  We develop a semi-analytical method for analyzing surface plasmon
interferometry using near-field scanning optical sources. We compare our
approach to Young double hole interferometry experiments using scanning
tunneling microscope (STM) discussed in the literature and realize experiments
with an aperture near-field scanning optical microscope (NSOM) source
positioned near a ring like aperture slit milled in a thick gold film. In both
cases the agreement between experiments and model is very good. We emphasize
the role of dipole orientations and discuss the role of magnetic versus
electric dipole contributions to the imaging process as well as the
directionality of the effective dipoles associated with the various optical and
plasmonic sources.
