Shear force distance control in near-field optical microscopy: experimental evidence of the frictional probe-sample interaction

The properties of the probe-surface contact for a near-field optical microscope driven in the shear force mode have been studied applying lateral amplitudes of the probing fiber tip larger than 15 nm. Electric current measurements between a conductive tip and a conductive sample reveal a pulsed current behavior at the very beginning of the approach curve. In the upper part of the approach curve it turns to the quasiconstant current. From this observation a conclusion is drawn about the presence of permanent mechanical contact between the probe and the surface in the shear force mode. A shift of the approach curve along the z-axis as a function of dither amplitude was discovered. These results are in contradiction to the established conception of possible physical mechanisms of shear force interaction. To settle this issue the friction model is proposed according to which the damping of the probe vibrations is caused by the friction between the tip and the surface.