A PHASE-LOCKED SHEAR-FORCE MICROSCOPE FOR DISTANCE REGULATION IN NEAR-FIELD OPTICAL MICROSCOPY

A nonoptical phase-locked shear-force microscope utilizing a quartz cr ystal tuning fork acting as a voltage-controlled oscillator in a phase -locked loop has been implemented. A tapered optical fiber is rigidly mounted on one of the prongs of the fork to serve as both a shear-forc e pickup and a near-field optical probe. The crystal is driven at its resonance frequency-through positive feedback of the monitored current through the crystal. This signal is used as the voltage-controlled os cillator in a phase-locked loop. The scheme allows for scan speeds far beyond the Q-limited amplitude sensor bandwidth and exhibits excellen t sensitivity for a high-Q resonator. Furthermore, given the small vib ration amplitude of the tip (< 0.5 nm) and the distance over which it is reduced (> 6 nm), it is unlikely that the tip is making direct cont act with the sample surface as has been suggested for the optical shea r-force detection scheme. (C) 1997 American Institute of Physics.