High-Aspect-Ratio Silicon Metasurfaces: Design, Fabrication, and Characterization

Unrestricted manipulations on terahertz (THz) waves are highly desired in integration-optics, but THz devices based on conventional materials are usually bulky in size. Although all-silicon metasurfaces have exhibited great capabilities in manipulating THz waves, most of them are less efficient and have limited functionalities. In this work, we first design a silicon meta-atom structure consisting of a high-aspect-ratio (AR) micro-pillar that exhibits nearly perfect transmission and large transmission phase of THz wave, and systemically study how the fabrication quality (e.g., steepness of the sidewall and the vertical thickness distribution) may influence the final performance of a functional metasurface constructed with such meta-atoms. After experimentally examining how two working phases in the deep-reactive-ion-etch technology (i.e., the etch and passivation phases) influence the quality of the fabricated meta-devices, we develop an optimized Bosch process to realize high-AR (~20:1) all-silicon metasurfaces by balancing two working phases. We finally design/fabricate a high-AR silicon metasurface and experimentally demonstrate that it behaves as a high-efficiency half-wave-plate for THz waves in transmission geometry. Our results pave the avenue to realize ultra-compact THz meta-devices with high performance in transmission geometry, which is highly desirable for THz applications.