Recently, we presented an experimental realization of a deterministic optical rocking ratchet [Arzola, A. V., et al. Phys
Rev. Lett. 106: 168104 (2011)]. We obtained a systematic motion of microparticles and demonstrated that it is possible
to control their average velocity and their direction of motion in real time by properly tuning experimental parameters.
We have extended our study in order to establish the conditions for observing the crucial effect of current reversals in
deterministic conditions, phenomenon predicted more than a decade ago, but experimentally demonstrated for the first
time in our system.
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.
We propose a technique for the characterization of a 1D-periodic optical potential by studying the dynamics of
non-brownian microscopic particles immerse in water (negligible thermal noise). It has been demonstrated that
in the Mie regime, a periodic light pattern applied to a particle acts as an effective potential that depends on
the size of the particle respect to the period of the optical landscape [I. Ricardez-Vargas, et.al. Appl. Phys.
Lett. 88, 121116 (2006)]. We verify this fact by studying the dynamics of a particle moving within the pattern
due to the effect of a known constant external force. The periodic light pattern is generated with interference
techniques whereas the external force is applied by means of a controlled inclination of the sample cell. We fit
the experimental results for the ensemble average of particle position against time with a theoretical model of
the physical situation. In this way we obtain a curve for the optical force as a function of particle's position for
We propose a model for a walker moving on an asymmetric periodic ratchet potential. The walker has two 'feet' represented as two finite-size particles coupled nonlinearly through a double-well potential. In contrast to linear coupling, the bistable potential admits a richer dynamics where the ordering of the particles can alternate. The transitions between the two stable points on the bistable potential, correspond to a walking with alternating particles. In our model, each particle is acted upon by independent white noises, modeling thermal noise, and additionally we have an external time-dependent force that drives the system out of equilibrium, allowing directed transport. This force can be common colored noise, periodic deterministic driving or fluctuations on the bistable potential. In the equilibrium case, where only white noise is present, we perform a bifurcation analysis which reveals different walking patterns available for various parameter settings. Numerical simulations showed the existence of current reversals and significant changes in the effective diffusion constant and in the synchronization index. We obtained an optimal coherent transport, characterized by a maximum dimensionless ratio of the current and the effective diffusion (Peclet number), when the periodicity of the ratchet potential coincides with the equilibrium distance between the two particles.