High-throughput diffraction-assisted surface-plasmon-polariton coupling by a super-wavelength slit.

We propose a novel SPP coupling scheme capable of high SPP throughput and high SPP coupling efficiency based on a slit of width greater than the wavelength, immersed in a uniform dielectric. The dispersive properties of the slit are engineered such that the slit sustains a low-loss higher-order waveguide mode just above cutoff, which is shown to be amenable to wavevector matching to the SPP mode at the slit exit. The SPP throughput and SPP coupling efficiency are quantified by numerical simulations of visible light propagation through the slit for varying width and dielectric refractive index. An optimal SPP coupling configuration satisfying wavevector matching is shown to yield an order-of-magnitude greater SPP throughput than a comparable slit of sub-wavelength width and a peak SPP coupling efficiency ≃ 68%. To our knowledge, this is the first investigation of coupling between higher-order waveguide modes in slits of super-wavelength width and SPP modes.