Optical antenna properties of scanning probe tips: Plasmonic light scattering, tip-sample coupling, and near-field enhancement

The optical near-field distribution and enhancement near the apex of model scanning probe tips are calculated within the quasistatic approximation. The optical tip-sample coupling sensitively depends on both the tip and sample material. This, in addition to the tip-sample distance and apex geometry, is found to affect the spatial resolution that can be obtained in scattering near-field microscopy (s-SNOM). A pronounced structural plasmon resonant behavior is found for gold tips, which redshifts upon tip-sample approach on the length scale given by the tip radius. This near-field tip-sample coupling also allows for surface plasmon excitation in the sample. With the critical dimensions of the tip apex in the range of 10 to several 10s of nanometers, the results are found to be in good agreement with experiment and more rigorous theoretical treatments.