Nonlinear Response of Multi-Segmented Photodetectors Used for Measurements of Microcantilever Motion over Large Dynamic Ranges

Abstract

The use of multi-segmented Position Sensitive Photodiodes (PSD) to measure microcantilever deflections have been found to produce nonlinear signal output, especially when the dynamic range is large. The reflected beam of the microcantilever may undergo intensity and shape modifications prior to reaching the PSD. In a multi-microcantilever sensor system the variation in the size of the individual spots plays an additional role contributing to the nonlinearities of detector output. Irrespective of the range of operation the merits of intensity normalization have been discussed. We show that the output is proportional to the width of the spot along the split line of the detector. This enables the determination of the shape of a spot. We show that the microcantilever vibrational spectrum can be obtained just using a single segment photodetector instead of using multiple segmented PSDs. These concepts will greatly facilitate interpretation of sensor data acquired from either single or multi-microcantilever experimental platforms