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 an d topography in the images obtained by illumination-mode scanning near-fiel d optical microscopy. We use a model for the emitting tip, which describes the polarization and confinement effects of a real tip. We analyze the imag e formation, especially the coupling between topographic and dielectric con trast. In the case of weakly scattering samples, we introduce rigorously th e concepts of impulse response and equivalent surface profile. This tool ma y be useful to describe and understand quantitatively experimental images. Finally, we study the presence of artifacts in the images, due to the coupl ing between optical scattering and the z motion of the tip in constant-dist ance operating mode. We put forward the difficulty of predicting the relati ve weight of the artifact and the purely optical contributions. (C) 1999 Am erican Institute of Physics. [S0021-8979(99)06513-5].