Extreme field enhancement in nano-gap plasmonic cavity via 90% efficient coupling with silicon waveguide.

We propose a novel design for a sub-5-nm-gap plasmonic cavity to couple it efficiently with an integrated low loss silicon waveguide. We numerically obtain over 90% efficient coupling between a nano-gap plasmonic cavity with a modal volume of less than 10^-7λ³ and a conventional silicon-on-insulator (SOI) waveguide by utilizing the anti-symmetric second-order resonance mode of the cavity and engineering its geometry to reduce the modal size to less than 5 nm. The electromagnetic field efficiently coupled to the small cavity, leading to extreme enhancement of the field intensity. For a 2-nm-gap cavity, the intensity enhancement was calculated to be more than 100,000,000 compared to that of light in an SOI waveguide.