Deep-subwavelength semiconductor nanowire surface plasmon polariton couplers

The increased importance of plasmonic devices has prompted a sizable research activity directed toward the development of ultracompact and high-performance couplers. Here, we present a novel scheme for efficient, highly localized, and directional sourcing of surface plasmon polaritons (SPPs) that relies on the excitation of leaky mode optical resonances supported by high-refractive index, semiconductor nanowires. High coupling efficiencies are demonstrated via finite difference frequency domain simulations and experimentally by leakage radiation microscopy. This efficiency is quantified by means of a coupling cross section, the magnitude of which can exceed twice the geometric cross section of the nanowire by exploiting its leaky resonant modes. We provide intuition into why the SPP coupling via certain wire modes is more effective than others based on their symmetry properties. Furthermore, we provide an example showing that dielectric scatterers may perform as well as metallic scatterers in coupling to SPPs.