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

We have enhanced the apertureless scattering-type scanning near-field optical 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 of higher-harmonic dither demodulation of the scattered signal on the other. Here we present the basic rationale for these techniques, and give examples measured with two different experiments, one in the infrared (10 mum wavelength), the other in the visible (633 nm). The latter operates in a fully heterodyne mode and displays simultaneous images of optical near-field phase and amplitude, at below 10 nm resolution.