Deterministic optical rocking ratchet: theory and experiment

Abstract

We present a theoretical model and the experimental demonstration of the rocking ratchet effect in the deterministic regime using an optical trapping device. Our system consists of a dielectric spherical particle in a 1D optical potential created by means of an interference pattern of asymmetric fringes. In order to achieve the asymmetry of the fringes, three light beams are interfered by pairs by controlling their relative polarization states, intensities and phases. A periodic time-dependent external force of zero average is introduced by moving the sample with respect to the optical pattern, for which the translation stage is driven sideways. The drag force acting on the particle due to this relative motion has the effect of tilting the optical potential periodically in opposite directions, providing the "rocking" mechanism. We show that an inversion of the asymmetry in the effective optical potential occurs as the size of the particle is varied, and therefore, we can observe opposite motion of different particles within the same optical pattern. The dynamics of the system is studied in terms of the different control parameters, such as the size of the particles, the period and asymmetry of the fringes, the amplitude and frequency of the rocking mechanism, and the power level in the sample.