Laser cooling below the Doppler limit

The experiment reported makes use of Na atoms in optical molasses formed at the intersection of three orthogonal pairs of counterpropagating laser beams. Laser cooling leads to a strongly damped motion of the atoms (viscous confinement) in this region. The molasses is loaded with slow atoms from a laser-cooled atomic beam, and the laser beams that confine the molasses are turned off, allowing the atoms to fall with ballistic trajectories through a probe beam some distance under the molasses. Fluorescence from the atoms falling through the probe produces a time-of-flight (TOF) signal that can be compared with a calculated TOF signal based on the initial spatial and assumed initial (Maxwell-Boltzmann) velocity distributions. Temperatures determined by this method are plotted as a function of a laser detuning red of resonance. It is clear that the temperatures measured are well below the Doppler cooling limit except when the detuning is quite small. Polarization gradient cooling, which involves optical pumping among magnetic sublevels, could explain the observations.