Pure optical contrast in scattering-type scanning near-field microscopy

We have enhanced the apertureless scattering-type scanning near-field optic al microscope by two improvements which together achieve a recording of the true near field without any height-induced artefact. These are the use of interferometric detection of the scattered light on one hand, and the use o f higher-harmonic dither demodulation of the scattered signal on the other. Here we present the basic rationale for these techniques, and give example s measured with two different experiments, one in the infrared (10 mum wave length), the other in the visible (633 nm). The latter operates in a fully heterodyne mode and displays simultaneous images of optical near-field phas e and amplitude, at below 10 nm resolution.