Piezo-electric tuning fork tip-sample distance control for near field optical microscopes

In a recent paper [Karrai and Grober, Appl. Phys. Lett. 66 (1995) 1842], a new technique was developed in order to control the distance separation between a tapered metal-coated optical fiber tip and the surface of a sample. This new technique is based on a piezo-electric tuning fork used as a shear-force detector. The fiber tip, which is attached along one of the arms of the tuning fork, acts as a shear-force pick-up. We present in this article the idealized model analysis that leads to the design parameters of a tuning fork optimized for near-field scanning optical microscopy.