The Measurement of Displacement and Optical Force in Multi-Optical Tweezers

We set up a system of multiple optical tweezers based on a spatial light modulator, and measured the displacement and optical force of the trapped particles simultaneously. All of the trapped particles can be clearly imaged in three dimensions by several CCDs. The displacement is obtained by calculating the gray weighted centroid in the trapped particle's image. The stiffness of the trapped particles in the optical traps is measured by oscillating the sample stage in a triangular wave based on Stokes fluid dynamics. The optical force of each trapped particle can be calculated by the measured displacement and stiffness.