Model for scanning near-field optical microscopy: a quasi-electrostatic approximation

A theoretical model for scanning near-field optical microscopy (SNOM) based on the consideration of the interaction between the probe tip and the sample is proposed. Both the optical probe tip and the sample protrusions are represented by polarizable dipole spheres. The induced polarization effects on the sample surface can be replaced by the image dipoles by adopting the quasi-electrostatic approximation. Applying the radiation theory of the dipole, we have established a set of equations to describe the field distribution at the sites of the probe tip and the sample protrusions. This set of field equations can be solved self-consistently. The results are exactly the same as those obtained by means of the dyadic electromagnetic propagator formalism, and also the derivation procedure is relatively simple. This method permits us to analyse the physical mechanisms of the interaction between the probe tip and the rough surface in SNOM intuitively.