Radiation pressure vortices in two crossed standing waves

We have observed the effect of microscopic radiation pressure vortices on atomic motion in two crossed, near-resonant, optical standing waves. When the two standing waves oscillate synchronously the resulting light field and the corresponding light forces possess a 90^∘{} rotation symmetry. However, when one of the standing waves' oscillations is delayed with respect to those of the other, a vortical radiation pressure force arises which can be identified by the breaking of this symmetry. Radiation pressure vortices should play a significant role for the atomic kinematics in optical molasses and magneto-optical traps.