A Method for Realizing Dynamic Color Holography Based on On-Chip Metasurfaces of Hydrogel Waveguides

Abstract

This invention discloses an on-chip metasurface based on a hydrogel waveguide and a method for realizing dynamic color holography. The invention utilizes single-step electron beam exposure direct writing technology to directly print rectangular nanopores on the surface of the hydrogel waveguide layer without additional processing steps. Leveraging the optically tunable properties of hydrogel, the on-chip metasurface achieves switching between optical hiding and dynamic dual-channel holographic display through a simple exhalation process. Utilizing the dispersion of the hydrogel waveguide layer and the humidity-driven dynamic extraction efficiency, the integrated on-chip metasurface can achieve dynamic encryption of color holography and successfully displayed color holographic images under specific humidity conditions. This on-chip metasurface surpasses traditional static on-chip monochrome holographic display technology, featuring high integration and no zero-order diffraction interference. It can realize the integration of micro- and nano-optical devices and is expected to be applied to active optical information hiding / encryption, full-color holographic display technology, and next-generation wearable AR devices.

Description

Technical Field

[0001] This invention relates to the fields of micro-nano fabrication technology and integrated photonics and optical display technology, specifically to an on-chip metasurface based on hydrogel waveguide and a method for realizing dynamic color holography. Background Technology

[0002] Metasurfaces are two-dimensional artificial materials that have shown great potential in the field of nanophotonics due to their superior light control capabilities, providing a more integrated and powerful alternative to traditional optical devices. Recently, researchers have integrated metasurfaces with waveguides to achieve a variety of complex functions, including free manipulation of guided waves, arbitrary beam deflection, holographic displays, and nanoprinting.

[0003] However, while traditional waveguide materials such as silicon dioxide or silicon nitride possess excellent stability properties, this also presents significant challenges for the dynamic control of devices. Several tunable schemes for free-space metasurface optics have been proposed, such as those using liquid crystals, chemically responsive materials, and phase-change materials. However, finding an effective and practical way to actively control guided waves remains a major challenge for on-chip integrated metasurface devices. With the advancement of thin-film optics research, hydrogel materials, due to their ability to dynamically expand / contract by absorbing / releasing water molecules, have recently emerged as an alternative material for fabricating active optical devices. This unique scalable property has been used to fabricate hydrogel metasurfaces in free space, exhibiting excellent dynamic optical display capabilities. Furthermore, due to their low refractive index and transparent optical properties, hydrogels also hold great potential for on-chip active optical operation as dynamically scalable waveguides with embedded metastructures, and are expected to drive the further development of highly integrated devices in optical displays. The application prospects of hydrogels in on-chip active optical operation are broad, but to fully realize their potential, many challenges need to be overcome, requiring further optimization of material properties, improvement of processing accuracy, enhancement of environmental adaptability, and functional integration capabilities. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides an on-chip metasurface based on hydrogel waveguides and a method for realizing dynamic color holography.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] In a first aspect, the present invention provides an on-chip metasurface based on a hydrogel waveguide. Using a single-step electron beam direct writing technique, rectangular nanopores of uniform size are directly obtained on the surface of the hydrogel waveguide layer, wherein the nanopores can couple guided waves into free space. Furthermore, as the hydrogel waveguide layer absorbs water molecules and its thickness increases, its guided wave extraction efficiency for different wavelength components changes.

[0007] To achieve the above functions and objectives, the hydrogel-on-sheet metasurface in this invention has a two-layer structure; the two-layer structure is a metal layer and a hydrogel waveguide layer; the metal substrate is any one of silver, zinc, aluminum or copper (preferably silver); the hydrogel waveguide layer is any one of polyvinyl alcohol, polyethylene glycol, acrylate and its derivatives (preferably polyvinyl alcohol); the dynamic optical response of the on-sheet metasurface depends on the thickness variation of the hydrogel waveguide layer.

[0008] Specifically, the metal substrate is 100-120nm thick to ensure total reflection of the guided wave at the lower interface, and the initial thickness of the hydrogel waveguide layer is 400-430nm to match the low extraction efficiency of green light (532nm) and red light (635nm) under indoor humidity, thus achieving the hiding of the holographic image.

[0009] Furthermore, the metasurface on the hydrogel sheet designed in this invention exhibits a gradually changing efficiency in extracting green light as humidity increases, showing a higher extraction efficiency when the relative humidity increases to 90%, and presenting the designed holographic image in the far field. For example, a guided wave (green light) with a wavelength of 532nm has a low extraction efficiency at an indoor relative humidity of 50%, and cannot present a holographic image in the far field; when the humidity increases to 90%, the waveguide thickness increases, resulting in an increased extraction efficiency of green light, allowing the target holographic image to be captured in free space using a screen or mobile phone camera, achieving dynamic display of the holographic image.

[0010] Secondly, this invention provides a method for achieving dynamic image switching and color holographic display. By manipulating the extraction efficiency of two different wavelength channels (green light and red light) respectively, holographic images of different colors / patterns can be encrypted and displayed alternately under their respective extraction humidity conditions, thereby achieving dynamic switching between dual-channel images. Utilizing the dispersion of the waveguide hydrogel and the correlation between extraction efficiency and relative humidity, the size and position of two pattern elements in the holographic image can be precisely controlled, enabling the simultaneous encryption of multi-color holographic images onto the on-chip hydrogel metasurface under specific humidity conditions, thereby dynamically displaying holographic images of mixed colors.

[0011] Thirdly, this invention provides an application of on-chip metasurfaces based on hydrogel waveguides. Utilizing methods for achieving dynamic image switching and color holographic display, highly integrated on-chip metasurfaces with hydrogel waveguide layers, as described above, have applications in numerous areas, including but not limited to gas sensing, active optical information encryption, and wearable AR displays, or applications in drug delivery, tissue engineering, biomedical imaging, and photomedicine.

[0012] In summary, the core technical points of this invention are as follows:

[0013] 1. Novel Coupling Mechanism: This invention achieves the coupling of guided waves to free space by etching nanopores on the surface of the hydrogel waveguide layer. This is a novel coupling mechanism that, compared with traditional metasurface designs, increases the possibility of dynamic control while maintaining high integration, and has significant innovation.

[0014] 2. Application of dynamic materials: By utilizing the absorption characteristics of water molecules by hydrogels, dynamic optical responses dependent on ambient humidity have been achieved. The application of this material provides a new approach to realizing dynamic holographic images and is an important supplement to traditional fixed optical elements.

[0015] 3. Multicolor Dynamic Holographic Technology: This technology enables dynamic switching of dual-channel images and color holographic display, representing a significant innovation in the field of holographic display. By precisely controlling the extraction efficiency of different wavelength channels, it achieves dynamic adjustment of the colors and patterns of the holographic image, a major breakthrough over traditional holographic technology.

[0016] 4. Precise control and design: By precisely controlling the dynamic extraction efficiency related to the dispersion and thickness of the hydrogel waveguide layer, the precise design of the pattern position and size in the holographic image is achieved. This control capability is technically innovative and makes it possible to achieve complex holographic display effects.

[0017] In summary, this invention demonstrates high technological innovation across multiple fields, including materials science, optical engineering, and holographic technology. It not only proposes new material application schemes but also develops novel optical element designs and control strategies, paving new paths for the development of dynamic optical elements and holographic display technology. These innovations are expected to drive technological progress in related fields, better serving future areas such as optical communication, information processing, and display technologies.

[0018] The technical principles and research process of this invention are as follows:

[0019] Unlike traditional metasurfaces that interact with free-space light, on-chip metasurfaces couple guided waves from the end face to a hydrogel waveguide layer, extracting them into free space through nanopores and imparting specific phase modulation. For each nanopore, the corresponding extracted phase consists of the cumulative propagating phase βP in the waveguide and the detour phase Δφ determined by its position in each period, where β represents the propagation constant of the guided mode and P is the period of the superatom along the waveguide propagation direction. For the design of the color hologram, different color components are decomposed and rescaled to reconstruct an image of the corresponding wavelength, and then phase holograms are generated using the Gerchberg-Saxton (GS) algorithm. A pre-compensation algorithm is also used to move pixels in the hologram to ensure seamless overlap between the two components of the image. Specifically, according to the diffraction formula, the exit angle of the extracted wave can be expressed as:

[0020] θ d =sin -1 (n PVA +mλ0 / P) (1)

[0021] Where m is the order of the diffracted beam, n PVA The refractive index of the hydrogel is ~1.51. Considering the incident light wavelengths of 532 nm and 635 nm, the zero-order diffracted beam is confined in the waveguide, and the -1st order diffraction angles are 75.5° and 59.9°, respectively. Therefore, the deflection angle of the color hologram can be set to θ. d =π / 2–sin -1 (0.4) = 66.4°. At this point, the pixel movement ΔN can be expressed as:

[0022] ΔN=–NPβsinθ (2)

[0023] Where θ = π / 2 – θ d N is a matrix related to the size of the metasurface. Finally, by superimposing the two sets of holographic calculation results, the target phase of the desired color hologram can be obtained.

[0024] The advantages and beneficial effects of this invention are as follows:

[0025] 1. The present invention proposes an on-chip metasurface based on hydrogel waveguide. Using hydrogel as a waveguide, it overcomes the difficulty of achieving dynamic control of traditional waveguide materials while maintaining high integration. It realizes arbitrary phase manipulation of externally coupled light waves and finally obtains dynamically switched holographic images.

[0026] 2. In addition, by using electron beam direct writing technology, nanopores with specific phase arrangements can be printed on the surface of hydrogel through a single-step exposure process. Compared with traditional on-chip metasurfaces, this eliminates the need for complex subsequent processing procedures and improves the success rate of processing.

[0027] 3. Utilizing the excellent water absorption and swelling properties of hydrogels, the waveguide thickness can be increased simply by blowing air in the experiment, achieving dynamic selectivity in the extraction efficiency of guided waves of different wavelengths. This provides a new paradigm for the dynamic control of on-chip metasurfaces.

[0028] 4. By calculating the dispersion relationship of different wavelength components of the guided wave in the hydrogel waveguide layer, a pre-compensation algorithm is used to reconstruct the target color image, so that different color components are accurately superimposed in the far field under specific humidity conditions, and a three-color dynamic holographic image is realized in two wavelength channels.

[0029] 5. The design scheme of on-chip propagation is applied, and the designed holographic image is free from zero-order diffraction interference. This on-chip metasurface based on hydrogel waveguide can realize the integration of micro and nano optical devices and is expected to be applied to active optical information hiding / encryption, full-color holographic display technology and next-generation wearable AR devices. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the on-chip metasurface based on hydrogel waveguide and the realization of dynamic color holography in this invention;

[0031] Figure 2 This is a schematic diagram of on-chip metasurface phase modulation based on hydrogel waveguide in this invention;

[0032] Figure 3 This is a simulation diagram showing the variation of the far-field emission angle of different wavelength guided waves with thickness in Embodiment 1 of the present invention;

[0033] Figure 4 This is a simulation diagram showing the variation of guided wave extraction intensity with thickness at different wavelengths in Embodiment 1 of the present invention;

[0034] Figure 5 This is a simulation effect diagram of the electric field extracted from the 532nm and 635nm wavelength channels in Embodiment 1 of the present invention;

[0035] Figure 6 This is a simulation effect diagram of the intensity change of the 532nm and 635nm wavelength channels when the thickness changes continuously in Embodiment 1 of the present invention;

[0036] Figure 7 This is a schematic diagram of the on-chip metasurface fabrication process and humidity response based on hydrogel waveguides in this invention;

[0037] Figure 8 This is an experimental result diagram of the atomic force microscopy test of the sample morphology in Embodiment 2 of the present invention;

[0038] Figure 9 This is an experimental effect diagram of the dynamic change of the 532nm channel monochromatic holography in Embodiment 2 of the present invention;

[0039] Figure 10 These are simulation and experimental results of the holographic dynamic switching between the 532nm and 635nm channels in Embodiment 2 of the present invention.

[0040] Figure 11 These are simulation and experimental results of the color holographic dynamic display in Embodiment 2 of the present invention. Detailed Implementation

[0041] To more clearly illustrate the embodiments of the present invention and its effects, the present invention will be further described below with reference to specific embodiments and accompanying drawings.

[0042] Example 1

[0043] This embodiment is a simulation verification of an on-chip metasurface based on hydrogel. Figure 1 To demonstrate the functionality of the designed metasurface on the hydrogel sheet, a two-layer hydrogel platform was constructed, consisting of a silver substrate and a top hydrogel waveguide layer. The refractive index of the hydrogel is approximately 1.51. Under indoor conditions, the extraction intensity of incident light at the nanopores on the sheet is low, and holographic information is hidden. By increasing the thickness of the hydrogel waveguide layer through air blowing, the extraction intensity of the guided waves at 532 nm and 635 nm gradually increases, revealing an encrypted target image in the far field. Its phase modulation capability of the guided waves is as follows: Figure 2 As shown, for each nanopore, the corresponding extraction phase consists of the cumulative propagation phase βP in the waveguide and the detour phase Δφ determined by its position in each period, where β represents the propagation constant of the guided mode and P is the period of the superatom along the waveguide propagation direction. To demonstrate the wavelength selectivity brought about by hydrogels of different thicknesses, a nanopore array with a period of 400 nm was used in the simulation. By changing the thickness of the waveguide (570 nm, 603 nm, 645 nm, and 708 nm, respectively), the far-field emission angle spectra at different wavelengths were observed. Based on the relative magnitude of the extraction intensity at the selected target wavelengths (532 nm and 635 nm), the hydrogel was encoded using binary "0" and "1" characters, as shown below. Figure 3 As shown.

[0044] By extracting wavelength and far-field emission intensity information, the wavelength selectivity of hydrogels of different thicknesses can be more intuitively represented, such as... Figure 4 As shown, the far-field emission intensity values ​​at the target wavelength are labeled. The significant difference in extraction intensity between the two channels further verifies the experimental feasibility of the designed metasurface dynamic control function. Meanwhile, Figure 5 The electric field distributions of four different thicknesses of hydrogel extracted at the target wavelength were recorded (top: 532 nm; bottom: 635 nm). The dashed lines show the electric field distribution at high extraction intensities in the two channels, clearly demonstrating a distinct wavefront and high electric field intensity distribution. Furthermore, Figure 6 The intensity fluctuations of the extracted light from the two channels were recorded when the thickness of the hydrogel changed continuously, providing a more intuitive prediction of the tunable optical response of the hydrogel waveguide layer under humidity changes.

[0045] Example 2

[0046] This embodiment is a solution for implementing dynamic holographic switching and color holographic display functions. For example... Figure 7As shown, a single-step direct printing technique based on the shrinkage mechanism of polyvinyl alcohol (PVA) induced by electron beam irradiation was employed to fabricate a hydrogel-based metasurface. High-energy electron radiation crosslinks the ends of the PVA molecular chains, causing their thickness to shrink, thereby forming a nanoporous structure embedded in the hydrogel waveguide layer at the nanoscale. Furthermore, due to the humidity sensitivity of the PVA metasurface, its volume / thickness can increase / decrease. During a simple exhalation process, the thickness of the hydrogel waveguide layer gradually increases, fundamentally determining the variation in the optical extraction efficiency of its local nanostructures under different humidity conditions.

[0047] Figure 8 Atomic force microscopy (AFM) images of the prepared hydrogel metasurface. In the experiment, the metasurface period was set to 1000 nm, the width and length of the rectangular nanopores were 400 nm and 800 nm, respectively, and the pixel count was 500 × 500. The microscopic images clearly show the outline of the nanopores with a tortuous phase arrangement and a depth of 10–20 nm.

[0048] To preliminarily verify the holographic switching capability, a dynamic on / off monochrome holographic image display with a wavelength channel of 532nm was demonstrated in the experiment, such as... Figure 9 As shown, the holographic image "boat" is hidden under normal indoor relative humidity of approximately 50%. However, through a simple exhalation process, the optical extraction intensity of the local nanopores increases, thus successfully extracting the hidden image from the on-chip metasurface. To further achieve dynamic switching between the dual-wavelength image channels, the extraction efficiency of the two wavelength channels (532nm and 635nm) was manipulated independently in the experiment. By carefully encoding the phase distribution of the dual-channel image into the metasurface pattern, the holographic image dynamically changes from a red "trophy" to a green "medal" during exhalation and can be captured by a mobile phone camera, as shown. Figure 10 As shown, the difference in extraction intensity at 532nm and 635nm leads to different emission angles, thus allowing the encoding of images of different colors to be displayed alternately within the same field of view. Furthermore, by precisely designing the position and size of the holographic image, dynamic display of a three-color holographic image under dual-wavelength channels can be achieved in experiments, such as... Figure 11 As shown, the holographic image "medal" was decomposed into two patterns through calculation, and its size and deflection were pre-designed. In the experiment, the colored "medal" image was successfully reconstructed at a position of 66.4° in space, presenting a three-color pattern display. At a relative humidity of 50%, the extraction intensity of both channels was weak. As the humidity increased, different components of the target image appeared alternately, and under specific humidity conditions, overlapping color holograms were displayed, realizing the dynamic display of color holographic images.

[0049] Embodiments 1-2 of this invention demonstrate through simulation and experiments that the designed on-chip metasurface can achieve free switching of holographic images, overcoming the limitations of static control of traditional on-chip metasurfaces. While maintaining high integration, it simplifies the on-chip metasurface processing technology and endows it with dynamic control capabilities.

[0050] In summary, the hydrogel-based on-chip metasurface proposed in this invention exhibits good robustness and excellent humidity response. The hydrogel-based on-chip metasurface design offers significant flexibility in device integration and manufacturing convenience, providing a novel design approach for nanophotonic devices used in active color holographic displays. Furthermore, the simpler fabrication process facilitates operation by skilled technicians, increasing the success rate of device fabrication and offering a new and feasible strategy for the future practical application of on-chip micro-nano optics in manufacturing.

Claims

1. An on-chip metasurface based on a hydrogel waveguide, characterized in that: The on-chip metasurface has a two-layer structure, including a metal substrate for providing total internal reflection and a hydrogel waveguide layer formed on the metal substrate using a water-absorbing and expanding material; the surface of the hydrogel waveguide layer is provided with an array of several nanopores, and the unit nanopores in the array can couple guided waves into free space; after the hydrogel waveguide absorbs water, the thickness change causes the guided wave extraction efficiency of different wavelength components to change.

2. The on-chip metasurface based on hydrogel waveguide according to claim 1, characterized in that: The metal substrate is any one of silver, zinc, aluminum or copper; the hydrogel waveguide layer is any one of polyvinyl alcohol, polyethylene glycol, acrylate and its derivatives; the unit nanopores are all rectangular structures; all unit nanopores in the array are the same size.

3. The on-chip metasurface based on hydrogel waveguide according to claim 2, characterized in that: The metal substrate has a thickness of 100-120nm to ensure total reflection of the guided wave at the lower interface; the initial thickness of the hydrogel waveguide layer is 400-430nm to match the low extraction efficiency of green light at 532nm and red light at 635nm under indoor humidity, thereby achieving the concealment of the holographic image.

4. The on-chip metasurface based on hydrogel waveguide according to claim 3, characterized in that: The arrangement of the unit nanopores in the array is subjected to detour phase encoding; the detour phase encoding information is a holographic image; the unit nanopores carry guided waves of different phases, which are extracted into free space and present a holographic image in the far field.

5. The on-chip metasurface based on hydrogel waveguide according to claim 4, characterized in that: The array of several nanopores was obtained on the surface of the hydrogel waveguide layer using a single-step electron beam exposure direct writing technique.

6. A method for realizing dynamic color holography on an on-chip metasurface based on hydrogel waveguides, characterized in that: The method, namely the dynamic image switching and color holographic display method, adopts the on-chip metasurface as described in any one of claims 1 to 5, and utilizes the dynamic extraction efficiency related to the dispersion and thickness of the hydrogel waveguide layer to design the position and size of different patterns in the holographic image, so as to realize dynamic pattern switching under two wavelength channels and three-color pattern display based on color superposition.

7. The method according to claim 6, characterized in that: The method for dynamic color holography is specifically as follows: By manipulating the extraction efficiency of two different wavelength channels, the holographic image encryption function of different colors / patterns can be achieved, and they can be displayed alternately under their respective extraction efficiencies, thereby realizing dynamic switching between dual-channel images; By utilizing the dispersion of waveguide hydrogels and the correlation between extraction efficiency and relative humidity, the size and position of two pattern elements in a holographic image can be controlled to simultaneously densify multicolor holographic images onto an on-chip hydrogel metasurface under specific humidity conditions, thereby dynamically displaying a holographic image with mixed colors.

8. The method according to claim 7, characterized in that: Holographic images obtained through dynamic color holography are not affected by zero-order diffraction, and changes in the humidity of the hydrogel waveguide layer can be achieved by blowing air.

9. An application of an on-chip metasurface based on a hydrogel waveguide, characterized in that: Applications of highly integrated on-chip metasurfaces of hydrogel waveguide layers as described in any one of claims 1-5 in gas sensing, active optical information encryption, and wearable AR displays.

10. An application of an on-chip metasurface based on a hydrogel waveguide, characterized in that: Applications of highly integrated on-chip metasurfaces of hydrogel waveguide layers as described in any one of claims 1-5 in drug delivery, tissue engineering, biomedical imaging, and photomedicine.

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