Directional-multiplexing holography by on-chip metasurface

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.