CN111158075B - Watermark anti-counterfeiting super-surface device and design method thereof - Google Patents

Abstract

The invention provides a watermark anti-counterfeiting metasurface device and a design method thereof. The metasurface consists of nanobrick unit structures capable of responding to both red and green light. The nanobrick unit structure is equivalent to a polarizer for the incident red and green polarized light, which can effectively adjust the ratio of the red and green components in the incident polarized light to form a color pattern in the near field (the color is between red and green); in addition, By clever use of Marius' law and its variants, when the incident light is monochromatic light, a bright or dark watermark can be superimposed on the original monochromatic image by rotating the polarizer in the light path. The invention can be applied to high-resolution image display, optical anti-counterfeiting, and information multiplexing, and is small in size, low in cost, light in weight, and simple in design idea, and is very suitable for application in micro-photoelectric systems.

Description

Watermark anti-counterfeiting metasurface device and design method thereof

technical field

The invention belongs to the fields of micro-nano optics and polarized optics, in particular to a watermark anti-counterfeiting metasurface device and a design method thereof.

Background technique

Image anti-counterfeiting is a very important direction in the field of optics. Most of the anti-counterfeiting watermarking using metasurface images is realized by the transformation of different images, but this is not a real "watermark", because the original image tends to disappear during the transformation process, instead of superimposing the watermark pattern on the original image; in addition, traditional images The display resolution is low, the image area is large and conspicuous, and because there is no corresponding encoding and decoding process, the security is low when applied to anti-counterfeiting.

SUMMARY OF THE INVENTION

Aiming at the deficiencies of traditional image anti-counterfeiting, the present invention provides a metasurface device capable of efficiently realizing watermark anti-counterfeiting and a design method thereof by designing a silver nano-brick array in combination with polarization theory.

One of the objectives of the present invention is to provide a metasurface device for realizing watermark anti-counterfeiting, the metasurface utilizes Marius' law I=I ₀ cos ² (θ) and its related variants, the rotation angle is optimized, and for the first time through a single geometry The nano-brick unit structure array of the size can realize near-field monochrome watermark pattern and watermark-free pattern display and color watermark pattern. In addition, the present invention has a simple structure and is easy to process. The invention has good application potential in optical anti-counterfeiting, image hiding, high-resolution monochrome and color image display, information multiplexing and the like;

The second purpose of the present invention is to provide a design method of a metasurface device for realizing watermark anti-counterfeiting, which can generate the same light intensity and the difference of light intensity under different incident conditions by using different rotation angles of Marius' law. The superposition of near-field monochrome watermark patterns and the display of color watermark patterns are realized;

For achieving the above object, scheme of the present invention is as follows:

In a first aspect, the present invention provides a metasurface device for realizing watermark anti-counterfeiting, characterized in that:

The metasurface is formed by nanobricks capable of responding to red light and green light simultaneously to form a nanobrick unit array, the interval between different nanobrick unit structures is the period CS, and the nanobrick units at different positions correspond to different turning angles;

The nanobrick unit structure acts as a polarizer for linearly polarized light; when the incident red and green linearly polarized light is polarized along the long axis of the nanobrick, the red light will be reflected and the green light will be transmitted; when the incident red and green linearly polarized light is polarized along the short axis of the nanobrick , the green light will be reflected and the red light will be transmitted;

When the incident light source is green light, passing through a polarizer with a polarization direction of 0°, the watermark anti-counterfeiting metasurface and an analyzer with a polarization direction of 90° in turn, a monochromatic image will be observed on the surface of the sample. Continuous grayscale pattern; turn the polarization direction of the analyzer to 135°, the monochrome grayscale pattern will remain unchanged, but a layer of anti-counterfeiting watermark is superimposed on the surface;

When the incident light source is red and green two-color light, a polarizer with a polarization direction of 0°, the watermark anti-counterfeiting metasurface and an analyzer with a polarization direction of 0° are sequentially passed through. Superimposed anti-counterfeiting watermark and color pattern;

the polarization direction is relative to the x-axis;

In a second aspect, the present invention provides a method for designing a metasurface device for realizing watermark anti-counterfeiting as described above, characterized in that it comprises the following steps:

(1) According to the selected two wavelengths of incident light, through electromagnetic simulation software, when the incident ray polarized light irradiates the nano-brick unit vertically, the reflection efficiency of red light polarized along the long axis is high, and the transmission efficiency of green light is high; With the goal of high red light transmission efficiency and high green light reflection efficiency, the period CS of the nano-brick unit structure, the width W, the length L, and the height H of the nano-brick unit structure are optimized;

观察光强I₁和I₂的函数图像，会发现0<θ<π/8时，I₁≈I₂；3π/8<θ<π/2时，I₁<I₂；7π/8<θ<π时，I₁>I₂。而对于光强I₁而言，区间0<θ<π/8，3π/8<θ<π/2，7π/8<θ<π对应的强度正好是相等的，因此当入射情况改为第二种时，我们在希望叠加暗水印的区域选择转角范围7π/8<θ<π；在希望叠加亮水印的区域，选择转角范围3π/8<θ<π/2；在希望不叠加水印的区域，选择转角范围0<θ<π/8；由此实现单色光入射情况下，检偏器从90°转到135°时，能够在原图不变的基础上叠加亮与暗两种水印；于此同时，由于所设计的纳米砖能够对红绿光同时响应，当入射光为红绿光，且起检偏器均为0°时，样片表面能够观察到一幅颜色介于红绿之间的彩色图案；(2) Considering the incident of monochromatic green light, define the rotation angle θ of the nanobrick as the angle between the long axis of the nanobrick and the x-axis, when the polarization direction of the polarizer is 0° and the direction of the analyzer is 90°; then the reflection The green light intensity is I ₁ =I ₀ sin ² (2θ), the reflected green light intensity I ₁ is a function of θ, and the period is π/2, which means between 0 and π, according to this incident situation In the monochromatic target pattern below, the grayscale of each pixel can have four different rotation angles as candidate rotation angles; if the analyzer is rotated to 135°, the reflected green light intensity is

Observe the function image of light intensities I ₁ and I ₂ , you will find that when 0<θ<π/8, I ₁ ≈I ₂ ; when 3π/8<θ<π/2, I ₁ <I ₂ ; 7π/8< When θ<π, I ₁ >I ₂ . For the light intensity I ₁ , the intensities corresponding to the interval 0<θ<π/8, 3π/8<θ<π/2, and 7π/8<θ<π are exactly equal, so when the incident situation is changed to the first In the second case, we select the corner range 7π/8<θ<π in the area where we want to superimpose the dark watermark; in the area where we want to superimpose the bright watermark, select the corner range 3π/8<θ<π/2; area, select the rotation angle range 0<θ<π/8; in this way, in the case of monochromatic light incident, when the analyzer is turned from 90° to 135°, the light and dark watermarks can be superimposed on the basis of the original image unchanged. At the same time, since the designed nano-brick can respond to red and green light at the same time, when the incident light is red and green light, and the analyzers are both 0°, a color between red and green can be observed on the surface of the sample. color pattern between;

(3) according to the silver nano-brick array structure determined in step (2), a photolithography process is used to prepare a watermark anti-counterfeiting metasurface;

(4) When the incident light is green light, the polarization direction of the polarizer is 0°, and the direction of the analyzer is 90°, a monochromatic pattern will be observed in the near field; rotate the analyzer to 135°, and the You can see that the light and dark watermarks are superimposed on the monochrome pattern; switch the incident light to red and green light, rotate the analyzer to 0°, and you will see a picture with a watermark that is different from the background color;

Advantages and beneficial effects of the present invention:

The watermarking anti-counterfeiting metasurface of the present invention can realize real watermarking through the transformation of the polarization angle of the analyzer, that is, the original pattern remains unchanged, and the pattern is superimposed on this basis; in addition, the present invention can also realize color watermarking under the illumination of two-color incident light pattern, and the resolution can reach 74000dpi, which exceeds the resolution of traditional image display and ordinary metasurface color patterns. Specifically:

(1) The provided watermark anti-counterfeiting metasurface can realize the disappearance and reproduction of the watermark on the target pattern by rotating the analyzer under the incident of monochromatic light, and has anti-counterfeiting function;

(2) The provided watermark anti-counterfeiting metasurface can realize a color watermark pattern under the incident of red and green light;

(3) Compared with the traditional image display, the resolution of the pattern provided by the present invention is greatly improved, and a decoding process is required, and there are different patterns under different incident conditions, which has certain confidentiality;

(4) Compared with the traditional image display device, the near-far-field multiplexing metasurface of the present invention has the advantages of small size, low cost, low weight, and can realize dynamic modulation;

(5) It has an ultra-micro-sized structure and can be widely used in the field of photonic integration;

(6) The metal nano-brick array structure can be processed by the standard photolithography process, and the process is simple;

(7) It provides a new idea for the design and preparation of image display and encryption, which has great reference value and application prospect.

Description of drawings

1 is a schematic diagram of the structural unit of the watermark anti-counterfeiting metasurface in Embodiment 1, and the rotation angle θ is defined as the angle between the long axis of the nanobrick and the x axis;

Fig. 2 is the three-dimensional structure schematic diagram of some silver nanobrick array structures in Example 1;

3 is a graph of the simulation result of the silver nanobrick array on incident light in Example 1, where the x-axis is the wavelength and the y-axis is the reflection efficiency, including the long-axis reflection efficiency and the short-axis reflection efficiency.

4 is a schematic diagram of an optical path for decoding a near-field pattern in Embodiment 1;

Fig. 5 is the nano-printing diagram when the monochromatic light is incident; Fig. (a) is the monochromatic diagram when the analyzer is 90°; Fig. (b) is the pattern of superimposing watermark when the analyzer is 135°;

Fig. 6 is the color band watermark pattern when two-color light is incident;

In the figure: 1. Nano-brick; 2. Substrate; L, Nano-brick long-axis dimension; W, Nano-brick short-axis dimension; H, Nano-brick height, CS, nano-brick spacing.

Detailed ways

The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

The embodiment of the present invention provides a metasurface for realizing watermark anti-counterfeiting. The metasurface is composed of a transparent substrate and a nano-unit array etched on the surface of the substrate. The nano-unit array includes a plurality of nano-unit structures (the nano-units described in this embodiment) The structure is silver nano-bricks), and the disappearance and reproduction of monochrome image watermarks and the display of color watermarked images are realized through design.

(1) The silver nanobrick array structure shown in FIGS. 1 to 2 includes two layers, which are silver nanobricks 1 and substrate 2 in order from top to bottom. Wherein, the silver nano-brick array 1 is constituted by the periodic arrangement of silver nano-brick unit structures, the silver nano-bricks are rectangular parallelepipeds, and their length, width and height are all sub-wavelength dimensions. The structure of a single silver nanobrick unit is shown in Figure 1. In this embodiment, the substrate 2 is a silicon dioxide substrate.

As an example, 510nm and 620nm are selected as the working wavelengths, and the existing CSTSTUDIO SUITE electromagnetic simulation tool is used to optimize the structure of the silver nanobrick unit at the working wavelength, so that when the incident ray polarized light is polarized along the long axis of the nanobrick, the red light reflection efficiency When the incident ray polarized light is polarized along the short axis of the nano-brick, the reflection efficiency of green light is the highest; by rotating the nano-brick array structure, the reflected light can be changed from red light to green light. In this embodiment, the optimized silver nano-brick has a length L=140nm, a width W=85nm, a thickness H=70nm, and within the unit structure, CS=340nm; the polarization efficiency diagram obtained by simulation is shown in Figure 3, and the polarization along the long axis is shown in Figure 3. The reflection efficiency of red light reaches 90%, and the reflection efficiency of green light polarized along the short axis reaches 70%;

观察光强I₁和I₂的函数图像，会发现0<θ<π/8时，I₁≈I₂；3π/8<θ<π/2时，I₁<I₂；7π/8<θ<π时，I₁>I₂。而对于光强I₁而言，区间0<θ<π/8，3π/8<θ<π/2，7π/8<θ<π对应的强度正好是相等的，因此当入射情况改为第二种时，我们在希望叠加暗水印的区域选择转角范围7π/8<θ<π；在希望叠加亮水印的区域，选择转角范围3π/8<θ<π/2；在希望不叠加水印的区域，选择转角范围0<θ<π/8；由此实现单色光入射情况下，检偏器从90°转到135°时，能够在原图不变的基础上叠加亮与暗两种水印；于此同时，由于所设计的纳米砖能够对红绿光同时响应，当入射光为红绿光，且起检偏器均为0°时，样片表面能够观察到一幅颜色介于红绿之间的彩色图案；(2) Considering the incident of monochromatic green light, define the rotation angle θ of the nanobrick as the angle between the long axis of the nanobrick and the x-axis, when the polarization direction of the polarizer is 0° and the direction of the analyzer is 90°; then the reflection The green light intensity is I ₁ =I ₀ sin ² (2θ), the reflected green light intensity I ₁ is a function of θ, and the period is π/2, which means between 0 and π, according to this incident situation In the monochromatic target pattern below, the grayscale of each pixel can have four different rotation angles as candidate rotation angles; if the analyzer is rotated to 135°, the reflected green light intensity is

Observe the function image of light intensities I ₁ and I ₂ , you will find that when 0<θ<π/8, I ₁ ≈I ₂ ; when 3π/8<θ<π/2, I ₁ <I ₂ ; 7π/8< When θ<π, I ₁ >I ₂ . For the light intensity I ₁ , the intensities corresponding to the interval 0<θ<π/8, 3π/8<θ<π/2, and 7π/8<θ<π are exactly equal, so when the incident situation is changed to the first In the second case, we select the corner range of 7π/8<θ<π in the area where we want to superimpose dark watermarks; in the area where we want to superimpose bright watermarks, select the range of rotation angle 3π/8<θ<π/2; area, select the rotation angle range 0<θ<π/8; thus, in the case of monochromatic light incident, when the analyzer is turned from 90° to 135°, the light and dark watermarks can be superimposed on the basis of the original image. At the same time, because the designed nano-brick can respond to red and green light at the same time, when the incident light is red and green light, and the analyzers are both 0°, a color between red and green can be observed on the surface of the sample. color pattern between;

(3) according to the silver nano-brick array structure determined in step (2), a photolithography process is used to prepare a watermark anti-counterfeiting metasurface;

(4) The optical path diagram is shown in Figure 4; when the incident light is green light, the polarization direction of the polarizer is 0°, and the direction of the analyzer is 90°, a monochromatic pattern will be observed in the near field; When the polarizer is rotated to 135°, two watermarks, bright and dark, are superimposed on the monochromatic pattern, as shown in Figure 5; when the incident light is switched to red and green light, the analyzer is rotated to 0°, and a watermark will be seen. A picture with different watermark and background color, as shown in Figure 6;

The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited to this. Any modifications made by any person skilled in the art within the technical scope disclosed by the present invention are equivalent Substitutions and improvements, etc., should all be included within the protection scope of the invention.

Claims (3)

1. a watermark anti-counterfeiting metasurface device, is characterized in that: The metasurface is composed of nano-brick unit arrays formed by nano-bricks that can respond to red light and green light at the same time, the interval between different nano-brick unit structures is the period CS, and the nano-brick units at different positions correspond to different rotation angles θ; The rotation angle θ is the angle between the long axis of the nanobrick and the x axis; The nano-brick unit structure acts as a polarizer for linearly polarized light; when the incident red and green linear polarized light is polarized along the long axis of the nano-brick, the red light will be reflected and the green light will be transmitted; When axially polarized, green light will be reflected and red light will be transmitted; The polarization direction refers to an edge of the upper surface of the substrate with respect to the x-axis: ① When the incident light source is green light, passing through a polarizer with a polarization direction of 0°, a watermark anti-counterfeiting metasurface and an analyzer with a polarization direction of 90° in turn, the watermark anti-counterfeiting metasurface will be observed on the upper surface of the watermark anti-counterfeiting metasurface. A monochromatic continuous grayscale pattern; turn the polarization direction of the analyzer to 135°, and the monochromatic grayscale pattern will remain unchanged, but a layer of anti-counterfeiting watermark is superimposed on the surface; ② When the incident light source is red and green dichromatic light, it will pass through a polarizer with a polarization direction of 0°, a watermark anti-counterfeiting metasurface and an analyzer with a polarization direction of 0° in turn, and the watermark anti-counterfeiting metasurface will be on the upper surface of the watermark anti-counterfeiting metasurface. A pattern in color with an anti-counterfeiting watermark superimposed was observed. 2. watermark anti-counterfeiting metasurface device according to claim 1, is characterized in that: When two-color light is incident, the near-field color printing image realizes continuous color adjustment between red and green light; When monochromatic light is incident, the near-field grayscale pattern realizes the continuous adjustment of grayscale; Using Marius' law I=I ₀ cos ² (θ) and related variants, the superposition and concealment of watermarks can be realized under the condition of monochromatic light illumination. 3. a design method of watermark anti-counterfeiting metasurface device as claimed in claim 1 or 2, is characterized in that: comprise the following steps: (1) According to the selected two wavelengths of incident light, through electromagnetic simulation software, when the incident ray polarized light irradiates the nano-brick unit vertically, the reflection efficiency of red light polarized along the long axis is high, and the transmission efficiency of green light is high; With the goal of high red light transmission efficiency and high green light reflection efficiency, the period CS of the nano-brick unit structure, the width W, the length L, and the height H of the nano-brick unit structure are optimized; (2) Considering the incident of monochromatic green light, define the rotation angle θ of the nanobrick as the angle between the long axis of the nanobrick and the x-axis, when the polarization direction of the polarizer is 0° and the direction of the analyzer is 90°; then the reflection The green light intensity is I ₁ =I ₀ sin ² (2θ), the reflected green light intensity I ₁ is a function of θ, and the period is π/2, that is, between 0 and π. Monochromatic target pattern, the grayscale of each pixel has four different corners as candidate corners; Rotate the analyzer to 135°, then the reflected green light intensity is

Observe the function image of light intensities I ₁ and I ₂ , when 0<θ<π/8, I ₁ ≈I ₂ ; when 3π/8<θ<π/2, I ₁ <I ₂ ; 7π/8<θ <π, I ₁ >I ₂ ; for the light intensity I ₁ , the corresponding intensities in the interval 0<θ<π/8, 3π/8<θ<π/2, 7π/8<θ<π are equal , so when the incident light source is changed to red and green two-color light, when the dark watermark area needs to be superimposed, select the corner range 7π/8<θ<π; when the bright watermark area needs to be superimposed, select the corner range 3π/8<θ<π /2; When the area without watermark needs to be superimposed, select the corner range 0<θ<π/8; In this way, when the monochromatic light is incident, when the analyzer is turned from 90° to 135°, the light and dark watermarks can be superimposed on the basis of the original image; In response, when the incident light is red and green light, and the analyzers are both 0°, a color pattern between red and green can be observed on the upper surface of the watermark anti-counterfeiting metasurface; (3) according to the silver nano-brick array structure determined in step (2), a photolithography process is used to prepare a watermark anti-counterfeiting metasurface; (4) When the incident light is green light, the polarization direction of the polarizer is 0°, and the direction of the analyzer is 90°, a monochromatic pattern will be observed in the near field; rotate the analyzer to 135°, and the You can see that the light and dark watermarks are superimposed on the monochrome pattern; switch the incident light to red and green light, rotate the analyzer to 0°, and you will see a picture with a watermark that is different from the background color.

Images