Observation of asymmetrically dynamic motion of single colloidal particles in a polarized optical trap

In this study we report on the dynamic motion of a nano-sized colloidal particle captured in a polarized optical trap. A polystyrene sphere (300nm-diameter) that is electrically charged in solution was trapped with an optical tweezers formed by a linearly polarized TEM00 Gaussian beam, while the Brownian displacements of the trapped particle in x and y directions were measured so that the position of the particle's mass center can be mapped on the transverse plane and the corss-correlation between x and y displacements can be calculated. We found that the position's fluctuation of the trapped nano-sized particle in the parallel direction to the laser polarization is significantly larger than that in the normal direction, which suggests that there exists an additional random electric force parallel to the laser polarization direction exerting on the charged particle beside the known radiation forces on the dielectric particle. This asymmetry in dynamic motion is significant when the particle size is well less than the wavelength of the trapping laser. However, in an optical trap formed by a circularly polarized beam, this asymmetry in dynamic motion was observed to disappear. We present both the experimental results and a theoretical analysis.