LASER COOLING AT LOW-INTENSITY IN A STRONG MAGNETIC-FIELD

We have studied theoretically and experimentally the effect of a relatively strong magnetic field on sub-Doppler laser cooling in a one-dimensional optical molasses. We used the operator description of laser cooling with the Larmor precession frequency omega(z) being much higher than the optical pumping rate. We found velocity-selective resonances (VSR) in the force at velocities v(r) = nomega(z)/K, with n = 0,+/-1,+/-2 for both the scattering and redistribution force operators. These depend on the relative direction of the magnetic field and the polarization vectors of the light beams. Analytical results for the force on the atom are obtained in two cases that illustrate the effect of the VSR on the force. These formulas are compared with numerical calculations of the force. We also discovered a redistribution mechanism that relies on the gradient of the eigenstates of the light-shift operator, with eigenvalues that are independent of position so that a ''Sisyphus cooling'' picture does not apply. The theory is compared with many experimental results and excellent agreement is found. We believe that all essential features of laser cooling at law intensity are well described by this operator theory.