A Surface Plasmon Microcavity Between the Toroidal and Flat Metallic Surfaces

We consider the formation of the surface plasmon polariton (SPP) mode in the structure with a metallic torus and a metallic flat surface separated by a dielectric medium. The energy of the wave field is mainly concentrated in the dielectric medium at the vicinity of the minimum thickness of the gap between the metallic surfaces. The dependence of the resonant frequency on parameters of the structure was determined. The strongly localized SPP mode in the transverse direction contributes to the increase in the Purcell factor that is crucial for enhancement of the spontaneous emission rate.