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

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-TE₀ 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.