Force spectroscopy with a large dynamic range using small cantilevers and an array detector

Abstract

The important characteristics of a detector for force spectroscopy measurements are sensitivity, linearity and dynamic range. The commonly used two-segment detector that measures the position of a light beam reflected from the force-sensing cantilever in an atomic force microscope becomes nonlinear when the beam shifts significantly onto one of the segments. For a detection setup optimized for high sensitivity, such as needed for the use with small cantilevers, it is shown both experimentally and theoretically that the dynamic range extends to an upper detection limit of only about 115 nm in cantilever deflection if <10% nonlinearity is required. A detector is presented that circumvents that limitation. This detector is based on a linear arrangement of multiple photodiode segments that are read out individually. With such an array detector, the irradiance distribution of the reflected beam is measured. The reflected beam not only shifts in position but also deforms when the cantilever deflects because the bent cantilever acts as a curved mirror. The mean of the distribution, however, is a linear function of cantilever deflection in both theory and experiment. An array detector is consequently well suited for force measurements for which both high sensitivity and a large dynamic range are required.