Localization phenomena in elastic surface-polariton scattering caused by surface roughness

Localization phenomena in the elastic scattering of surface plasmon polaritons (SPP's), i.e., in the SPP scattering in the surface plane, are related to the appropriate characteristics of SPP and surface roughness. Optical fields of SPP's, which are excited at two different wavelengths (488 and 633 nm) used in turn for each surface region of four different metal (silver and gold) films, are imaged simultaneously with surface topography by use of a photon scanning tunneling microscope with shear force feedback. Single scattering and weak and strong localization of SPP's are observed depending on the sample and the light wavelength. This is a direct demonstration of different regimes of the elastic SPP scattering realized at the same surface region for different light wavelengths. It is shown that the regime of SPP scattering and, consequently, near-field optical images, are determined by the topography of a local surface area with the size limited by the SPP propagation length.