CN114690304A - A near-far-field dual-channel image display method based on metasurface materials - Google Patents

Abstract

The invention discloses a near-far-field dual-channel image display method based on metasurface materials, which utilizes two-dimensional code image recognition redundancy, intensity modulation redundancy and holographic design redundancy. The additional design freedom conferred by carefully designing the steering angles of the nanostructures enables the reproduction of twin-free holographic images in the far field while the structural surface of a piece of metasurfaces composed of a single nanostructure encodes a QR code pattern. The method of the invention has strong expansibility and robustness, and is not only extended to other optical platforms and working bands, but also applicable to large-area processing and production. And because the imaging modes in the near field and the far field are different, the decoding conditions are different, so the present invention has wide application prospects in the fields of high-end anti-counterfeiting, image display and the like.

Description

A near-far-field dual-channel image display method based on metasurface materials

technical field

The invention belongs to the technical field of micro-nano optics, and in particular relates to a near-far-field dual-channel image display method based on a metasurface.

Background technique

Metasurfaces are the first choice for high-performance, large-capacity, and multi-functional optical platforms due to their superior ability to manipulate light waves, enabling precise control of amplitude, phase, and polarization. In recent years, image display technology based on metasurface materials has received extensive attention from scholars at home and abroad due to its lightweight, miniaturization, large capacity, and high density. For example, in 2018, Bao et al. encoded two two-dimensional codes on the surface of a metasurface material by designing related pixels composed of various nanostructures, and decoded the two two-dimensional code images through specific wavelengths and incident angles. In the same year, Zhang et al. encoded a two-dimensional code image in a beam of laser light, and needed an analyzer to decode the image. In addition, some researchers have proposed a number of holographic image display technologies based on metasurface materials. By optimizing the design of the material, size and arrangement of nanostructures, the target holographic image can be reproduced in the far field. Since then, some researchers have fully exploited the various degrees of freedom in nanostructures, and realized the display of near-field images and far-field images simultaneously based on a piece of metasurface composed of a single nanostructure, variable-sized nanostructures, or stacked structures.

The invention proposes a new metasurface near-far-field dual-channel image display technology. A single nanostructure is used to design a metasurface array, so that while encoding a two-dimensional code image on the surface (near field) of the metasurface material, a holographic image without twinning can be reproduced at a distance. This new type of image display technology enriches the research field of image display, and also has good development prospects in the fields of high-end anti-counterfeiting, image hiding and so on.

SUMMARY OF THE INVENTION

In order to solve the limitation of the current near-far-field image display technology based on a single nanostructured metasurface, the purpose of the present invention is to provide a near-far-field dual-channel image display method based on a metasurface, which utilizes image recognition, intensity modulation and The redundancy of holographic design, by designing the steering angle of a single nanostructure, realizes the fusion of binary images and holographic images without twinning, thus realizing a new near-far-field dual-channel image display technology.

To achieve the above object, the present invention realizes through the following technical solutions:

The metasurface is composed of a plurality of nanobrick structural units arrayed on a plane, and the nanobrick structural units are composed of a transparent substrate and nanobricks deposited thereon. The steering angle of the nanobrick structural unit is θ, and the value range of θ is [0, π]. The side on which the nano-brick is deposited on the transparent substrate is a square working surface with side length C, and the side length C is a sub-wavelength level; the nano-brick length L, width W and height H are all sub-wavelength levels; according to the selected The working wavelength and the desired electromagnetic response characteristics are optimized by electromagnetic simulation to obtain specific geometric dimensions. The xoy coordinate system is established with the right-angled sides of the unit structure as the x-axis and the y-axis, the long side of the nanobrick is the long axis, the short side is the short axis, and the angle between the long axis and the x axis of the nanobrick is the steering angle θ of the nanobrick.

Based on the above technical solutions, preferably, the transparent substrate is made of fused silica glass material, and the nano-bricks are made of gold, silver, aluminum, silicon and other materials or use SOI materials to design nano-units.

Utilizing the redundancy of intensity regulation of nanostructures, the redundancy of QR code and holographic image design, and by arranging the steering angle of each nanostructure in the array structure, QR code images and no twinning can be realized on a piece of metasurface. Fusion of holographic images. Moreover, the information of the two channels is independent of each other, can be arbitrarily designed, and has strong flexibility. The invention can be applied to the fields of high-end anti-counterfeiting, polarization display, image hiding and the like.

A near-far-field dual-channel image display method based on a metasurface material provided by the present invention includes the following processes:

1) The metasurface is composed of a plurality of nano-brick structural units arrayed on a plane. Using the outgoing light intensity modulation function of the nano-brick structural units and the grayscale information of the two-dimensional code image, the outgoing light intensity and the turning of the nano-brick structural units are obtained. The corresponding relationship of the angle in the range of [0,180°];

2) Encoding a two-dimensional code image in the near field of the metasurface using nano-brick structural units. Since the two-dimensional code has redundancy in recognition and detection, the white pixels in the encoded two-dimensional code image are introduced into some noise to make Its gray value is converted into noise pixels between (0, 1], the two-dimensional code image can still be recognized and detected, and the modulated two-dimensional code image is obtained;

3) Since the outgoing light intensity modulation function has redundancy, that is, the outgoing light intensity modulation function is a non-monotonic function within the value range of the nano-brick steering angle [0,180°], and the same outgoing intensity corresponds to multiple nano-brick steering angles. (If the intensity modulation function is cos ² θ or sin ² θ, it corresponds to 2; if the intensity modulation function is cos ² 2θ or sin ² 2θ, it corresponds to 4); use the nanobrick structural unit according to the method of step 2). The outgoing light intensity modulation function realizes the encoding of the modulated two-dimensional code image, and introduces noise pixels in the two-dimensional code image, so that the gray value types of the white pixels in the two-dimensional code image are increased, resulting in the corresponding modulation of the two-dimensional code image. The steering angle types of the nano-brick array are increased, so that the candidate information of various nano-brick steering angles can be correspondingly introduced, and the introduction of multiple nano-brick steering angles will not change the intensity of the near-field two-dimensional code image. However, when the circularly polarized light is incident on the metasurface, the phase distribution options brought about by it become more (the phase change of the geometric phase has a linear relationship with the turning angle of the nanobricks), which can also be introduced accordingly. The candidate information of the geometric phase distribution is holographically transformed according to the obtained various geometric phase distributions to obtain a series of corresponding designed holographic images;

4) Based on the redundancy of the holographic design, the same holographic image corresponds to various geometric phase distributions; the fidelity of the holographic design and the fidelity of the two-dimensional code image are used as evaluation indicators (that is, the design holographic image and the target holographic image are used. The minimum error is the evaluation index), using the simulated annealing optimization algorithm to select the most suitable noise introduction method from a variety of candidate nano-brick steering angle values, so as to achieve near-field QR code images and far-field twin-free Design and implementation of holographic images of images.

In step 3), since the phase change of the geometric phase is twice the turning angle of the nano-brick, various candidate nano-brick turning angles are correspondingly introduced into various candidate information of the geometric phase distribution.

Based on the above technical solutions, each nano-unit structure in the nano-unit array is equivalent to an intensity modulator, and the output light intensity can be continuously changed by changing the steering angle of the nano-structure. By designing the geometric size of the nanostructure or setting the deflection angle of the polarizer and the analyzer, the functional relationship between the outgoing light intensity and the nanobrick deflection angle can be cos ² θ, sin ² θ, cos ² 2θ, sin ² 2θ and other relationships. To achieve the target intensity distribution, nanobricks with different steering angles can be arranged.

Specifically, if the polarization direction is α ₁ and the linearly polarized light passes through the nanostructure, the output light intensity I ₁ can be expressed as:

I ₁ =I ₀ [A ² cos ² (θ-α ₁ )+B ² sin ² (θ-α ₁ )]

Among them, I ₀ is the incident light intensity, and A and B are the complex transmission coefficients or reflection coefficients of the long and short axes of the nanobricks, respectively. When the nano-brick is a polarizer, that is, A=1, B=0 or A=0, B=1, by designing the polarization direction of the incoming polarized light, the outgoing light intensity modulation function can be cos ² θ, sin ² theta.

If the polarization direction is α ₁ and the linearly polarized light passes through the nanostructure, and then passes through the analyzer with the polarization direction α ₂ , the output light intensity I ₂ can be expressed as:

Then, by designing the values of α ₁ , α ₂ and A and B, the output light intensity modulation function can be realized as cos ² 2θ and sin ² 2θ. For example, α ₁ =45°, α ₂ =-45°, A=1, B=-1 (the nano-brick is a half-wave plate), then the outgoing light intensity function is cos ² 2θ.

If the light intensity modulation functions cos ² θ and sin ² θ are selected, it is necessary to optimize the design to make the nano-bricks have the characteristics of polarization splitting, that is, the linearly polarized light incident along the long axis direction is almost completely reflected when passing through the nano-bricks. The linearly polarized light incident in the axial direction is almost completely transmitted when passing through the nanobricks. Or (and) the linearly polarized light incident along the long axis direction is almost completely transmitted when passing through the nanobricks, and the linearly polarized light incident along the short axis direction is almost completely reflected when passing through the nanobricks.

If the light intensity modulation functions cos ² 2θ and sin ² 2θ are selected, the nanostructure can be any anisotropic structure, and the near-far field dual-channel image display can be realized. If the half-wave plate structure is preferred, the highest output efficiency can be obtained.

的关系如下式When the nanostructure acts as a phase modulator, based on the geometric phase principle, the steering angle of the nanobrick and the amount of phase change brought by it

The relationship is as follows

Therefore, the nanostructure can be regarded as an intensity modulator in near-field imaging. In far-field imaging, it can be seen as a phase modulator. Therefore, in order to take into account the two functions, additional design freedom needs to be found to ensure that both can coexist in one structure.

The redundancy of two-dimensional code identification and detection means that adding some errors or noises to the two-dimensional code does not affect the identification and detection of the two-dimensional code. The redundancy of holographic design means that the corresponding phase distribution of the design target hologram is not unique. The light intensity modulation redundancy of the nanostructure unit refers to that when the nanostructure acts as an intensity modulator, in the range of the nanobrick’s steering angle [0, π], the same outgoing light intensity can simultaneously correspond to multiple steering angles θ. . Comprehensive utilization of the design freedom brought by these three redundancy can solve the problems of the existing near and far field image display technology based on simple structure. On a metasurface, the simultaneous realization of two-dimensional code images and twin-free holographic images is achieved.

Specifically, the two-dimensional code image contains two parts: black pixels and white pixels (the gray value of black pixels is 0, and the gray value of white pixels is 1). The redundancy of the two-dimensional code can be guaranteed. When some white pixels are added to the white pixels, that is, the pure white pixels with a grayscale value of 1 are converted into noise pixels with a grayscale value of 0.25, 0.5, 0.75, and 1. The code image can still be recognized.

After adding noise pixels to the ^two -dimensional code image, the gray value of the noise pixels modulated by the ^{two -} dimensional code image is converted into the outgoing light intensity. 2θ, sin ² 2θ redundancy shows that within the value range of nano-bricks [0, π], the nano-brick steering angles corresponding to the same emission intensity have become multiple, not only one, so its band The amount of geometric phase change has also become various, which can be beneficial to the design of multi-step phase-type holography and eliminate twinning images.

Based on the holographic design redundancy, various geometric phase distributions of the same holographic image can be designed. For example, when each white pixel in the QR code has 4 candidates for grayscale noise, if the intensity modulation function is selected as cos ² θ, and one grayscale noise corresponds to two nano-brick corners, the total corresponding far field is 8 The phase change amount of the species is introduced. For a QR code image with 200x200 white pixels, there are a total of 8 ^200x200 noise introduction schemes, corresponding to 8 ^200x200 phase distributions, with the fidelity of the holographic design and the fidelity of the QR code image (The error between the design image and the target image is the smallest) as the evaluation index, and the simulated annealing algorithm is used to select the most suitable noise introduction scheme, so as to realize the design and implementation of the near-field two-dimensional code image and the far-field twin-free image.

A near-far-field dual-channel image display method based on metasurface material designed by the present invention has the following advantages and positive effects:

1. On the basis of not changing the function of the QR code, a holographic image without twinning is encoded in the far field. This method overcomes the limitation of the existing near and far-field image display methods based on a simple structure, and provides a brand new method and approach for near and far-field image display.

2. The method of the present invention can be realized only by a single nano-brick structure, the processing difficulty is low, and the used structure is simple, so it is suitable for large-area integration and processing.

3. The nanostructure required by the present invention is not limited to a specific material, and various materials and various schemes can be realized.

4. The present invention has strong robustness to the geometric dimensions of the nanostructures, so it has a high tolerance to machining errors and is more convenient for practical application.

5. Two-dimensional code images and holographic images are reproduced by imaging methods based on two different principles of nano-printing and holography, respectively, so their decoding conditions are different. Near-field images need to be observed with the aid of a magnifying system or microscope, and far-field images need to be observed by building a holographic optical path. The complex observation conditions make them have strong application prospects in the fields of encryption and high-end anti-counterfeiting.

6. The multifunctional optical device of the present invention can work in both transmission mode and reflection mode, which has great convenience in practical application.

7. Since the geometric size of the metasurface is very small, only in the sub-wavelength level, and encoding two images of about 500x500 pixels, only a 200x200μm metasurface is required, so it is miniaturized, lightweight, and highly integrated. It is suitable for the large-scale development of miniaturization and miniaturization in the future.

8. The method proposed in the present invention is not limited to nanometer size and visible light waveband, and can be extended to traditional optical devices and multiple working wavebands.

Description of drawings

FIG. 1 is a schematic diagram of the structure of the nano-brick unit in Example 1. FIG.

FIG. 2 is a scanning diagram of the transmittance and reflectivity of the unit structure of silver nanobricks in Example 1. FIG.

FIG. 3 is a flow chart of the design of the steering angle of the nano-bricks in Example 1. FIG.

FIG. 4 is an image of the two-dimensional code encoded by channel 1 in the first embodiment.

FIG. 5 is a holographic image without twin images encoded by channel 2 in Embodiment 1. FIG.

FIG. 6 is a schematic diagram of the structure of the nanobrick unit in the present embodiment 1. FIG.

FIG. 7 is a scanning diagram of the reflectivity of the nano-brick unit structure in Example 2. FIG.

FIG. 8 is an image of the two-dimensional code encoded by channel 1 in the second embodiment.

FIG. 9 is a holographic image without twin images encoded by channel 2 in the second embodiment.

Detailed ways

In order to more clearly illustrate the embodiments of the present invention and/or the technical solutions in the prior art, specific embodiments of the present invention will be described below with reference to the accompanying drawings. Obviously, the accompanying drawings in the following description are only embodiments of the present invention. For those of ordinary skill in the art, other drawings and other implementation.

The present invention will be further described below with specific embodiments in conjunction with the accompanying drawings.

In Figure 1, L is the length of the nano-brick, W is the width of the nano-brick, H is the height of the nano-brick, C is the unit size of the nano-brick, θ is the direction angle of the nano-brick, and is the angle between the long axis of the nano-brick and the x-axis .

Embodiments 1 and 2 are specific implementation processes of a near-far-field dual-channel image display method based on a metasurface material.

Example 1

In this embodiment, the nano-unit structure is composed of silver nano-bricks and a silicon base layer, and the design wavelength is selected as λ=633 nm. For this wavelength, the electromagnetic simulation software CST is used to optimize and simulate the nano-unit structure to obtain the optimized silver nanometer. The size parameters of the brick are: length L=160nm, width W=80nm, height H=70nm, and the side length of the base of the unit structure is C=300nm. The transmission and reflection efficiency of the nanobricks for linearly polarized light incident along the long and short axes of the nanobricks under this structural parameter is shown in Figure 2, where R _l and R _s represent the reflections along the long and short axes of the nanobricks, respectively light efficiency. Specifically, at the operating wavelength of 633 nm, the reflectivity R _l in the long axis direction of the nanobricks and the transmittance T _s in the short axis direction of the nanobricks reach 92.6% and 95.3%, respectively, while the reflectivity along the short axis direction of the nanobricks reaches 92.6% and 95.3%, respectively. The R _s and the transmittance T _l along the long axis of the nanobricks are suppressed below 4% and 2%. Therefore, at 633 nm, the optimized nanobricks can be regarded as ideal polarizers in both reflection and transmission modes.

Taking into account the redundancy characteristics of nanostructure intensity modulation, two-dimensional code identification and holographic design, the steering angle of the nanobricks is determined according to the design flow in Figure 3. Firstly, according to the target image of the two-dimensional code, the candidate value of the steering angle of the participating nanostructures is determined. Due to the redundancy of the two-dimensional code image and the intensity modulation function, having a variety of nanobrick steering angles can satisfy the construction of the two-dimensional code image. Then, according to the target holographic image and the far-field diffraction calculation formula, a feasible steering angle distribution that not only satisfies two-dimensional code recognition but also realizes holographic reproduction is found from the optional space of nanostructure steering angle. Finally, on a metasurface composed of nanostructures of the same size and different steering angles, the simultaneous encoding of two-dimensional codes and twin-free holographic images is realized, as shown in Figure 4 and Figure 5.

Example 2

In this embodiment, the nano-unit structure is composed of SOI materials, wherein the nano-bricks are silicon, and the base materials are silicon dioxide and silicon materials, as shown in FIG. 6 . The design wavelength is selected as λ=610nm. For this wavelength, the nano-turn unit structure is optimized and simulated by the electromagnetic simulation software CST. The size parameters of the optimized SOI nano-brick are: length 180nm, width 100nm, height 220nm, unit structure size is 400. The reflection efficiency of the nanobricks for the linearly polarized light incident along the long and short axes of the nanobricks under this structural parameter is shown in Figure 7. It is obvious that at 610 nm, the light incident along the long axis of the nanobricks is hardly reflected. , and most of the light incident along the short axis of the nanobrick is reflected, so it can be regarded as a reflective polarizer.

The steering angle of the nanobricks is determined according to the design flow of Figure 3. Firstly, the nanostructures involved in the holographic design are determined according to the target image of the QR code, and then, according to the target holographic image and the far-field diffraction calculation formula, from the optional space of the steering angle of the nanostructures, it is found that both the QR code encoding and the holographic reproduction can be realized. feasible steering angle distribution. Finally, on a metasurface composed of nanostructures of the same size and different steering angles, the simultaneous encoding of the two-dimensional code and the holographic image without twinning is realized, as shown in Figure 8 and Figure 9.

The above-mentioned embodiments are used to explain the present invention, rather than limit the present invention. Within the spirit of the present invention and the protection scope of the claims, any modifications and changes made to the present invention all fall into the protection scope of the present invention.

Claims (7)

1. a near-far-field dual-channel image display method based on metasurface material, is characterized in that comprising the following process: 1) The metasurface is composed of a plurality of nano-brick structural units arrayed on a plane. Using the outgoing light intensity modulation function of the nano-brick structural units and the grayscale information of the two-dimensional code image, the outgoing light intensity and the turning of the nano-brick structural units are obtained. The corresponding relationship of the angle in the range of [0,180°]; 2) Encoding a two-dimensional code image in the near field of the metasurface using nano-brick structural units. Since the two-dimensional code has redundancy in recognition and detection, the white pixels in the encoded two-dimensional code image are introduced into some noise to make Its gray value is converted into noise pixels between (0, 1], the two-dimensional code image can still be recognized and detected, and the modulated two-dimensional code image is obtained; 3) Since the outgoing light intensity modulation function has redundancy, that is, the outgoing light intensity modulation function is a non-monotonic function within the value range of the nano-brick steering angle [0,180°], and the same outgoing intensity corresponds to multiple nano-brick steering angles. ; According to the method of step 2), the output light intensity modulation function of the nano-brick structural unit is utilized to realize the encoding of the modulated two-dimensional code image, and noise pixels are introduced in the two-dimensional code image, so that the gray value type of the white pixel in the two-dimensional code image is more, and correspondingly introduce candidate information for obtaining various steering angles of nano-bricks; when circularly polarized light is incident, correspondingly introduce candidate information for various geometric phase distributions; 4) Based on the redundancy of the holographic design, the same holographic image corresponds to various geometric phase distributions; the fidelity of the holographic design and the fidelity of the two-dimensional code image are used as the evaluation indicators, and the simulated annealing optimization algorithm is used to obtain the results from various candidates. The most suitable noise introduction method is selected from the value of the nano-brick steering angle, so as to realize the design and realization of the near-field two-dimensional code image and the far-field twin-free holographic image. 2. a kind of near-far-field dual-channel image display method based on metasurface material as claimed in claim 1, is characterized in that described nano-brick structural unit comprises transparent base and nano-brick, and transparent base is placed on described plane, The nano-bricks are deposited on the transparent substrate; the turning angle of the nano-brick structural unit is θ, and the value range of θ is [0, π]; the side of the transparent substrate on which the nano-bricks are deposited is a square working surface with side length C, and the side length is C. The length C is a sub-wavelength level; the length L, width W and height H of the nano-brick are all sub-wavelength levels; the L, W and H are obtained by electromagnetic simulation optimization according to the selected incident light wavelength; Establish an xoy coordinate system for the x-axis and the y-axis, the long side of the nanobrick is the long axis, the short side is the short axis, and the angle between the long axis of the nanobrick and the x axis is the steering angle θ of the nanobrick; The transparent substrate is made of fused silica glass material, and the material of the nano-bricks includes gold, silver, aluminum or silicon. 3. a kind of near-far-field dual-channel image display method based on metasurface material as claimed in claim 1, is characterized in that in step 1), introduce polarizer and analyzer by designing the geometrical dimension of nano-structure unit or setting According to the deflection angle of the device, the functional relationship between the output light intensity and the steering angle θ is obtained, that is, the output light intensity modulation function is cos ² θ, sin ² θ, cos ² 2θ or sin ² 2θ. 4. a kind of near and far-field dual-channel image display method based on metasurface material as claimed in claim 3, it is characterized in that after the linearly polarized light of polarization direction α ₁ passes through the nano-structure unit, the exit light intensity I ₁ can represent for: I ₁ =I ₀ [A ² cos ² (θ-α ₁ )+B ² sin ² (θ-α ₁ )] Among them, I ₀ is the incident light intensity, A and B are the complex transmission coefficients or reflection coefficients of the long and short axes of the nanobricks, respectively; when the nanobricks are polarizers, that is, A=1, B=0 or A=0, When B=1, by designing the polarization direction of the incoming ray polarized light, the intensity modulation function of the outgoing light can be realized as cos ² θ or sin ² θ. 5. a kind of near-far-field dual-channel image display method based on metasurface material as claimed in claim 4, it is characterized in that selecting the output light intensity modulation function to be cos ² θ or sin ² θ, then it is necessary to optimize the design so that Nanobricks have the characteristics of polarization splitting, that is, when the incident light wave passes through the nanobricks at the working wavelength, the linearly polarized light with the polarization direction along the long axis of the nanobricks is reflected, and the linearly polarized light along the short axis of the nanobricks is transmitted at the same time, or is the polarization direction. The linearly polarized light along the long axis of the nanobrick is transmitted, while the linearly polarized light whose polarization direction is along the short axis of the nanobrick is reflected. 6. a kind of near and far-field dual-channel image display method based on metasurface material as claimed in claim 3, it is characterized in that after the linearly polarized light that polarization direction is α ₁ passes through nano-structure unit, passes through polarization direction is α ₂ again. The analyzer, the outgoing light intensity I ₂ can be expressed as:

Among them, I ₀ is the incident light intensity, A and B are the complex transmission coefficients or reflection coefficients of the long and short axes of the nanobricks, respectively; then by designing the values of α ₁ , α ₂ and A and B, the outgoing light can be realized. The emphasis modulation function is cos ² 2θ or sin ² 2θ. At this time, the nanostructure unit is any anisotropic structure, which can realize near-far-field dual-channel image display. If the half-wave plate structure is preferred, the highest output efficiency can be obtained. 7. a kind of near-far-field dual-channel image display method based on metasurface material as claimed in claim 1, is characterized in that in step 2), the white pixel in the two-dimensional code image is introduced some noise points to make its gray value Converted to 0.25, 0.5, 0.75 and 1 noise pixels.

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