Chip-scale hollow photonic coupler cum mode transformer based on 2-D taper

Abstract Efficient, low-loss coupling between standard lensed fibers and deeply subwavelength on-chip waveguides is essential for high-density photonic and plasmonic integration, yet remains a significant challenge due to severe mode-mismatch. We present a hollow taper coupler that converts the fiber mode into a deeply confined dielectric spot. The device comprises a silicon linear 2-D hollow taper, whose width narrows from several microns to the submicron region. This configuration forces the optical field to concentrate at the silicon–air interface, creating a hybrid plasmonic mode at the output end. With its compact footprint (≤ 20 μm), low loss, and CMOS-compatible fabrication, the hollow taper coupler provides a streamlined, high-efficiency interface for seamless fiber-to-chip integration. The tapering of the structure is along the thickness and width, which makes dual axis confinement of light in silicon at the input end, indicating better control. Coupling efficiency using s-parameter (S 21 ) indicates better transfer of light from the input end to the output. Due to the gradual scaling of the taper, field enhancement occurs at the output end, enhancing light confinement in nanoscale devices and creating a hybrid plasmonic mode.