Tuning the stiffness asymmetry of optical tweezers via polarization control

We report an experimental demonstration of tailoring the landscape of the optical potential by using the polarization state of a trap laser beam as a tuning knob. Two groups of polystyrene spheres, i.e., 300 nm and 1 μm, are optically trapped individually to investigate the effect of the intrinsic and a strongly-modified optical field distribution on the resulting trap stiffness, respectively. The angular variations of the orthogonal pair of stiffnesses, k_x and k_y, with the polarization state of the trap laser beam were systematically analyzed, and the results show that polarization control could provide a wide tuning range of trap-stiffness asymmetry (s_T = 1− k_x/k_y). The implication of asymmetry control of the optical potential is briefly discussed.