Unidirectional coupler for surface plasmon polariton using total external reflection of high index material

The surface plasmon polaritons (SPPs) have been researched intensively, the very low coupling efficiency of the SPPs brings about difficulty in practical experiment. We proposed a unidirectional coupler for the SPPs using total external reflection. If the high-index dielectric is located on the left side of the slit, the SPPs toward the left side cannot propagate due to the cut-off property, and they should be totally reflected. By properly designing the structure parameter for constructive interference between the reflected Spp mode and the right-ward propagating SPP mode can happen, resulting in enhancement of the coupling efficiency of the SPPs.