Radiation forces on a two-level atom in a sigma(+)-sigma(-) configuration of Laguerre-Gaussian beams

We investigate the radiation forces associated with the J=0-->J=1 transition of a two-level atom in a laser configuration of two counter-propagating Laguerre-Gaussian (LG) beams with opposite circular polarisations. It is shown that, in addition to the usual dissipative and dipole forces, the atom experiences a torque about the beam axis. The latter arises from the orbital angular momentum properties of the Laguerre-Gaussian beams. The atom experiences either a static torque or a purely velocity-dependent torque, depending on the relative signs of the orbital angular momenta of the two LG beams. The trajectories of the atom are calculated in each case by solving the optical Bloch equations together with the classical equation of motion for the atom.