Coupling light into and out from the surface plasmon polaritons of a nanometer-thin metal film with a metal nanostrip

Abstract

Scattering of light by a metal nanostrip placed near a nanometer-thin metal film in a homogeneous dielectric environment is considered. For a plane wave incident on the geometry from outside we study the scattering cross sections governing the excitation of symmetric and antisymmetric surface plasmon polariton (SPP) waves and out-of-plane (OUP) propagating waves. For a symmetric (long-range) or antisymmetric (short-range) SPP propagating along the metal film and being incident on the nanostrip, we study the fraction of power coupled into forward and backward propagating symmetric and antisymmetric SPP waves, and into OUP propagating waves, respectively. Resonance peaks in the scattering cross sections, or dips in the relative power going into all scattering channels due to resonant absorption, are related to standing-wave resonances of counterpropagating gap SPP waves in the gap between the metal nanostrip and the metal film. The resonances are studied with respect to the width of the metal nanostrip and the size of the gap between the metal film and nanostrip. For a 15-nm-thick film it is found that coupling of light in and out of the symmetric SPP is weak compared with coupling to and from OUP and antisymmetric SPP waves. Improving the in and out coupling of power in the symmetric SPP waves with thicker films (50 nm) and larger scattering objects is considered.