Flexible Fishbone Waveguides Made of Single-Crystal Silicon for Terahertz-Wave Path Switching

Abstract

Integrated systems for signal control are essential for advancing terahertz applications including 6G communication systems, medical diagnosis, and security checks. Photonic integrated circuits (PICs) enable broadband and high efficiency control in the terahertz range. Optical switches are key components for signal routing. Among various approaches, direct-path-switching-type optical switches based on micro-mechanical systems (MEMS) offer superior modulation depth, bandwidth, and optical efficiency. However, conventional optical switches using ribbon waveguides are too large to be realized in the terahertz range, unlike in the infrared region. In this study, we propose a “fishbone” waveguide consisting of a subwavelength structure and a beam to achieve both high mechanical flexibility and sufficient optical confinement. We clarified the relationship between the waveguide parameters and its mechanical and optical characteristics. A $1\times 2$ optical switch incorporating a 10-mm-long fishbone waveguide was fabricated using micromachining processes. This footprint is equivalent to 1/88 that of a previously reported compact optical switch. Experimental results demonstrated a $475~\mu $ m displacement at a driving voltage of 65 V, and optical switching with an average crosstalk of −10 dB across the 270–320 GHz band. To the best of our knowledge, this is the first demonstration of a direct-path-switching-type optical switch in the terahertz range. This study contributes to further miniaturization and low-voltage operation of MEMS optical switches and enhances functionality in terahertz PICs. [2026-0061]