Low‐Loss Routing of Strongly Confined Light in Ultra‐Compact Silicon Slot‐Waveguide Bends

ABSTRACT Photonic slot waveguides confine light in subwavelength low‐index gaps and are useful for compact light–matter interaction devices, but tight bending strongly distorts the slot‐confined mode and limits dense on‐chip routing. Conventional routing strategies typically avoid strong curvature by using large bend radii or strip–slot conversion, thereby increasing the circuit footprint. Here, we demonstrate an ultra‐compact slot waveguide bend in which independent rail‐width optimization gives rise to asymmetric mode evolution within the bending region, enabling efficient output‐mode recovery under extreme curvature. The optimized geometry develops a rail‐width asymmetry, leading to field redistribution toward the wider outer rail followed by efficient coupling to the output slot mode. Implemented on a 220‐nm‐thick silicon‐on‐insulator platform, the inverse‐designed 90° bend has a bending radius of 1.5 μm and a 100 nm slot gap. Three‐dimensional finite‐difference time‐domain simulations predict quasi‐TE 0 modal transmission exceeding 95% across the C‐band. Experimental measurements further confirm broadband transmission in routing structures containing up to 80 cascaded ultra‐tight 90° bends. From the accumulated routing loss, we extract an effective excess loss of approximately 0.29 dB per 90° turn. These results demonstrate asymmetric mode evolution as a practical design strategy for ultra‐compact slot‐waveguide routing in high‐density photonic integrated circuits.