Enhanced dielectric contrast in scattering-type scanning near-field optical microscopy

'Apertureless' probe tips have a much higher resolution potential compared to the traditional aperture tips of scanning near-field optical microscopes (SNOM), yet when illuminated by a laser focus a large amount of unwanted background scattering occurs both at the probe shaft and at the sample. Here we study in detail how this background can be suppressed by dithering the probe-sample distance, and thereby demonstrate how to enhance the optical near-field contrasts. We find from theory that the coupling of probe dipole and its image in the sample causes a steep increase of scattering cross-sections at small probe-sample distances. This strongly non-linear behavior produces higher harmonics when modulating the distance. Demodulation at higher harmonics, therefore, enables an effective probe tip 'sharpening' and improves both resolution and image contrast. This effect is experimentally confirmed by imaging purely dielectric contrast of a topogaphically flat pn(+)-nanostructured semiconductor, realizing lambda/100 resolution at 10 mu m infrared wavelength. (C) 2000 Elsevier Science B.V. All rights reserved.