Topographical artifacts and optical resolution in near-field optical microscopy

Starting from general principles of near-field optical microscopy, I demonstrate that an optical image obtained with any near-field microscope operating in a constant (probe–surface) distance mode contains pure topographical artifacts, topographically induced features, and pure optical contrast. Scanning a sharp topographical step at an otherwise homogeneous sample surface is shown to provide a sensitivity window that determines the scale on which the near-field optical image would represent mostly pure optical contrast. I suggest that the sensitivity window can be regarded as the upper limit of optical resolution of a near-field microscope.