CN110927830A - Super-surface image anti-counterfeiting method based on space-frequency multiplexing - Google Patents
Super-surface image anti-counterfeiting method based on space-frequency multiplexing Download PDFInfo
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Abstract
The invention discloses a super-surface image anti-counterfeiting method based on space-frequency multiplexing, which is realized by a super-surface with a polarizer function, wherein the super-surface is composed of a substrate and a nano brick array etched on the substrate, and can regulate and control the polarization direction and the intensity of incident linearly polarized light and realize continuous gray image display. By skillful design of spatial frequency, the same super surface displays two different images under two different cut-off frequencies, and has the advantages of high resolution and high fidelity. The invention provides a new image anti-counterfeiting method, and can be widely applied to the fields of information multiplexing, encryption display and the like.
Description
Technical Field
The invention belongs to the technical field of micro-nano optics, and particularly relates to a super-surface image anti-counterfeiting method based on space-frequency multiplexing.
Background
Due to the increasing sophistication of counterfeiting technologies and means, anti-counterfeiting technologies are becoming more and more important in order to protect the interests of enterprise brands and consumers. In order to achieve the anti-counterfeiting purpose, the anti-counterfeiting technology needs to be capable of accurately identifying authenticity and simultaneously is not easy to copy. In recent years, due to its ultra-thin sub-wavelength structure, the advantage of precisely controlling the amplitude, phase, polarization state, etc. of the electromagnetic field has attracted much attention. As a two-dimensional plane material, the super surface can realize a plurality of functions by combining some control modes, for example, super surface holograms of different images can be generated by changing the polarization state of incident light, a two-stage zooming function of a lens is realized by utilizing the change of the circular polarization state of the incident light, and the like. At present, many researchers use the super-surface material to control the polarization state to realize some specific functions, but the design based on super-surface nano printing and simultaneously overlapping spatial frequency multiplexing has not been proposed and researched. Because the method is difficult to copy, the method is considered to be applied as an image anti-counterfeiting method, and the spatial frequency reuse-based super-surface anti-counterfeiting method has good application and development prospects in the future.
Disclosure of Invention
The invention aims to solve the technical problem of providing a super-surface image anti-counterfeiting method based on space-frequency multiplexing, which can provide a reliable anti-counterfeiting method and means as image anti-counterfeiting and greatly improve the anti-counterfeiting safety.
The technical scheme adopted by the invention for solving the technical problems is as follows: the super-surface image anti-counterfeiting method based on space-frequency multiplexing combines two different images in different spatial frequency domains into a mixed image, combines a super-surface with a polarizer function, displays a continuous gray scale mixed image with high resolution in a near field, can extract the two different images after the mixed image passes through two different filters, and the super-surface comprises a substrate and a nano brick array etched on the substrate.
According to the technical scheme, the super surface adopts a silver-silicon dioxide material structure, the silver on the top layer is used for etching the nano brick array, and the silicon dioxide on the bottom layer is used as a substrate.
According to the technical scheme, the substrate can be divided into a plurality of working faces with the side length of C, each nano unit structure is composed of a square working face and a nano brick etched on the working face, the structural size length L, the width W and the height H of each nano brick are set according to the wavelength of incident light, are obtained through electromagnetic simulation optimization, and are all sub-wavelength levels. And a coordinate system is established by taking the right-angle side of the unit structure as an X axis and a Y axis, the long side of the nano brick is a long axis, the short side of the nano brick is a short axis, and an included angle theta between the long axis of the nano brick and the X axis is a steering angle of the nano brick.
According to the technical scheme, each nano unit structure in the nano unit array has the function of a polaroid, and the long axis direction of the nano brick is equivalent to the transmission axis of the polaroid. Taking the incidence of the X-ray polarized light as an example, the polarization direction of the emergent ray polarized light is parallel to the long axis of the nano brick, namely, forms an angle theta with the X axis.
According to the technical scheme, the emergent linearly polarized light meets the Malus theorem after passing through the analyzer, namely
I＝I0(cosθ)2
Wherein, I0The intensity of emergent light after passing through the super-surface is shown as theta, the included angle between the polarization direction of incident linearly polarized light and the long axis direction of the nano brick is shown as theta, and the intensity of the linearly polarized light after passing through the analyzer is shown as I, so that any gray level adjustment can be realized by changing the size of the theta.
According to the technical scheme, high-frequency information and low-frequency information of two different images are respectively extracted, components of the two space frequency domains are superposed to generate a mixed image, and the high-pass filter and the low-pass filter are utilized to obtain the image corresponding to the high-frequency component or the low-frequency component from the mixed image.
According to the technical scheme, when linearly polarized light enters the super-surface, the light intensity and the polarization direction of the linearly polarized light are modulated and then emitted as reflected light, and the continuous gray scale mixed image with high resolution can be displayed in a near field by designing the distribution of the steering angles of the nano-brick array.
According to the method, high-frequency information and low-frequency information of two different images are respectively extracted, the two kinds of spatial frequency information are superposed in a pair of mixed patterns, the principle of spatial frequency multiplexing and the function of the super surface are combined, the polarization direction of linearly polarized light can be controlled by the super surface with the function of the half-wave plate, and the display of continuous gray level images can be realized in a near field by combining the Malus law. When the super-surface image extraction method is applied to image anti-counterfeiting, the mixed pattern displayed on the super-surface can be used for extracting two different high-resolution images in two high-frequency and low-frequency spatial frequency domains respectively.
The invention has the following beneficial effects: 1. the super-surface image anti-counterfeiting method provided by the invention can obtain two different image information from a high-frequency channel and a low-frequency channel respectively while obtaining a mixed image with high resolution by utilizing spatial frequency multiplexing, the image information corresponding to the two channels is not influenced mutually, and the extraction and verification can be carried out by utilizing a specific mode.
2. The super-surface image anti-counterfeiting method provided by the invention has two information patterns, and the extraction space-frequency channel of the super-surface image anti-counterfeiting method also needs special setting, so that the super-surface image anti-counterfeiting method has an encryption function and high copying and counterfeiting difficulty, can provide a reliable anti-counterfeiting method and means as image anti-counterfeiting, and greatly improves the anti-counterfeiting safety.
3. The structure size of the super-surface material adopted by the invention is sub-wavelength level, so that the super-surface material has the characteristics of small volume, light weight, high integration and the like, and is suitable for the development of miniaturization in the future. In addition, the super surface is a two-dimensional plane material, so that the processing and manufacturing are simple and the cost can be saved.
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The invention will be further described with reference to the accompanying drawings and examples, in which:
FIG. 1 is a schematic diagram of a nano-cell structure in an embodiment of the present invention;
FIG. 2 is a schematic diagram of the principle of a polarizing plate of a silver nanobead in the embodiment of the present invention;
FIG. 3 is a graph of transmittance and reflectance scans of a half-wave plate based metasurface in an embodiment of the present invention;
fig. 4 is a schematic diagram of space frequency multiplexing according to an embodiment of the present invention.
FIG. 5 is a schematic diagram of an embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
The embodiment of the invention provides a design principle of a super-surface image anti-counterfeiting method based on space-frequency reuse, and high-security image anti-counterfeiting is realized by combining space-frequency reuse and super-surface nano printing.
The super-surface material adopted in the embodiment of the invention is a silver-silicon dioxide structure, the silver on the top layer is used for etching the nano brick array, and the silicon dioxide on the bottom layer is used as a substrate. The single nanometer unit structure is shown in figure 1, the super surface material is composed of a 1-substrate and a 2-nanometer brick, the substrate of the unit structure is a square working surface with the side length of C, a nanometer brick is etched on the square working surface, the long side of the nanometer brick is a long axis, the short side of the nanometer brick is a short axis, the length L, the width W and the height H of the structure are all sub-wavelength levels, and theta is the steering angle of the nanometer brick. The super surface adopted by the image anti-counterfeiting method provided by the invention is a nano unit array formed by a plurality of nano unit structures, and the sizes and the central intervals of all adjacent nano bricks in the nano unit array are the same.
Taking the working wavelength as λ 633nnm as an example, electromagnetic simulation software is used for modeling and simulation, linear polarized light is incident perpendicular to the working surface, and as an example, the long axis of the nano brick in the built nano unit structure model is incident along the X axis and the short axis along the Y axis and is incident as X linear polarized light and Y linear polarized light respectively. The structural parameters of the scanning nano-unit under the working wavelength include L, W, H, C, the highest transmittance of the x-linear polarized light along the long axis of the nano-brick in the polarization direction and the highest reflectance of the y-linear polarized light along the short axis of the nano-brick in the polarization direction are the optimized objects, and the scanning result is shown in fig. 3. When the working wavelength is 633nm, the reflectivity of the x-linear polarized light and the transmissivity of the y-linear polarized light are both higher than 90%, and the reflectivity of the x-linear polarized light and the reflectivity of the y-linear polarized light are both lower than 10%, and the structural parameters of the nano unit are as follows: 300nm, 160nm, 80nm and 70 nm.
Each nano unit structure in the super surface adopted in the invention is equivalent to a polaroid, a single nano unit structure is taken as an example, as shown in figure 2, two right-angle sides of the working surface of the substrate are taken as an X axis and a Y axis to establish a coordinate system, and for simplifying the description, the strength is taken as I0The x-ray polarized light is incident, the polarization direction of the emergent ray polarized light is changed after passing through the nano-brick, and the polarization direction of the reflected ray polarized light deflects by an angle theta along the long axis direction of the nano-brick relative to the original polarization direction, so that the steering angle theta of the nano-brick of each nano-unit structure can be designed and adjusted, and the polarization directions of the ray polarized light passing through the nano-unit structures are different.
According to the Malus theorem, namely:
wherein, I0The light intensity of emergent light after passing through the super-surface,
the included angle between the polarization direction of incident linearly polarized light and the long axis direction of the nano brick is shown as I, the light intensity of the linearly polarized light after passing through the analyzer can be changed
When the polarization direction of the analyzer is kept unchanged, one nano unit structure is used as one pixel, and the continuous gray scale of each pixel can be regulated by controlling the nano brick steering angle theta of each nano unit structure, so that the display of a high-resolution continuous gray scale image is realized.
Based on the principle, the gray level image displayed by the super-surface nano printing can be designed at will. The invention provides an image anti-counterfeiting method based on space-frequency multiplexing, which extracts and superposes components of different space frequency domains of two images to synthesize a mixed image. As an example, two images with the same pixel size of 784 × 743, namely "tiger" and "panda", are selected. FIG. 3 is a scanned graph of transmittance and reflectance of a half-wave plate based meta-surface in an embodiment of the invention. As shown in fig. 3 and 5. Extracting the low-frequency components of the image "tiger" and the high-frequency components of the image "panda" in a spatial frequency domain, setting the cut-off frequencies of the low-frequency components and the high-frequency components to be 35c/i (period/image) and 60c/i respectively, wherein the low-frequency components of the image contain more contour information, and the high-frequency components contain more detail information, as shown in fig. 4, wherein d and e are spatial domain images of the image "tiger" and the image "panda" after low-frequency filtering and high-frequency filtering respectively. Combining the extracted high frequency component and low frequency component, a mixed image can be obtained, as shown by f, which contains both the low frequency information of "tiger" and the high frequency information of "panda". Since the human visual system has different degrees of sensitivity to different spatial frequencies at different distances, it can be seen that the mixed image f can observe two different images, namely the 'tiger' image and the 'panda' image at a far distance and a near distance respectively.
In the method, based on the principle, the half-wave plate-based super-surface nano printing and space-frequency multiplexing method are combined, and the mixed image is displayed by utilizing the super-surface, so that the method can be applied to image anti-counterfeiting. The high-frequency information and the low-frequency information provided by the two images can be used as two verification modes, the mixed image of the small picture is used as a mark to be etched on a product needing anti-counterfeiting, the observed pattern is a 'tiger' image with low-frequency information, when the image is amplified by an amplifying device, the 'panda' image with high-frequency information and fine patterns of the 'panda' image can be further observed, and the display mode of the super-surface nano printing is utilized, so that the anti-counterfeiting mode has high imitation difficulty and high safety, and therefore, the anti-counterfeiting method has a good application development prospect in the future.
It will be understood that modifications and variations can be made by persons skilled in the art in light of the above teachings and all such modifications and variations are intended to be included within the scope of the invention as defined in the appended claims.
Claims (7)
1. A super-surface image anti-counterfeiting method based on space-frequency multiplexing is characterized in that two different images in different spatial frequency domains are combined into a mixed image, a super-surface with a polarizer function is combined, a continuous gray scale mixed image is displayed in a near field, the two different images can be extracted from the mixed image after passing through two different filters, and the super-surface comprises a substrate and a nano brick array etched on the substrate.
2. The space-frequency multiplexing-based super-surface image anti-counterfeiting method according to claim 1, wherein the super-surface adopts a silver-silicon dioxide material structure, the silver on the top layer is used for etching the nano-brick array, and the silicon dioxide on the bottom layer is used as a substrate.
3. The space-frequency multiplexing-based super-surface image anti-counterfeiting method according to claim 1 or 2, wherein the substrate is divided into a plurality of working surfaces with side lengths of C nanometer unit structures, each nanometer unit structure is composed of a square working surface and a nanometer brick etched on the working surface, a coordinate system is established by taking the right-angle side of the unit structure as an X axis and a Y axis, the long side of the nanometer brick is a long axis, the short side of the nanometer brick is a short axis, and an included angle theta between the long axis of the nanometer brick and the X axis is a steering angle of the nanometer brick.
4. The space-frequency multiplexing-based super-surface image anti-counterfeiting method according to claim 3, wherein each nano-unit structure in the nano-unit array has a function of a polarizer, and the long axis direction of the nano-brick is equivalent to the transmission axis of the polarizer.
5. The spatial-frequency-multiplexing-based super-surface image anti-counterfeiting method according to claim 4, wherein the linearly polarized emergent light meets Malus' theorem after passing through the analyzer
I＝I0(cosθ)2
Wherein, I0The intensity of emergent light after passing through the super-surface is shown, theta is the included angle between the polarization direction of incident linearly polarized light and the long axis direction of the nano brick, I is the intensity of linearly polarized light after passing through the analyzer, and the intensity can be changedThe magnitude of θ enables arbitrary gray scale adjustment.
6. The spatial-frequency multiplexing-based super-surface image anti-counterfeiting method according to claim 1 or 2, characterized in that high-frequency information and low-frequency information of two different images are extracted respectively, components of the two spatial frequency domains are superposed to generate a mixed image, and images corresponding to the high-frequency components or the low-frequency components can be obtained from the mixed image by using a high-pass filter and a low-pass filter.
7. The space-frequency multiplexing-based super-surface image anti-counterfeiting method according to claim 6, wherein when linearly polarized light enters the super-surface, the light intensity and the polarization direction of the linearly polarized light are modulated and then emitted as reflected light, and the continuous gray scale mixed image with high resolution can be displayed in a near field by designing the distribution of the turning angles of the nano-brick array.
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