Coupling of multiple LSP and SPP resonances: interactions between an elongated nanoparticle and a thin metallic film.

We study the coupling interactions between a progressively elongated silver nanoparticle and a silver film on a glass substrate. Specifically, we investigate how the coupling between localized surface plasmons (LSPs) and propagating surface plasmon polaritons (SPPs) is influenced by nanoparticle length. Although the multiple resonances supported by the nanoparticle are effectively standing wave surface plasmons, their interaction with the SPP continuum of the underlying Ag film indicates that their spectral response is still localized in nature. It is found that these LSP-SPP interactions are not limited to small particles, but that they are present as well for extremely long particles, with a transition to the SPP coupling interactions of a bilayer metallic film system beginning at a particle length of approximately 5 μm.