Optical contrast, topographic contrast and artifacts in illumination-mode scanning near-field optical microscopy

We use a two-dimensional exact numerical simulation and a three-dimensional perturbative analysis to study the coupling between dielectric contrast and topography in the images obtained by illumination-mode scanning near-field optical microscopy. We use a model for the emitting tip, which describes the polarization and confinement effects of a real tip. We analyze the image formation, especially the coupling between topographic and dielectric contrast. In the case of weakly scattering samples, we introduce rigorously the concepts of impulse response and equivalent surface profile. This tool may be useful to describe and understand quantitatively experimental images. Finally, we study the presence of artifacts in the images, due to the coupling between optical scattering and the z motion of the tip in constant-distance operating mode. We put forward the difficulty of predicting the relative weight of the artifact and the purely optical contributions. (C) 1999 American Institute of Physics. [S0021-8979(99)06513-5].