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摘要:
片上超表面是将超表面引入到集成光波导上以实现对导波的任意调制，它为导波与自由空间光波之间的转换提供了一个方便且通用的平台。尽管之前在片上全息方面已经做出了一些探索，但在扩展编码自由度和多路复用方面仍需突破。本文提出并实验验证了一种基于片上超表面的多路复用全息显示器件。通过迂回相位和几何相位的融合,使片上超表面将导波以圆偏光的形式耦合到自由空间中，以打破此前片上超表面沿传播方向上相位简并的局限性，进一步扩展了编码自由度。同时使用模拟退火相位优化算法和多路复用技术，实现了可独立编码的四通道远场全息显示复用。本文的设计方法以片上超表面的多功能集成，为具有高信息存储容量的集成光通信提供了一种新的途径。
Abstract:The on-chip metasurface is introduced into integrated optical waveguides to achieve arbitrary modulation of guided waves, which provides a convenient and versatile platform for the conversion between guided waves and free-space functions. Despite previous explorations in on-chip holography demonstration, it still faces critical challenges to expand the encoding freedom and multiplexing. Here, we propose and experimentally demonstrate a quad-fold multiplexed holographic display optics device based on an on-chip metasurface. By mixing the detour phase and Pancharatnam-Berry (PB) phase, the on-chip metasurface couples the guided waves into free space in circular polarization, destroying the phase degeneracy that exists in the wavevector directions with only the detour phase. Moreover, by utilizing simulated annealing phase optimization algorithm and multiplexing, we achieved a quad-fold multiplexed far-field holographic display with independent encoding capability. The proposed method in this paper opens up a new prospect for multifunctional integration of on-chip metasurfaces and provides an alternative approach for integrated optical communication with high information storage capacity.
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Overview: Photonic integrated circuits (PIC) serve as an essential and promising candidate to eventually replace electronic circuits for the next-generation information processing. However, traditional PIC devices based on optical waveguides are usually bulky and lack full control at the subwavelength scale to achieve arbitrary wavefront-shaping functionalities. Recently, the invention of on-chip metasurface promotes the connection between guided and free-space optics and realizes the arbitrary conversion of guided waves and free-space light. As a new type of on-chip nanophotonic device, the introduction of metasurface onto the optical waveguide has made significant progress and exhibited multi-functional conversion from the guided waves to free-space, including directional beam-steering emitters, mode-conversion, on-chip lensing, optical router, and on-chip holography, etc. These on-chip nanophotonics devices provide new avenues for photonic chip-scale devices and miniature on-chip systems. For instance, meta-holography is an emerging and universal strategy based on engineered nanoantennas array to construct an optical-field image. However, on-chip far-field holographs are limited for realizing multiplexing for multiple directions due to a lack the arbitrary-encoding capability because their detour phases are complementarily related when the source propagates and excites the on-chip array from either positive or negative direction. Here, we propose and experimentally demonstrate a quad-fold multiplexed far-filed holographic display optics device based on an on-chip metasurface. This optics device is composed of silicon nanopillar arrays on top of a planar waveguide of Si3N4, in which a relatively thick layer of silica serves as the bottom cladding substrate. By mixing the detour phase and Pancharatnam-Berry (PB) phase, the on-chip metasurface could couple the guided waves into free space in circular polarization. The phase degeneracy in the positive and negative directions could be decoupled by selecting the desired circular polarization. Subsequently, utilizing a simulated annealing phase optimization algorithm to optimize the phase required by holograms and the multiplexing technology of on-chip directional, we achieved a quad-fold multiplexed far-field holographic display with independent encoding capability. Eventually, to verify the on-chip quad-fold multiplexed holography performance, we fabricated an on-chip metasurface sample by the conventional electron-beam lithography technique and the reactive ion etching processing. Through end-fire coupling from the laser source at λ = 650 nm into the on-chip metasurface sample along ±x/±y - directions, the far-field holographic images of the four letters (“A”, “B”, “C”, and “D”) multiplexing are successfully observed at their corresponding areas. The method proposed here opens up new prospects for the multifunctional integration of on-chip metasurfaces and provides an alternative approach for integrated optical communication with high information storage capacity.
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图 1 片上四通道复用全息原理示意图。当光源分别沿±x, ±y方向边缘入射时，集成在波导上的片上超表面分别在远场显示四幅不同的全息图像(“A/B/C/D”)
Figure 1. Schematic diagram of the on-chip four-channel multiplexed holography. Four diverse holographic images (“A/B/C/D”) in the far-field could be displayed by the on-chip metasurface integrated on the waveguide, when the laser source is end-fire coupled into the waveguide along the ±x and ±y directions, respectively
图 2 用于控制和提取导波的片上超表面工作机理。(a) 构成片上超表面的单元结构示意图；(b) 通过结合迂回相位和几何相位来提取导波的原理；(c) 离散迂回相位与单元内Si纳米柱位置对应关系；(d) 离散几何相位调控量与Si纳米柱方向角的关系
Figure 2. On-chip metasurface mechanism for controlling the phase of extracted guided waves. (a) Schematic of the unit cell to compose the on-chip metasurface; (b) The principle of guided wave extraction by combining the detour phase and geometric phase; (c) The correspondence between discrete detour phases and the positions of meta-atoms in the unit cell; (d) The relationship between discrete geometric phase delay and the orientation angle of the meta-atoms
图 3 片上四通道复用全息相位优化流程图
Figure 3. Phase optimization flowchart for the on-chip quad-fold multiplexed holography
图 4 片上四重复用全息超表面样品制作工艺流程图
Figure 4. The sample fabrication flow chart of the on-chip quad-fold multiplexing holographic metasurface
图 5 片上四重全息实验验证。(a) 制备样品的整体SEM图像和局部放大SEM图像；(b) 测量片上四重全息的光学实验装置图；(c) 导波分别沿±x/±y方向入射时的目标全息图像及与目标图像对应的区域测量的全息图像
Figure 5. Experimental verification for the on-chip quad-fold holography. (a) The overall SEM image and zoom-in SEM image of the fabricated sample; (b) Optical experimental setup sketch for measuring on-chip quad-fold holography; (c) Target and experimental holograms measured in the area corresponding to the target for guided waves incidence from the ±x/±y direction, respectively
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