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Detection of probe dither motion in near-field scanning optical microscopy.

The probe-to-sample separation in near-field scanning optical microscopes can be regulated by a noncontact shear-force sensing technique. The technique requires the measurement of a minute dither motion applied to the probe. We have characterized an optical detection method for measuring this motion to determine the optimum detection configuration in terms of sensitivity and stability. A scalar diffraction model of the detection method is developed for calculating sensitivity, and experimental results are found to be in good agreement with the theoretical predictions. We find that maximum sensitivity and stability cannot be achieved simultaneously, and it may be desirable in practice to trade sensitivity for enhanced stability.