Tunable near-field and far-field color holographic metasurface

Abstract

The invention provides a tunable near-field and far-field color holographic metasurface. The super surface is a nano unit array formed by staggered arrangement of Si nano unit structures responding to blue light and phase change material Sb2S3 nano unit structures responding to red light or green light, and a substrate is made of SiO2. By controlling the crystalline state and the amorphous state of the phase change material Sb2S3, dynamic regulation and control of the image color under the condition of red light or green light incidence are achieved. The nanometer unit array is equivalent to a half-wave plate for incident ray polarization, the polarization direction and light intensity of emergent rays can be effectively adjusted, so that a colored pattern is formed in a near field, Fourier holography is achieved in a far field when incident light is monochromatic circular polarization, and the two image display modes are mutually independent. According to the invention, color near-field display and far-field holographic multiplexing can be realized through one metasurface, dynamic regulation and control of image colors are realized, and the metasurface has great potential in multiple fields such as data storage, holographic display, information encryption and the like.

Description

Technical Field

[0001] This invention relates to a tunable near-field and far-field color holographic metasurface, belonging to the field of micro-nano optics and optical holography. Background Technology

[0002] Metasurfaces can flexibly and effectively control the amplitude, phase, and polarization state of light fields at subwavelength scales, offering advantages such as small size, light weight, and easy integration. They have been widely applied in various fields of optics, including holographic displays, optical encryption, nanoprinting, edge detection, diffraction neural networks, and optical cloaking. Currently, numerous studies have utilized metasurfaces to achieve near-field and holographic image displays. However, research on metasurface devices that achieve color near-field image display and far-field holographic image multiplexing using a single metasurface, and that tune the operating wavelength using phase change materials, is still limited. This invention achieves color near-field image display and far-field holographic image multiplexing using a metasurface, showing significant application prospects in areas such as improving information density, integrating multifunctional devices, and achieving information encryption.

[0003] Currently, while various metasurface technologies can achieve color near-field image display and far-field holographic images, they generally suffer from a problem: low transmittance for blue, green, and red light, with some metasurfaces exhibiting transmittance as low as around 20%. This severely limits the quality of holographic and near-field imaging. To address this issue, the nano-unit structure array designed in this invention significantly improves the transmittance for the three primary colors of light, reaching a maximum of over 60% and a minimum of 48%, further enhancing imaging quality. Summary of the Invention

[0004] To address the shortcomings of current holographic metasurfaces, such as low transmittance for the three primary colors of light and lack of dynamic tunability, the present invention includes designing the size and shape of the unit structure and changing the metasurface's response to red and green light by adjusting the state of the phase change material Sb2S3, thereby generating near-field and far-field holographic images of different colors.

[0005] The objective of this invention is achieved as follows:

[0006] The exhibit is a tunable near-field and far-field color holographic metasurface, characterized in that: the metasurface is composed of a periodically arranged array of nanounits with a lateral period P of 460 nm, which is etched onto the working surface of a substrate (1) with a substrate thickness of H = 200 nm. The nanounit array can be further divided into two nanounit structures, namely unit structure 1 (2) and unit structure 2 (3). The working surface of each unit structure is rectangular. The length of unit structure 1 is l1 = 92 nm, the width is w1 = 34 nm, the height is h = 400 nm, the rotation angle is α, and the material is Si; the length of unit structure 2 is l2 = 145 nm, the width is w2 = 75 nm, the height is h = 400 nm, the rotation angle is β, and the material is Sb2S3; the substrate material is SiO2.

[0007] Among them, unit structure 1 can respond to blue light incident with a transmittance of 54%, generating a blue image in the near field and a blue holographic image in the far field; unit structure 2 uses the phase change material Sb2S3. When it is in the amorphous state, it can respond to green light incident with a transmittance of 60%, generating a green image in the near field and a green holographic image in the far field; when it is in the crystalline state, it can respond to red light incident with a transmittance of 48%, generating a red image in the near field and a red holographic image in the far field.

[0008] The operating wavelength of the blue light is λ. B =473nm; the working wavelength of green light is λ G =568nm; Red light operating wavelength is λ R =688nm.

[0009] Wherein, the near field is the surface of the metasurface, and the far field is beyond 1m from the metasurface.

[0010] Furthermore, according to the formula for the intensity of the emitted light after the incident polarized light passes through the nanounit structure, I0sin 2 (2θ) It can be seen that since the domain [0,π] of the rotation angle θ of the nanounit structure is non-monotonic, when linearly polarized light of intensity I0 passes through a rotation angle of θ, When using the four unit structures, the emitted light intensity is always I0sin 2 (2θ).

[0011] Based on the PB phase principle, when right-handed circularly polarized light passes through a rotation angle of θ, When the four nanounit structures are used, the emitted left-handed circularly polarized light will be accompanied by 2θ, 2θ+π、 The phase change amount, while ensuring continuous near-field intensity adjustment, obtains additional phase control degrees of freedom.

[0012] Based on the above principle, firstly, all n possible angle combinations are calculated according to the original image to be displayed in the near field. Then, another m possible angle combinations are calculated according to the original image to be displayed in the far field. Finally, combined with the GS phase retrieval algorithm, the angle arrangement of the nano-unit structure is determined according to the color pattern to be generated in the far field.

[0013] The method constructs a new mapping relationship between the emitted light intensity and the rotation angle of the nanounit structure, namely a one-to-four mapping relationship, which gives new phase design freedom while ensuring near-field color control, so as to realize far-field color Fourier phase holography.

[0014] The beneficial effects of this invention are as follows:

[0015] The metasurface designed in this invention can simultaneously realize two completely different imaging technologies: near-field color patterns and far-field color holography. Moreover, the two control methods can be controlled independently, and the resulting near-field patterns and far-field patterns are completely uncorrelated.

[0016] The metasurface unit structure designed in this invention has high transmittance for the three basic colors of light, which significantly improves the image quality.

[0017] The metasurface material designed in this invention has the characteristics of miniaturization, lightweight, high integration, and low manufacturing cost, and is suitable for the large-scale development of miniaturization and micro-miniaturization in the future.

[0018] The metasurface designed in this invention applies phase change materials to a color holographic metasurface, enabling tuning of the metasurface's operating wavelength. Attached Figure Description

[0019] Figure 1 This is a three-dimensional schematic diagram of a nano-unit structure array according to an embodiment of the present invention.

[0020] Figure 2 This is a top view of the nano-unit structure array according to an embodiment of the present invention.

[0021] Figure 3 This is a transmittance curve of nanounit structure 1 and unit structure 2 in an embodiment of the present invention.

[0022] Figure 4 This is a near-field color image of x-linearly polarized light incident in the blue light band according to an embodiment of the present invention.

[0023] Figure 5 This is a far-field color hologram of right-handed circularly polarized light incident in the blue light band according to an embodiment of the present invention.

[0024] Figure 6This is a near-field color image of x-linearly polarized light incident in the green band according to an embodiment of the present invention.

[0025] Figure 7 This is a far-field color hologram of right-handed circularly polarized light incident in the green light band according to an embodiment of the present invention.

[0026] Figure 8 This is a near-field color image of x-linearly polarized light incident in the red band according to an embodiment of the present invention.

[0027] Figure 9 This is a far-field color hologram of right-handed circularly polarized light incident in the red band according to an embodiment of the present invention. Detailed Implementation

[0028] The present invention will now be described in further detail with reference to the accompanying drawings and specific examples.

[0029] The specific implementation steps of this invention include:

[0030] Figure 1 and Figure 2 This is a schematic diagram of the unit structure of a tunable near-field and far-field color holographic metasurface. The metasurface is composed of a nano-unit array with a period of P = 460 nm. The substrate (1) is SiO2 with a thickness of H = 200 nm. The nano-unit array can be further divided into two nano-unit structures, namely unit structure 1 (2) and unit structure 2 (3). The working surface of each unit structure is rectangular. Unit structure 1 is composed of Si with a length of l1, a width of w1, a height of h = 400 nm, and a rotation angle of α. Unit structure 2 is composed of phase change material Sb2S3 with a length of l2, a width of w2, a height of h = 400 nm, and a rotation angle of β.

[0031] Narrow-band response wavelengths of 473nm and 568nm were selected for blue and green light, respectively. Simulations were then performed on the parameters of unit structures 1 and 2 for each wavelength, with the phase change material Sb₂S₃ in an amorphous state. The simulations optimized the conversion efficiency and response bandwidth of right-handed circularly polarized light transmitted perpendicularly at the incident blue and green light. The lengths l₁ and l₂, and widths w₁ and w₂ of the two unit structures were scanned under blue and green light, respectively. The requirement was that each unit structure responds only to a narrow bandwidth near the dominant wavelength, and that the cross-polarization (right-hand → left-hand) conversion efficiency is highest. After fixing the length l₂ and width w₂ of unit structure 2, the temperature was changed to bring the Sb₂S₃ to a crystalline state. At this point, unit structure 2 responds to a narrow bandwidth near the dominant wavelength of red light (688nm), and the cross-polarization (right-hand → left-hand) conversion efficiency is highest. After optimization, the length of unit structure 1 is l1 = 92nm and the width is w1 = 34nm; the length of unit structure 2 is l2 = 145nm and the width is w2 = 75nm. Figure 3The graph shows the polarization conversion efficiency of the optimized unit structure 1 and unit structure 2 as a function of wavelength. It can be seen that high-efficiency cross-polarization conversion is achieved within the corresponding narrow band of the center wavelength, while almost no response is generated to light waves outside the narrow band of the center wavelength.

[0032] When linearly polarized light of intensity I0 is incident on a unit cell with a rotation angle θ, the light intensity is modulated, and the modulation method conforms to the following formula:

[0033] I1=I0sin 2 (2θ)

[0034] Where θ is the angle between the major axis of the unit structure and the polarization direction of the x-ray polarized light, and I1 is the intensity of the emitted x-ray polarized light, it can be seen that arbitrary transmission intensity modulation can be achieved by changing the value of θ, thereby realizing near-field imaging, and the incident light passes through a rotation angle of θ. When using the four unit structures, the emitted light intensity is always I0sin 2 (2θ); When right-handed circularly polarized light passes through a rotation angle of θ, When the four nanounit structures are used, the emitted left-handed circularly polarized light will be accompanied by 2θ, 2θ+π、 The amount of phase change.

[0035] First, based on the blue target image and the green or red target image in the near-field color pattern, according to the formula I1 = I0sin 2 The non-monotonicity of the (2θ) function was used to calculate all the nano-unit array arrangements. Combined with the GS phase retrieval algorithm, additional phase degrees of freedom were utilized to design a phase-type holographic pattern for unit structure 1 that responds to blue light and unit structure 2 that responds to green or red light.

[0036] Therefore, when linearly polarized light with a wavelength of 473 nm is incident on a tunable near-field and far-field holographic multiplexed color holographic metasurface, the phase change material Sb₂S₃ is in an amorphous or crystalline state, and a surface similar to the one on the metasurface will form. Figure 4 The blue near-field image shown; when a tunable near-field and far-field holographic multiplexing color holographic metasurface with a wavelength of 473nm is used, the following will be observed at a position in the far field (more than 1m away from the metasurface): Figure 5 The blue Fourier hologram shown; when linearly polarized light with a wavelength of 568 nm is incident on a tunable near-field and far-field holographic multiplexing color holographic metasurface, the phase change material Sb₂S₃ is in an amorphous state, and a hologram like the one shown is formed on the surface of the metasurface. Figure 6The green near-field image shown; when a tunable near-field and far-field holographic multiplexing color holographic metasurface with a wavelength of 568 nm is used, the following will be observed at a position in the far field (more than 1 m away from the metasurface): Figure 7 The green Fourier hologram shown; when linearly polarized light with a wavelength of 688 nm is incident on a tunable near-field and far-field holographic multiplexing color holographic metasurface, the phase change material Sb₂S₃ is in a crystalline state, and a hologram like the one shown is formed on the surface of the metasurface. Figure 8 The red near-field image shown; when a tunable near-field and far-field holographic multiplexing color holographic metasurface with a wavelength of 688nm is used, the following will be observed at a position in the far field (more than 1m away from the metasurface): Figure 9 The red Fourier hologram shown.

[0037] In summary, this metasurface can simultaneously achieve near-field imaging and far-field color phase-type Fourier holography through two independent responses: intensity modulation and phase modulation. In this embodiment, the generated near-field and far-field images are also uncorrelated, so it is impossible to infer one holographic image from another. Therefore, this metasurface can be applied to fields such as color display, information encryption, and anti-counterfeiting.

Claims

1. A tunable near-field and far-field color holographic metasurface, characterized in that: The metasurface is composed of a periodically arranged array of nano-units with a lateral period and a longitudinal period of P. The array is etched onto the working surface of the substrate (1). The nano-unit array can be further divided into two nano-unit structures, namely unit structure 1 (2) and unit structure 2 (3). The working surface of each unit structure is rectangular. The length of unit structure 1 is l1, the width is w1, the height is h, and the material is Si. The length of unit structure 2 is l2, the width is w2, the height is h, and the material is Sb2S3. The substrate material is SiO2.

2. The tunable near-field and far-field color holographic metasurface according to claim 1, characterized in that: The period of the nano-unit array is P = 460 nm; the thickness of the substrate is H = 200 nm; the length of unit structure 1 is l1 = 92 nm, the width is w1 = 34 nm, the height is h = 400 nm, and the rotation angle is α; the length of unit structure 2 is l2 = 145 nm, the width is w2 = 75 nm, the height is h = 400 nm, and the rotation angle is β.

3. The tunable near-field and far-field color holographic metasurface according to claim 1, characterized in that: When blue light is incident, the phase change material Sb2S3 is in an amorphous or crystalline state, displaying a blue image in the near and far fields; when green light is incident, the phase change material Sb2S3 is in an amorphous state, displaying a green image in the near and far fields; when red light is incident, the phase change material Sb2S3 is in a crystalline state, displaying a red image in the near and far fields.

4. A tunable near-field and far-field color holographic metasurface according to claim 1, characterized in that: The blue light operating wavelength of the invention is λ. B =473nm; the working wavelength of green light is λ G =568nm; Red light operating wavelength is λ R =688nm.

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