Experimental Investigation on How Sampling Frequency Affects the Measured Stiffness of an Optical Trap

The stiffness of an optical trap is an extremely important parameter in measuring mechanical properties of biological micro-particles or colloids. Few attentions are paid to experimental study of the stiffness v.s. sampling frequency dependence. We studied experimentally how the sampling frequency of a detector influences the measured transversal stiffness of a single beam optical trap. The stiffness of an optical trap was calibrated by detecting the reflected light signal by polystyrene spheres through a quadrant photodiode detector (QPD), and then calculated using the Boltzmann statistics method. In our experiments, polystyrene spheres had been used to conclude that in the directions both parallel (p) and perpendicular (s) to the laser polarization direction, the measured stiffnesses are different and all varies with the sampling rate of the QPD used in our experiment. For a certain sampling frequency range, the measured stiffness decreases while the sampling frequency increases. Our results provide researchers information for sampling frequency selection in related experiments.