Optical lever detection in higher eigenmode dynamic atomic force microscopy

Abstract

The optical lever detection scheme is widely used in atomic force microscopy for the detection of the cantilever deflection. Laser spot size as well as adjustment of the laser along the cantilever determine the zeros of the transfer function of the signal path from the tip-sample forces to the optical readout. This can cause (almost) pole-zero cancellations which lead to a significantly reduced sensitivity in the detection of higher mode vibrations of the cantilever. Physically, the light lever detection integrates over the slope of the cantilever. However, the sign of the slope of higher flexural modes varies along the cantilever. Thus, integration can lead to a significantly decreased sensitivity to higher eigenmode vibrations. Illuminating only the area between the free end and the next zero crossing of the slope of the modal shape provides a good compromise between high and low frequency sensitivity.