NOVEL DIGITAL FEEDBACK SCHEME OF SHEAR-FORCE CONTROL IN NEAR-FIELD SCANNING OPTICAL MICROSCOPY

We have developed a novel digital feedback scheme of distance control via shear-force measurement in the near-field scanning optical microsc ope. Using simple comparators, full digital control is achieved. In th is new scheme, the probe tip stops at a reference point as it approach es the sample and then draws back when the scanning piezoelectric tube executes lateral translation of the sample by one step. The resulting feedback motion is similar to tapping at each point, which removes th e feedback oscillation and the hysteresis effect in the conventional a nalog feedback scheme. Because the control program monitors the tip po sition at each step and there is no settling time of the feedback, the scanning speed can be improved without oscillation and tip-sample col lision. (C) 1997 American Institute of Physics.