Influence of non-conservative optical forces on the dynamics of optically trapped colloidal spheres: The fountain of probability

We demonstrate both experimentally and theoretically that a colloidal sphere trapped in an optical tweezer does not come to equilibrium, but rather reaches a steady state in which its probability flux traces out a toroidal vortex. This non-equilibrium behavior can be ascribed to non-conservative optical forces and constitutes a particularly simple thermal ratchet. We briefly discuss ramifications of this effect for previous experiments in which optical tweezers have been treated as conservative potential energy wells.