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 b ackground 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 n ear-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-sec tions at small probe-sample distances. This strongly non-linear behavior pr oduces higher harmonics when modulating the distance. Demodulation at highe r harmonics, therefore, enables an effective probe tip 'sharpening' and imp roves both resolution and image contrast. This effect is experimentally con firmed by imaging purely dielectric contrast of a topogaphically flat pn(+) -nanostructured semiconductor, realizing lambda/100 resolution at 10 mu m i nfrared wavelength. (C) 2000 Elsevier Science B.V. All rights reserved.