used in fields such as image encryption, holographic imaging, and planar optical lenses.

[0003] Currently, existing work has utilized Malus's law to perform multi-step modulation of the light field intensity of nonlinear optical metasurfaces to achieve image encryption. However, this method is prone to leaking encrypted image information in linear optical imaging. In other words, related technologies suffer from the problem of easy leakage of encrypted information and poor confidentiality when optical metasurfaces are used for image encryption. Summary of the Invention

[0004] To address the shortcomings of related technologies, this application provides a nonlinear optical metasurface, its design and fabrication method, and an image encryption system, aiming to solve the problems of easy leakage of encrypted information and poor confidentiality when optical metasurfaces are used for image encryption in related technologies.

[0005] The technical solution is as follows:

[0006] According to one aspect of this application, a nonlinear optical metasurface includes a C3 metal layer, a dielectric layer, and an ENZ layer; the dielectric layer includes a dielectric thin film with a multi-step thickness; the C3 metal layer includes a plurality of metal plasmon metasurface units with triple rotational symmetry; the dielectric layer is located on the surface of the ENZ layer, and the C3 metal layer is located on the surface of the dielectric layer.

[0007] In one exemplary embodiment, the metal plasmon metaunit includes three metal nanorods, the ends of which are connected at a single point.

[0008] In one exemplary embodiment, the ENZ layer is an indium tin oxide thin film.

[0009] In one exemplary embodiment, the dielectric film is a silicon dioxide film.

[0010] According to one aspect of this application, an image encryption system based on a nonlinear optical metasurface includes a pump light source, a nonlinear optical metasurface, and an imaging sensor; the thickness of the dielectric film of the nonlinear optical metasurface is determined according to the encrypted image; the pump light source emits pump light, and after the pump light irradiates the nonlinear optical metasurface, the transmitted light field is received by the imaging sensor to read out the encrypted image.

[0011] According to one aspect of this application, a method for designing the thickness of a dielectric thin film on a nonlinear optical metasurface includes: acquiring an encrypted image; the encrypted image being a grayscale image; determining the pixel values ​​of the encrypted image; determining the thickness of the dielectric thin film in a corresponding region based on the pixel values; and a pixel value corresponding to a dielectric thin film thickness.

[0012] According to one aspect of this application, a method for fabricating a nonlinear optical metasurface includes: fabricating a near-zero dielectric film on a substrate; fabricating dielectric films of various thicknesses at different locations on the near-zero dielectric film; and fabricating a plurality of metal plasmon metaunits with triple rotational symmetry on the dielectric film, wherein the near-zero dielectric film, the dielectric films of various thicknesses, and the plurality of metal plasmon metaunits constitute the nonlinear optical metasurface.

[0013] In an exemplary embodiment, the preparation of a near-zero dielectric thin film on a substrate includes: preparing a near-zero dielectric thin film on a substrate by magnetron sputtering.

[0014] In an exemplary embodiment, the step of preparing dielectric films of various thicknesses at different locations on the near-zero dielectric constant thin film includes: performing multiple dielectric film preparation operations on the near-zero dielectric constant thin film to prepare dielectric films of various thicknesses; each dielectric film preparation operation prepares a dielectric film of one thickness.

[0015] In an exemplary embodiment, the fabrication of a plurality of metal plasmon metaunits with triple rotational symmetry on the dielectric thin film includes: spin-coating photoresist on the dielectric thin film; etching a design pattern into the photoresist using electron beam lithography; developing the photoresist after electron beam lithography; fabricating a metal thin film using thin film fabrication techniques; and stripping off the remaining photoresist to fabricate a plurality of metal plasmon metaunits with triple rotational symmetry.

[0016] According to one aspect of this application, an electronic device includes: at least one processor, at least one memory, and at least one communication bus, wherein a computer program is stored in the memory, and the processor reads the computer program from the memory via the communication bus; when the computer program is executed by the processor, it implements a method for designing the dielectric thin film thickness of a nonlinear optical metasurface as described above.

[0017] According to one aspect of this application, a storage medium storing a computer program thereon, which, when executed by a processor, implements a method for designing the dielectric thin film thickness of a nonlinear optical metasurface as described above.

[0018] According to one aspect of this application, a computer program product includes a computer program stored in a storage medium, a processor of a computer device reads the computer program from the storage medium, and the processor executes the computer program such that the computer device, when executed, implements a method for designing the dielectric thin film thickness of a nonlinear optical metasurface as described above.

[0019] This application has the following beneficial effects:

[0020] The nonlinear optical metasurface provided in this application includes a C3 metal layer, a dielectric layer, and an ENZ layer. The C3 metal layer comprises multiple metal plasmon metaunits with triple rotational symmetry. The dielectric layer comprises a dielectric thin film with multi-step thickness. The ENZ layer is composed of an epsilon-near-zero (ENZ) material. When light enters the material with a near-zero dielectric constant, according to the continuity condition of the electric displacement vector, the electric field component perpendicular to the ENZ layer interface is greatly enhanced in the ENZ layer, thereby enhancing the nonlinear optical effect. Moreover, the metal plasmon metaunits with triple rotational symmetry can efficiently couple the electric field component of light parallel to the interface to the direction perpendicular to the interface, thus significantly improving the nonlinear conversion efficiency and obtaining stronger nonlinear emitted light (emitted frequency-doubled light). The dielectric layer can reduce the intensity of the nonlinear emitted light. Based on this, the intensity of the nonlinear emitted light can be controlled by using a dielectric thin film with multi-step thickness. This allows for the reading of grayscale images encrypted onto the optical metasurface based on the nonlinear emitted light field distribution, enabling image encryption applications. Furthermore, the nonlinear optical metasurface provided in this application exhibits a uniform light field distribution under linear optical measurement, meaning it will not leak encrypted information under linear optical imaging. This improves the confidentiality and decryption difficulty of image encryption using the nonlinear optical metasurface, solving the problem of easy leakage of encrypted information and poor confidentiality when optical metasurfaces are used for image encryption in related technologies. Attached Figure Description

[0021] Figure 1This is a schematic diagram of a nonlinear optical metasurface provided in an embodiment of this application;

[0022] Figure 2 This is a schematic diagram of an image encryption system based on a nonlinear optical metasurface provided in an embodiment of this application;

[0023] Figure 3 This is a flowchart illustrating a method for designing the dielectric thin film thickness of a nonlinear optical metasurface, as provided in an embodiment of this application.

[0024] Figure 4 This is a flowchart of a method for preparing a nonlinear optical metasurface provided in an embodiment of this application;

[0025] Figure 5 This is a schematic diagram illustrating the fabrication of dielectric thin films of various thicknesses in one example;

[0026] Figure 6 yes Figure 4 In one embodiment, step 440 is shown in a flowchart.

[0027] Figure 7 This is a schematic diagram of a design graphic in an example;

[0028] Figure 8 This is a schematic diagram illustrating the specific implementation of the nonlinear optical metasurface provided in this application displaying encrypted images under pump light illumination in an application scenario.

[0029] Figure 9 This is a structural block diagram of a device for designing the dielectric thin film thickness of a nonlinear optical metasurface, according to an exemplary embodiment.

[0030] Figure 10 This is a structural block diagram of an electronic device according to an exemplary embodiment. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application. Furthermore, the technical features involved in the various embodiments described below can be combined with each other as long as they do not conflict with each other.

[0032] Those skilled in the art will understand that, unless specifically stated otherwise, the singular forms “a,” “an,” “the,” and “the” used herein may also include the plural forms. It should be further understood that the term “comprising” as used in this application means the presence of the stated features, integers, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. It should be understood that when we say an element is “connected” or “coupled” to another element, it can be directly connected or coupled to the other element, or there may be intermediate elements. Furthermore, “connected” or “coupled” as used herein can include wireless connections or wireless coupling. The term “and / or” as used herein includes all or any units and all combinations of one or more associated listed items.

[0033] Before describing the various embodiments shown in this application, several concepts involved in this application will be introduced first.

[0034] Epsilon-near-zero (ENZ) materials are those whose real part of the dielectric constant approaches zero within a specific wavelength range.

[0035] Nonlinear optical effects refer to the phenomenon where light of a certain frequency, when incident on a medium, may generate second, third, or higher harmonics through interaction with the medium, and may also produce light with a periodic frequency distribution in the spectrum. Among these, the optical second harmonic is also called optical frequency doubling, and in this application, the term nonlinear light will be used to refer to the second harmonic (frequency doubling light).

[0036] As mentioned earlier, the related technologies suffer from the problem that optical metasurfaces are prone to leaking encrypted information and have poor confidentiality when used for image encryption.

[0037] There are two main methods for image encryption using optical metasurfaces. The first method is to encode the image information to be encrypted onto the optical metasurface using holographic imaging technology, and then recover the image information under specific reproduction conditions. The second method is to encrypt different image information into different polarization channels using Malus's law, and then extract the encrypted image under a specific polarization state.

[0038] However, both of these methods are easily cracked and copied, meaning they lack confidentiality. The problem of optical metasurfaces easily leaking encrypted information and lacking confidentiality when used for image encryption urgently needs to be solved.

[0039] Therefore, the nonlinear optical metasurface provided in this application can effectively improve the confidentiality of image encryption and the difficulty of decryption.

[0040] Please see Figure 1This application provides a nonlinear optical metasurface, including a C3 metal layer 101, a dielectric layer 103, and an ENZ layer 105.

[0041] The dielectric layer 103 includes a dielectric thin film 1031 with a multi-step thickness, the C3 metal layer 101 includes a plurality of metal plasmon metaunits 1011 with triple rotational symmetry, and the ENZ layer 105 is composed of a material with a near-zero dielectric constant.

[0042] The dielectric layer 103 is located on the surface of the ENZ layer 105, and the C3 metal layer 101 is located on the surface of the dielectric layer 103.

[0043] The metallic plasmon metaunit 1011 has triple rotational symmetry, which means that rotating the metallic plasmon metaunit 1011 by 120 degrees with the center of the metallic plasmon metaunit 1011 as the rotation axis will make it coincide with the original metallic plasmon metaunit 1011.

[0044] Multi-step thickness refers to a variety of different thicknesses. Multi-step thickness dielectric film 1031 means that dielectric film 1031 has multiple thicknesses, from... Figure 1 It can be seen that the thickness of the dielectric film 1031 is different at different locations.

[0045] The ENZ layer 105 is a thin film of a material with a near-zero dielectric constant, such as an indium tin oxide film or an aluminum-doped zinc oxide film, without any specific limitation here.

[0046] In one possible implementation, the metallic plasmonic metaunit 1011 can be made of gold.

[0047] In one possible implementation, the dielectric thin film 1031 can be a silicon dioxide thin film or a dielectric material thin film, without any specific limitation.

[0048] In one possible implementation, the metal plasmon metaunit 1011 includes three metal nanorods, the ends of which are connected at a single point, and the included angle between the three metal nanorods is 120 degrees. The three metal nanorods have the same length, width, and thickness, that is, the metal plasmon metaunit 1011 has triple rotational symmetry.

[0049] The metal nanorods have a length of 100 to 300 nm, a width of 50 to 100 nm, a thickness of 20 to 80 nm, and an operating wavelength of 800 to 1600 nm. Specifically, the length, width, and thickness of the metal nanorods are generally determined based on the actual required operating wavelength.

[0050] In one possible implementation, the thickness of the thin film of ENZ layer 105, a material with a near-zero dielectric constant, is greater than 10 nm.

[0051] In one possible implementation, the thickness of the dielectric film 1031 is determined based on the encrypted image.

[0052] In one possible implementation, the thickness of the multi-step thickness dielectric film 1031 is 0 to 100 nm. Specifically, the thickness of the dielectric film 1031 in the corresponding region needs to be determined according to the grayscale image to be encrypted. For example, if the grayscale range of the encrypted image is 0-255, when a pixel in the grayscale image is 0, the thickness of the dielectric film 1031 in the corresponding region is the thickest; if the pixel value is larger, i.e. closer to 255, the thickness of the dielectric film 1031 in the corresponding region is thinner; if the pixel value is 255, the thickness of the dielectric film 1031 in the corresponding region is 0.

[0053] It should be understood that by adjusting the thickness of the dielectric film 1031, the amplitude of the emitted frequency-doubled light at different locations on the metasurface can be controlled. The amplitude (intensity) of the emitted frequency-doubled light corresponds to different pixel values. For example, when the thickness of the dielectric film 1031 is 0, the emitted frequency-doubled light intensity is at its maximum, and the corresponding pixel value (grayscale value) is at its maximum. As the thickness of the dielectric film 1031 increases, the emitted frequency-doubled light intensity weakens. When it reaches 100 nm (maximum thickness), the emitted frequency-doubled light intensity weakens to less than one percent of the maximum emitted frequency-doubled light intensity, and the corresponding pixel value (grayscale value) is at its minimum. In other words, the intensity of the nonlinear light field is controlled in multiple stages by using a dielectric film 1031 with multi-stage thickness, thereby enabling image encryption applications.

[0054] In the above embodiments, when light enters a material with a near-zero dielectric constant, according to the continuity condition of the electric displacement vector, the electric field component perpendicular to the interface of the ENZ layer 105 is greatly enhanced in the ENZ layer 105, thereby enhancing the nonlinear optical effect and causing the ENZ layer 105 to generate nonlinear harmonics. Moreover, the metal plasmon metaunit 1011 with triple rotational symmetry can generate an extremely strong local electric field at the interface of the C3 metal layer 101-ENZ layer 105, and can also efficiently couple the electric field component of light parallel to the interface to the direction perpendicular to the interface, thus significantly improving the nonlinear conversion efficiency and obtaining stronger nonlinear emitted light (emitted frequency-doubled light). The dielectric layer 103 can reduce the intensity of the nonlinear emitted light. Based on this, the intensity of the nonlinear emitted light can be controlled by the dielectric thin film 1031 with multi-step thickness. Different light intensities produce different grayscale values, thereby reading the grayscale image encrypted to the optical metasurface based on the nonlinear emitted light field distribution, realizing image encryption applications. Furthermore, since the energy of a linear single-pulse light is too low to produce nonlinear optical effects, the nonlinear optical metasurface provided in this application exhibits a uniform light field distribution under linear optical measurements, meaning it will not leak encrypted information under linear optical imaging. Therefore, the nonlinear optical metasurface provided in this application can improve the confidentiality and decryption difficulty of image encryption, solving the problem in related technologies where optical metasurfaces used for image encryption easily leak encrypted information and have poor confidentiality.

[0055] Please see Figure 2 This application provides an image encryption system based on a nonlinear optical metasurface, including a pump light source 201, a nonlinear optical metasurface 203, and an imaging sensor 205. The pump light source 201 emits pump light, and after the pump light illuminates the nonlinear optical metasurface 203, the transmitted second harmonic light field is recorded by the imaging sensor 205, and the encrypted image is read out.

[0056] The pump light is near-infrared light, such as a femtosecond laser with a wavelength of 1200 nm. When the pump light irradiates the nonlinear optical metasurface 203, the nonlinear optical metasurface 203 generates nonlinear light (frequency-doubled light). For example, if the pump light has a side length of 1200 nm, it undergoes frequency doubling after passing through the nonlinear optical metasurface 203, producing light with a wavelength of 600 nm.

[0057] Compared to ordinary light sources (such as fluorescent lamps and sunlight), each emitted pulse in a femtosecond laser has a stronger peak intensity, meaning each pulse has more energy. This allows nonlinear metasurfaces to produce nonlinear effects, thus generating nonlinear light. Ordinary light sources, due to their low single-pulse energy, cannot induce nonlinear effects on nonlinear metasurfaces.

[0058] Because the emitted frequency-doubled light is very weak, the imaging sensor 205 needs to be a high-sensitivity imaging sensor, such as a high-sensitivity detector or spectrometer.

[0059] Nonlinear optical metasurface 203 can be used as follows Figure 1 The nonlinear optical metasurface shown.

[0060] Through the above embodiments, encrypted images can be read from the emitted frequency-doubled light field distribution of the nonlinear optical metasurface under the illumination of pump light.

[0061] In one exemplary embodiment, nonlinear optical metasurfaces can be measured using linear and nonlinear optical measurement systems. Specifically, linear transmission spectroscopy measurements are performed using a continuous white light source and a portable spectrometer, and the frequency doubling response and nonlinear imaging of the nonlinear metasurfaces are studied using a femtosecond laser system, an optical parametric oscillator, a high-sensitivity spectrometer, and an sCMOS detector.

[0062] In the measurement, a nonlinear optical measurement system can be used to perform real-space imaging of the nonlinear optical metasurface to extract encrypted images, while simultaneously characterizing the nonlinear conversion efficiency and image characteristics of the nonlinear optical metasurface. In linear optical measurement, the light field distribution is uniform, making it impossible to extract encrypted images, thus improving the confidentiality of image encryption and the difficulty of decryption.

[0063] Please see Figure 3 This application provides a method for designing the thickness of a dielectric thin film on a nonlinear optical metasurface, comprising the following steps:

[0064] Step 300: Obtain the encrypted image.

[0065] The encrypted image is a grayscale image.

[0066] Step 320: Determine the pixel values ​​of the encrypted image.

[0067] The pixel values ​​of a grayscale image can be 0 to 255, 0 to 7, or 0 to 3; no specific limitation is made here.

[0068] Step 340: Determine the thickness of the dielectric film in the corresponding area based on the pixel value.

[0069] Each pixel value corresponds to a specific dielectric film thickness. The thickness of the dielectric film ranges from 0 to 100 nm. A larger pixel value results in a thinner dielectric film in the corresponding area; conversely, a smaller pixel value results in a thicker dielectric film in the corresponding area. For example, with grayscale values ​​ranging from 0 to 255, there are 256 grayscale levels (pixel values), and correspondingly, 256 different dielectric film thicknesses. The thickness of the dielectric film in the corresponding area is then determined based on the pixel value.

[0070] Through the above embodiments, the thickness of the dielectric film is determined based on the encrypted image, and the intensity of the emitted frequency-doubled light is continuously controlled by using a dielectric film with multi-step thickness. Different pixel values ​​are generated in the emitted frequency-doubled light field distribution, thereby encrypting the encrypted image onto the nonlinear optical metasurface.

[0071] Please see Figure 4 This application provides a method for preparing a nonlinear optical metasurface, comprising the following steps:

[0072] Step 400: Prepare a near-zero dielectric thin film on the substrate.

[0073] In one possible implementation, a near-zero dielectric thin film is prepared on a substrate by magnetron sputtering.

[0074] A near-zero dielectric constant thin film can be an indium tin oxide thin film with a thickness greater than 10 nm.

[0075] The substrate serves as a supporting base, and can be, for example, a transparent glass sheet.

[0076] Step 420: Prepare dielectric films of various thicknesses at different locations of the near-zero dielectric constant film.

[0077] In one possible implementation, based on electron beam lithography, dielectric films of various thicknesses are prepared at different locations on films with near-zero dielectric constants.

[0078] The dielectric thin film can be made of dielectric materials such as silicon dioxide, and its thickness can range from 0 to 100 nm. The specific thickness needs to be determined based on the encrypted image. For example, a dielectric film can be used... Figure 3 The design method for dielectric film thickness shown determines the specific thickness, which includes multiple different thickness values. For example, if an encrypted image has 256 grayscale values ​​(pixel values), the specific thickness would include 256 different thickness values.

[0079] In one possible implementation, multiple dielectric film preparation operations are performed on a near-zero dielectric constant film to prepare dielectric films of various thicknesses; each dielectric film preparation operation prepares a dielectric film of a certain thickness.

[0080] For example, if there are 256 different thicknesses of dielectric thin films, then 256 dielectric thin film preparation operations are required, with each operation producing a dielectric thin film of a specific thickness.

[0081] The dielectric thin film preparation operations may include electron beam lithography, electron beam evaporation, lift-off processes, etc., and are not specifically limited here.

[0082] like Figure 5It shows a schematic diagram of the preparation of dielectric thin films of various thicknesses in one example.

[0083] Figure 5 In the first step, electron beam lithography and electron beam evaporation are used to prepare the positioning marks required for subsequent overlay on the near-zero dielectric thin film (indium tin oxide thin film). The positioning marks can also be prepared by laser direct writing exposure and other methods. This is not a specific limitation. The positioning marks are the "+" icons located at the four corners of the near-zero dielectric thin film.

[0084] In the second step, an electron beam lithography-development-thin film preparation-lifting operation is performed according to the design requirements (thickness) to prepare a dielectric thin film of a certain thickness.

[0085] In the third, fourth, and fifth steps, an electron beam lithography-development-thin film preparation-lifting operation is performed once each to prepare a dielectric thin film of one thickness.

[0086] Finally, dielectric films of four different thicknesses were fabricated on films with near-zero dielectric constants.

[0087] Step 440: Prepare multiple metal plasmon superunits with triple rotational symmetry on a dielectric thin film.

[0088] Nonlinear optical metasurfaces are composed of near-zero dielectric films, dielectric films of various thicknesses, and multiple metallic plasmon metaunits.

[0089] The metallic plasmon metaunit comprises three metallic nanorods, whose endpoints are connected at a single point. The included angle between the three nanorods is 120 degrees. The three nanorods have the same length, width, and thickness, meaning the resulting metallic plasmon metaunit exhibits triple rotational symmetry. The metallic plasmon metaunit can be... Figure 1 The metal plasmon metaunit 1011 shown is shown.

[0090] Triple rotational symmetry means that, with the center of the metallic plasmon metaunit as the rotation axis, rotating the metallic plasmon metaunit by 120 degrees will allow it to coincide with the original metallic plasmon metaunit.

[0091] The metal nanorods have a length of 100 to 300 nm, a width of 50 to 100 nm, a thickness of 20 to 80 nm, and an operating wavelength of 800 to 1600 nm. Specifically, the length, width, and thickness of the metal nanorods are generally determined based on the actual required operating wavelength.

[0092] Specifically, such as Figure 6 As shown, step 440 may also include the following steps:

[0093] Step 600: Spin-coat photoresist onto the dielectric film and use electron beam lithography to etch the design pattern into the photoresist.

[0094] In one possible implementation, a spin coater is used to spin-coat electron-sensitive photoresist onto a dielectric film of multi-step thickness.

[0095] In one possible implementation, the design pattern is etched into the photoresist using electron beam lithography via positioning markers, where the positioning markers can be... Figure 5 The positioning marks are located at the four corners.

[0096] Design graphics refer to the two-dimensional planar shape of metallic plasmonic metaunits. For example... Figure 7 The illustration shows a schematic diagram of an example design pattern, wherein the design pattern is a two-dimensional planar shape of a metal plasmonic metaunit, and the metal nanorod in the design pattern has a length of L and a width of W.

[0097] Step 620: After developing the photoresist subjected to electron beam lithography, a metal thin film is prepared using thin film preparation technology.

[0098] Thin film preparation technology can be electron beam evaporation.

[0099] Step 640: Remove the remaining photoresist to prepare multiple metal plasmon metaunits with triple rotational symmetry.

[0100] Each region of a dielectric thin film of a specific thickness has one or more metallic plasmon metaunits.

[0101] Through the above embodiments, a nonlinear optical metasurface with subwavelength thickness containing encrypted image information is prepared, which can realize multi-step control of the intensity of the nonlinear light field, and thus read the encrypted image from its nonlinear light field distribution to realize the application of image encryption; at the same time, its linear light field distribution is uniform, and the encrypted image will not be leaked under linear optical measurement. Moreover, compared with linear optical image encryption, nonlinear optical image encryption involves frequency conversion, so it is much more difficult to crack and copy than general optical holographic anti-counterfeiting technology, thus having better concealment and higher decryption difficulty. The nonlinear optical metasurface provided in this application can realize image encryption, and has high confidentiality and is not easy to crack.

[0102] Figure 8 This is a schematic diagram illustrating the specific implementation of the nonlinear optical metasurface provided in this application displaying encrypted images under pump light irradiation in an application scenario. In this application scenario, the material of the metallic plasmon metaunits of the nonlinear optical metasurface is gold, the material of the dielectric thin film is silicon dioxide, and the material of the near-zero dielectric constant thin film is indium tin oxide (ITO).

[0103] Figure 8 As can be seen, under the illumination of pump light, the emitted second harmonic of the nonlinear optical metasurface can produce an encrypted image, such as an image of a flower.

[0104] In this application scenario, encrypted images are read based on the light field distribution of the emitted second harmonic of the nonlinear optical metasurface.

[0105] The following are embodiments of the apparatus described in this application, which can be used to execute the method for designing the dielectric film thickness of the nonlinear optical metasurface involved in this application. For details not disclosed in the apparatus embodiments of this application, please refer to the method embodiments of the method for designing the dielectric film thickness of the nonlinear optical metasurface involved in this application.

[0106] Please see Figure 9 This application provides a device 900 for designing the thickness of a dielectric thin film on a nonlinear optical metasurface, including but not limited to: an image acquisition module 910, a pixel value determination module 930, and a thickness determination module 950.

[0107] The image acquisition module 910 is used to acquire an encrypted image; the encrypted image is a grayscale image.

[0108] The pixel value determination module 930 is used to determine the pixel values ​​of the encrypted image.

[0109] The thickness determination module 950 is used to determine the thickness of the dielectric film in the corresponding area based on the pixel value; one pixel value corresponds to one dielectric film thickness.

[0110] It should be noted that the above-described device for designing the dielectric film thickness of nonlinear optical metasurfaces is only illustrated by the division of the above-described functional modules when designing the dielectric film thickness. In practical applications, the above functions can be assigned to different functional modules as needed. That is, the internal structure of the device for designing the dielectric film thickness of nonlinear optical metasurfaces will be divided into different functional modules to complete all or part of the functions described above.

[0111] Furthermore, the embodiments of the apparatus for designing the dielectric film thickness of nonlinear optical metasurfaces and the method for designing the dielectric film thickness of nonlinear optical metasurfaces provided in the above embodiments belong to the same concept. The specific way in which each module performs its operation has been described in detail in the method embodiments, and will not be repeated here.

[0112] Please see Figure 10 This application provides an electronic device 4000, which may include: a desktop computer, a laptop computer, a server, etc.

[0113] exist Figure 10 The electronic device 4000 includes at least one processor 4001, at least one communication bus 4002, and at least one memory 4003.

[0114] The processor 4001 and memory 4003 are connected, for example, via a communication bus 4002. Optionally, the electronic device 4000 may also include a transceiver 4004, which can be used for data interaction between the electronic device and other electronic devices, such as sending and / or receiving data. It should be noted that in practical applications, the transceiver 4004 is not limited to one, and the structure of the electronic device 4000 does not constitute a limitation on the embodiments of this application.

[0115] Processor 4001 may be a CPU (Central Processing Unit), a general-purpose processor, a DSP (Digital Signal Processor), an ASIC (Application Specific Integrated Circuit), an FPGA (Field Programmable Gate Array), or other programmable logic devices, transistor logic devices, hardware components, or any combination thereof. It can implement or execute the various exemplary logic blocks, modules, and circuits described in conjunction with the disclosure of this application. Processor 4001 may also be a combination that implements computational functions, such as including one or more microprocessor combinations, a combination of a DSP and a microprocessor, etc.

[0116] The communication bus 4002 may include a path for transmitting information between the aforementioned components. The communication bus 4002 may be a PCI (Peripheral Component Interconnect) bus or an EISA (Extended Industry Standard Architecture) bus, etc. The communication bus 4002 can be divided into an address bus, a data bus, a control bus, etc. For ease of representation, Figure 10 The bus is represented by a single thick line, but this does not mean that there is only one bus or one type of bus.

[0117] The memory 4003 may be ROM (Read Only Memory) or other types of static storage devices capable of storing static information and instructions, RAM (Random Access Memory) or other types of dynamic storage devices capable of storing information and instructions, or EEPROM (Electrically Erasable Programmable Read Only Memory), CD-ROM (Compact Disc Read Only Memory) or other optical disc storage, optical disc storage (including compressed optical discs, laser discs, optical discs, digital universal optical discs, Blu-ray discs, etc.), magnetic disk storage media or other magnetic storage devices, or any other medium capable of carrying or storing desired program code in the form of instructions or data structures and accessible by a computer, but not limited thereto.

[0118] The memory 4003 stores a computer program, and the processor 4001 reads the computer program stored in the memory 4003 through the communication bus 4002.

[0119] When the computer program is executed by the processor 4001, it implements the method for designing the dielectric film thickness of the nonlinear optical metasurface in the above embodiments.

[0120] Furthermore, this application provides a storage medium storing a computer program, which, when executed by a processor, implements the method for designing the dielectric thin film thickness of the nonlinear optical metasurface in the above embodiments.

[0121] This application provides a computer program product comprising a computer program stored in a storage medium. A processor of a computer device reads the computer program from the storage medium and executes the computer program, causing the computer device to perform the method for designing the dielectric thin film thickness of the nonlinear optical metasurface described in the above embodiments.

[0122] Compared with related technologies, this application can produce the following beneficial effects:

[0123] 1. The nonlinear optical metasurface provided in this application includes a C3 metal layer, a dielectric layer, and an ENZ layer. The C3 metal layer comprises multiple metal plasmon metaunits with triple rotational symmetry. The dielectric layer comprises a dielectric thin film with multi-step thickness. The ENZ layer is composed of an epsilon-near-zero (ENZ) material. When light enters the material with a near-zero dielectric constant, according to the continuity condition of the electric displacement vector, the electric field component perpendicular to the ENZ layer interface is greatly enhanced in the ENZ layer, thereby enhancing the nonlinear optical effect. Moreover, the metal plasmon metaunits with triple rotational symmetry can efficiently couple the electric field component of light parallel to the interface to the direction perpendicular to the interface, thus significantly improving the nonlinear conversion efficiency and obtaining stronger nonlinear emitted light (emitted frequency-doubled light). The dielectric layer can reduce the intensity of the nonlinear emitted light. Based on this, the intensity of the nonlinear emitted light can be controlled by using a dielectric thin film with multi-step thickness. This allows for the reading of grayscale images encrypted onto the optical metasurface based on the nonlinear emitted light field distribution, enabling image encryption applications. Furthermore, the nonlinear optical metasurface provided in this application exhibits a uniform light field distribution under linear optical measurement, meaning it will not leak encrypted information under linear optical imaging. This improves the confidentiality and decryption difficulty of image encryption using the nonlinear optical metasurface, solving the problem of easy leakage of encrypted information and poor confidentiality when optical metasurfaces are used for image encryption in related technologies.

[0124] 2. According to the method for preparing a nonlinear optical metasurface provided in this application, a nonlinear optical metasurface with a subwavelength thickness containing encrypted image information can be prepared. This allows for multi-step control of the intensity of the nonlinear light field, thereby enabling the reading of encrypted images from its nonlinear light field distribution and realizing image encryption applications. Simultaneously, its linear light field distribution is uniform, and the encrypted image will not be leaked under linear optical measurement. Furthermore, compared with linear optical image encryption, nonlinear optical image encryption involves frequency conversion, making it far more difficult to crack and copy than general optical holographic anti-counterfeiting technology. Therefore, it has better concealment and higher decryption difficulty. The nonlinear optical metasurface provided in this application can realize image encryption with high confidentiality and is not easy to crack.

[0125] 3. The thickness of the dielectric film is determined based on the encrypted image (pixel value). Specifically, the larger the pixel value, the smaller the thickness of the dielectric film in the corresponding area, and the stronger the intensity of the emitted frequency-doubled light; conversely, the smaller the pixel value, the larger the thickness of the dielectric film in the corresponding area, and the weaker the intensity of the emitted frequency-doubled light. Thus, by using a dielectric film with multi-step thickness, the intensity of the emitted frequency-doubled light can be continuously controlled, generating different pixel values ​​in the emitted frequency-doubled light field distribution, thereby encrypting the encrypted image onto the nonlinear optical metasurface.

[0126] It should be understood that although the steps in the flowcharts of the accompanying figures are shown sequentially as indicated by the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless explicitly stated herein, there is no strict order restriction on the execution of these steps, and they can be executed in other orders. Moreover, at least some steps in the flowcharts of the accompanying figures may include multiple sub-steps or multiple stages. These sub-steps or stages are not necessarily completed at the same time, but can be executed at different times, and their execution order is not necessarily sequential, but can be performed alternately or in turn with other steps or at least some of the sub-steps or stages of other steps.

[0127] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A nonlinear optical metasurface, characterized in that, It includes a C3 metal layer, a dielectric layer, and an ENZ layer; the dielectric layer includes a dielectric thin film with multi-step thickness; the C3 metal layer includes multiple metal plasmon metaunits with triple rotational symmetry; each metal plasmon metaunit includes three metal nanorods, the ends of the three metal nanorods are connected at a point, and the included angle between the three metal nanorods is 120 degrees, and the three metal nanorods have the same length, width, and thickness; The ENZ layer is a thin film of a material with a near-zero dielectric constant. The dielectric layer is located on the surface of the ENZ layer, and the C3 metal layer is located on the surface of the dielectric layer.

2. The nonlinear optical metasurface as described in claim 1, characterized in that, The ENZ layer is an indium tin oxide thin film.

3. The nonlinear optical metasurface as described in claim 1, characterized in that, The dielectric film is a silicon dioxide film.

4. An image encryption system, characterized in that, Includes the nonlinear optical metasurface, pump source, and imaging sensor as described in any one of claims 1 to 3; the thickness of the dielectric thin film is determined based on the encrypted image; The pump light source emits pump light, which illuminates the nonlinear optical metasurface. The transmitted light field is recorded by the imaging sensor, and the encrypted image is read out.

5. A method for designing the dielectric thin film thickness of a nonlinear optical metasurface as described in any one of claims 1 to 3, characterized in that, include: Obtain an encrypted image; the encrypted image is a grayscale image; Determine the pixel values ​​of the encrypted image; The thickness of the dielectric film in the corresponding region is determined based on the pixel value; one pixel value corresponds to one dielectric film thickness.

6. A method for preparing a nonlinear optical metasurface as described in any one of claims 1 to 3, characterized in that, include: A near-zero dielectric thin film is prepared on a substrate; Dielectric thin films of various thicknesses were prepared at different locations of the near-zero dielectric constant thin film; Multiple metal plasmon metaunits with triple rotational symmetry are fabricated on the dielectric thin film to obtain the nonlinear optical metasurface.

7. The preparation method according to claim 6, characterized in that, The preparation of a near-zero dielectric thin film on a substrate includes: A near-zero dielectric thin film was prepared on a substrate by magnetron sputtering.

8. The preparation method according to claim 6, characterized in that, The preparation of dielectric films of various thicknesses at different locations of the near-zero dielectric constant film includes: Multiple dielectric film preparation operations are performed on the near-zero dielectric constant film to prepare dielectric films of various thicknesses; each dielectric film preparation operation produces a dielectric film of a certain thickness.

9. The preparation method according to any one of claims 6 to 8, characterized in that, The fabrication of multiple metal plasmon metaunits with triple rotational symmetry on the dielectric thin film includes: Photoresist is spin-coated onto the dielectric film, and the design pattern is etched into the photoresist using electron beam lithography. After developing the photoresist subjected to electron beam lithography, a metal thin film is prepared using thin film preparation technology; The remaining photoresist was stripped off to prepare multiple metal plasmon metaunits with triple rotational symmetry.

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