Local Excitation of Surface Plasmons by TNOM

A point light source, such as the aperture scanning near-field optical microscope (SNOM) probe in emission mode, can excite surface plasmon polaritons (SP) in metallic films by means of its near field. We demonstrated this using a tunnel near-field optical microscope (TNOM) with thin silver films as samples. Light is transmitted through the film predominantly into the directions defined by the dispersion relation of the SP, forming a sharply confined conical sheet of radiation in the far field. The angle of emission is larger than the critical angle theta_c for total reflection, hence not accessible to standard SNOM. The weak emission into allowed directions mainly stems from SP scattering at film imperfections. With TNOM, it is possible to record ``forbidden'' (theta >/= theta_c) and ``allowed'' (theta >/= theta_c) light separately but simultaneously. The resulting scan images represent the intensities of SP excitation and SP scattering at imperfections of the silver film as a function of probe position. Their most conspicuous features are concentric circles and confocal hyperbolae, respectively, suggestive of interactions between plasmons originating from the tip and from nearby scattering centers. Our findings indicate that plasmon scattering/reflection can be studied quantitatively and on a local scale. (Dept480)