Force detection in optical tweezers using backscattered light

In force-measuring optical tweezers applications the position of a trapped bead in the direction perpendicular to the laser beam is usually accurately determined by measuring the deflection of the light transmitted through the bead. In this paper we demonstrate that this position and thus the force exerted on the bead can be determined using the backscattered light. Measuring the deflection for a 2.50 mum polystyrene bead with both a position sensitive detector (PSD) and a quadrant detector (QD) we found that the linear detection range for the PSD is approximately twice that for the QD. In a transmission-based setup no difference was found between both detector types. Using a PSD in both setups the linear detection range for 2.50 mum beads was found to be approximately 0.50 mum in both cases. Finally, for the reflection-based setup, parameters such as deflection sensitivity and linear detection range were considered as a function of bead diameter (in the range of 0.5-2.5 mum). 140pN was the largest force obtained using 2.50 mum beads.