Surface-plasmon vortices in nanostructured metallic films

Light scattering by a small protrusion on a metal surface is analyzed within the framework of perturbation theory. Upon normal incidence of linearly-polarized monochromatic wave, slight deviations of the protrusion shape from a circularly symmetric one lead to formation of optical vortices in the near-field region due resonant excitation of circular surface plasmons. This agrees with the results of scanning near-field optical microscopy experiments revealing distinct spiral patterns in the in-plane near-field intensity distribution for metallized nanostructured polymer substrates.