Highly efficient on-chip excitation of orthogonal-polarized gap plasmons for a dense polarization multiplexing circuit

We propose an on-chip excitation structure for orthogonal-polarized gap plasmons in polarization multiplexing photonic integrated circuits. The structure consists of a Au nanostripe and tapered gap for refractive index matching to a nano-scale gap plasmonic waveguide; it was fabricated on the top surface of a dielectric-stripe-type waveguide. The excitation ratio from the dielectric-stripe-surface mode to the metallic-gap mode was estimated to be 0.79 using the finite-difference time-domain method for a 100-nm-wide, 100-nm-thick gap waveguide. We experimentally observed the gap mode plasmonic intensity distribution using scanning near-field optical microscopy and confirmed the conversion.