Design of a Metal-Dielectric Sub-Wavelength Slit Structure for High Efficiency Coupling of Surface Plasmon Polaritons

We propose a high efficiency coupling scheme of surface plasmon polaritons (SPP) in a metal-dielectric slit structure. The design includes a narrrow silver-slit structure, which is immersed in a dielectric with refractive index 1.5. We map the dependence of SPP coupling efficiency on the dielectric layer thickness (d). By varying d the dispersion behavior of the SPP mode is tuned enabling minimized wavevector mismatch between the light exiting the slit and the SPP mode. A dielectric layer of thickness in the range 50 nm < d < 150 nm yields coupling efficiencies of approximately 80%, representing a nearly four-fold enhancement relative to the coupling efficiency without a dielectric layer.