<title>Mechanical oscillator tip-to-sample separation control for near-field microscopy</title>

Abstract

We describe a new system for controlling the tip-to-sample separation in a Near Field Scanning Optical Microscope. A tapered Al coated fiber was glued to a high-Q Si paddle mechanical oscillator. The paddle is capacitively driven at one of its resonances, and the amplitude of the movement is detected through another electrode. As the tip approaches the surface, the viscous drag acting on its increases, causing the amplitude of oscillation of the paddle-tip system to reduce. A signal proportional to the amplitude of oscillation is used as feedback to control the tip-to-sample distance. This is accomplished in the 0 - 200 nm range, with a stability better than 1 nm. We present a complete characterization of the system. In order to determine the capabilities of our setup we provide shear force images of different samples as well as simultaneous topographic and near field images of GaAs/AlGaAs 1.55 micrometers waveguides.