Tip-sample distance control for near-field scanning optical microscopes

In a recent paper we have developed a new technique allowing to control the distance separation between a tapered metal coated optical fiber tip and the surface of a sample. This technique is based on a piezo-electric tuning fork as a shear force sensor and is found to be suitable for the operation of near-field scanning optical microscope. We present in this article a review of the recent results concerning this new technique. In particular, we present hydrodynamic measurements performed in order to evaluate the shear (or viscous drag) forces picked up by the piezo-electric tuning fork sensor.