Plasmonic interactions between a metallic nanoshell and a thin metallic film

The plasmonic interaction and optical properties of a metallic nanoshell near a metallic film are investigated theoretically and experimentally. The plasmon hybridization method is extended to include the screening effects mediated by dielectric backgrounds and a realistic description of the dielectric properties of the metal. We show that the plasmonic structure of the system depends strongly on the aspect ratio of the nanoshell (the ratio of inner to outer radius) and the thickness of the film. In the thin film limit, the plasmonic coupling between the nanoshell and the film induces a low-energy virtual state (VS) mainly consisting of delocalized thin film plasmons. We show that the energy and intensity of this state can both be controlled by the aspect ratio of the nanoshell and the thickness of the film. The theoretical results are found to agree well with experimental observations. Using finite-difference time-domain simulations, we show that the electromagnetic field enhancements induced by excitation of the VS can be very large and that the nanoshell/thin film system could serve as a tunable plasmonic substrate for surface enhanced spectroscopies.