Modelling topographical artifacts in scanning near-field optical microscopy

Scanning probe techniques such as atomic force microscopy and scanning tunnelling microscopy have been used extensively for the study of surface properties on the nanoscale. A valuable, but less commonly used technique is scanning near-field optical microscopy (SNOM). This scanning probe technique allows the optical properties of supramolecular structures to be probed at resolutions better than allowed by the diffraction limit. This means that resolution better than 100 nm is achievable even with visible radiation. However, the geometry of the SNOM probe causes images obtained by this method to contain a mixture of optical and topographical information. Where there is no optical contrast on a surface, topography can introduce contrast to the image, which is obviously undesirable. Additionally, the topographical artifacts can cause apparent distortion or displacement of features in the optical image. To have confidence in results obtained by SNOM, the nature of topographical artifacts must be fully understood. Here we present one-dimensional numerical simulations of SNOM images and an investigation into several observable effects. In addition to the occurrence of topography-induced optical contrast we show that a low topographical feature near a higher one can be visible in an image but with low intensity and can appear displaced and distorted if it is approximately a probe diameter from the larger feature. (C) 2004 Published by Elsevier B.V.