Plasmonic Fabry-Pérot nanocavity.

We experimentally demonstrate a novel, all-plasmonic nanoscopic cavity exhibiting Q-factors up to 200 at visible frequencies. The Fabry-Pérot type resonator uses tall metallic fins that reflect up to 98% of incident surface plasmon to concentrate light within a subwavelength cavity mode. High aspect ratio metal fins, constructed using lithography and electroplating, reduce surface plasmon scattering out of the surface, while a short cavity length reduces the propagation loss. A simple Fabry-Pérot cavity model adapted for surface plasmon dispersion and reflection describes the underlying physics of the nanocavities and the results agree well with Johnson's and Christie's permittivity data. The occurrence of an optimum wavelength for plasmon storage in these cavities allows us to clearly visualize the fundamental trade-off between propagation loss and the spatial extent of surface plasmon polaritons. The subwavelength optical mode area within these cavities enables the enhancement of weak optical processes such as spontaneous emission and nonlinear optics at nanoscale dimensions.