Influence of nonconservative optical forces on the dynamics of optically trapped colloidal spheres: the fountain of probability

Phys Rev Lett. 2008 Sep 19;101(12):128301. doi: 10.1103/PhysRevLett.101.128301. Epub 2008 Sep 16.


We demonstrate both experimentally and theoretically that a colloidal sphere trapped in a static optical tweezer does not come to equilibrium, but rather reaches a steady state in which its probability flux traces out a toroidal vortex. This nonequilibrium behavior can be ascribed to a subtle bias of thermal fluctuations by nonconservative optical forces. The circulating sphere therefore acts as a Brownian motor. We briefly discuss ramifications of this effect for studies in which optical tweezers have been treated as potential energy wells.