Strip and gap plasmon polariton optical resonators

A rectangular silver strip and two strips separated by a narrow gap are considered as optical resonators. A surface integral equation method is used to study theoretically their scattering cross and local enhancement of the electric field relative to the incident wave. Peaks in the scattering spectra are shown to be related to the excitation of standing waves caused by forward and backward travelling slow plasmon polaritons (PPs) in the silver strip or in the gap between two strips. This is shown via analysis of the electric near-field inside and outside the strips, and by showing that an increase of the length of strips equal to half the slow PP wavelength results in a scattering resonance at app. the same wavelength. We study the broadening of resonances with strip thickness or gap size, respectively, for the two types of resonators. For field enhancement purposes we consider taking advantage of the fact that the electric field component of slow PPs along the axis of PP propagation is dominating inside the metal strips. Due to this fact and the electromagnetics boundary conditions forcing the surface normal electric field component to jump across the silver-air interface a small (5 nm) gap introduced in one of the strips leads to a factor similar to 25 enhancement of the field magnitude.