A novel digital feedback scheme of shear-force control in near-field scanning optical microscopy

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Abstract
We have developed a novel digital feedback scheme of distance control via shear-force measurement in the near-field scanning optical microscope. Using simple comparators, full digital control is achieved. In this new scheme, the probe tip stops at a reference point as it approaches 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 the feedback oscillation and the hysteresis effect in the conventional analog feedback scheme. Because the control program monitors the tip position at each step and there is no settling time of the feedback, the scanning speed can be improved without oscillation and tip–sample collision.

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