Near-field microscopy of surface-plasmon polaritons: Localization and internal interface imaging

Using a scanning near-field optical microscope with shear force feedback we directly probe optical fields of surface-plasmon polaritons (SPP's) excited at different interfaces of gold films while simultaneously imaging surface relief structures. We observe that near-field optical images, which are generated due to the SPP propagating along a rough surface, exhibit spatially localized (within {~}250 nm) field enhancement by {~}5 times, whereas those due to the SPP at a relatively smooth surface show a well-pronounced interference pattern related to the interference between the excited and scattered SPP's. We observe also that near-field optical images ascribed to the SPP excited at an external (gold-air) interface and those due to the SPP at an internal (gold-fluoride) interface exhibit not only some common features but also distinctly different features, which we believe are related to the topography of the internal interface.