Brownian particle in an optical potential of the washboard type

Experimental observations of the fluctuation-driven net transport of silica microspheres are presented in a two-dimensional optical potential of circular symmetry created by a Bessel light beam. The optical field is tailored to break symmetry and create a static tilted periodic (washboard) potential. As the tilt of potential exceeds a threshold, a transition between locked and running modes is observed. The running mode manifests itself by the rapid accumulation of particles at the beam center.