Graded index surface-plasmon-polariton devices for subwavelength light management

We propose and numerically demonstrate a parabolic graded index (GRIN) waveguide for surface-plasmon polaritons (SPPs) allowing subwavelength self-imaging and focusing of SPP waves bound at a metal-dielectric interface. The GRIN structure is applied to design ultracompact multimode-interference devices for SPPs. In particular, a 4 x 4 cross-connect device and an efficient waveguide launcher for dielectric-loaded SPP waveguides are demonstrated by full-wave numerical simulations.