Efficient excitation of surface plasmon polaritons by the scattering of a volume electromagnetic beam from a circularly symmetric defect of cosinusoidal form and finite size on a planar metal surface

Abstract

In a previous investigation we have studied the excitation of a surface plasmon polariton (SPP) when a volume electromagnetic wave in the form of a beam illuminates a circularly symmetric protuberance or indentation of Gaussian form on an otherwise planar metal surface in contact with vacuum. The fraction of the incident flux that was scattered into a SPP was rather small, of the order of one percent. In this paper, we propose a different form for a circularly symmetric surface defect and show that it is possible to achieve a much higher efficiency for the excitation of a SPP. The surface profile function we employ is of cosinusoidal form along the radial coordinate up to a radius R0, and vanishes outside this radius. Here R0 is chosen such that the profile function is continuous. By exploiting the circular symmetry of the problem we expand the reduced Rayleigh equation for the p- and s-polarized components of the electromagnetic field above and on a vacuum-metal interface into a set of one-dimensional integral equations that we then solve numerically. The solution of the integral equations in the first Born approximation shows that the scattering amplitude is related to the Bragg vector of the periodic part of the surface. Thus, a specific scattering geometry can be optimized by adjusting the periodicity and consequently the Bragg vector. We report excitation efficiencies that are about 15 times larger than those achieved with a Gaussian profile.