Excitation and focusing of surface plasmon polaritons by nanostructuring

Excitation, focusing, and directing of surface plasmon polaritons (SPPs) with curved chains of bumps located on a metal surface is investigated both experimentally and theoretically. We demonstrate that, by using a relatively narrow laser beam (at normal incidence) interacting only with a portion of a curved stripe or chain of nanoparticles, one can excite an SPP beam whose divergence and propagation direction are dictated by the incident light spot size and its position along the structure. It is also found that the SPP focusing regime is strongly influenced by the chain inter-particle distance. Extensive numerical simulations of the configuration investigated experimentally are carried out for a wide set of system parameters by making use of the Green's tensor formalism and dipole approximation. Comparison of numerical results with experimental data shows good agreement with respect to the observed features in SPP focusing and directing, providing the guidelines for a proper choice of the system parameters. It was found that the focusing regime of SPPs is strongly influenced by the chain inter-bump distance, so that the focusing and directing effects with optimal properties can be obtained only when the chain inter-bump distance is smaller than the SPP wavelength. Following the experimental conditions, we have studied the role of the size of light spot exciting SPPs. Spectral dependence of the focusing waist is also numerically studied for gold surface taking into account the ohmic loss.