Plasmon resonances and near-field optical microscopy: a self-consistent theoretical model.

Electromagnetic optical interactions between a small metal sphere and a metallic surface are studied by using a self-consistent approach in the presence of an external field. The intensity scattered by the metal particle is given for different polarizations of the incident field. This quantity, determined from a local treatment of the response function of the two interacting systems, exhibits a spatial dependence with respect to the approach distance close to that obtained from recent experimental studies. Moreover, at large separation, retardation effects included from a dipolar propagator give rise to pseudoperiodic oscillations such as the ones observed in reflection near-field optical microscopy. In the near-field range, plasmon modes of the whole system probe surface introduce narrow resonances in the scattered intensity versus the probe-sample separation.