Optical spectroscopy of a surface at the nanometer scale: A theoretical study in real space

The near-field optical interaction between a pointed detector and dielectric or metallic substrates can be exploited to perform nanometer-scale surface spectroscopy. This paper presents a general framework for a realistic description of the optical near field with application to local spectroscopy of metallic aggregates deposited on a transparent sample. The treatment is based on the field-susceptibility method associated with perturbation theory. The practical solution of this model by discretization in real space is obtained by a self-consistent procedure which takes all multiple-scattering effects into account. Numerical results illustrate the evolution of direct space images when scanning metallic aggregates at various frequencies. Our simulations clearly show the interest of such a local spectroscopy for morphologic studies and for the characterization of surface mesoscopic structures.