Non‐optical bimorph‐based force sensor for scanning near‐field optical microscopy of biological materials: characteristics, design and applications

Abstract

AbstractA non‐optical force sensor that allows operation both in lateral (shear) and in vertical (tapping) force detection modes has been introduced for dynamic tip–sample distance regulation in scanning near‐field optical microscopy (SNOM) of biological samples. The sensor is based on a rectangular bimorph cantilever consisting of two thin piezoceramic layers bonded to a brass centre shim. One of the piezo layers serves as the probe dither and another as the responder of the sensed forces. The sensor is driven with a home‐made Q‐control electronics so that its sensitivity and bandwidth can be adjusted. The dynamics, characteristics and design considerations of the sensor are theoretically and experimentally discussed. Driving the bimorph cantilever at its eigenfrequency with appropriate force feedback allows one to obtain a quality factor (Q‐factor) up to 103 in water, suitable for different sample softness and imaging environments. The high sensitivity of the sensor is demonstrated both by shear force and by tapping mode imaging of soft biological samples in their natural state. Near‐field optical resolution of better than 100 nm on red blood cells in water has been obtained. The experimental results suggest that this SNOM sensor would be a promising set‐up for biological applications. Copyright © 2007 John Wiley & Sons, Ltd.