Force, diffusion, and channeling in sub-Doppler laser cooling

We present an extensive set of measurements on one-dimensional sub-Doppler cooling and channeling in counterpropagating light beams. The experimental method consists of the measurement of the profile of an initially subrecoil collimated atomic beam, which is deflected by the interaction with the light field. The initial velocity of the atoms in the direction of the laser beams is varied in the range -1≤${\mathit{v}}_{\mathrm{\perp }}$(${\mathrm{ms}}^{\mathrm{-}1}$)≤1 by changing the angle between atomic and laser beams. For the orthogonal circular polarization (${\mathrm{\sigma }}^{+}$ ${\mathrm{\sigma }}^{\mathrm{-}}$) sub-Doppler cooling configuration, the force and the diffusion coefficient characterizing the cooling process have been determined as a function of the initial velocity ${\mathit{v}}_{\mathrm{\perp }}$ from the average deflection and broadening of the atomic beam profile. We observe transient effects due to the slow evolution of the distribution over the magnetic sublevels to an equilibrium. The experimental results agree very well with quantum Monte Carlo simulations and semiclassical calculations. For the orthogonal linear polarization (${\mathrm{\pi }}^{\mathit{x}}$ ${\mathrm{\pi }}^{\mathit{y}}$) configuration, we demonstrate the validity of the well-known Sisyphus picture for the cooling mechanism by comparing the experimental data to the results of a simple semiclassical Monte Carlo model incorporating only the dipole force and optical pumping. In weak standing waves of either circular or linear polarization, we demonstrate the characteristic features of channeling. © 1996 The American Physical Society.