Zero-Spacing Photonic Channels via Perturbation Engineering.

Optical waveguides conventionally rely on wavelength-scale low-index spacing or cladding to isolate neighboring channels, fundamentally limiting photonic integration density. Here, we show that such spatial separation is not a prerequisite for independent waveguiding. By introducing deep-subwavelength metal perturbations (∼λ_{0}/25) onto the surfaces of a dielectric slab, we realize multiple dielectric guiding channels with zero physical spacing. This behavior arises from perturbation-engineered suppression of supermode index splitting, achieved by selectively modifying the symmetric mode while leaving the antisymmetric mode nearly unchanged. We validate this mechanism through full-wave simulations and microwave experiments in both straight and bent geometries. Extending this concept to the optical regime, we design silicon-on-insulator waveguides incorporating hybrid silver-silicon perturbations, achieving ultralong coupling lengths (L_{c}≥1000λ_{0}) across a broad bandwidth from 1495 to 1565 nm. Owing to the minimal metal volume, propagation losses remain significantly lower than those of conventional hybrid plasmonic systems. This metal-dielectric hybrid waveguide system enables ultradense photonic integration and provides new insights into controlling optical coupling.