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

We have studied theoretically and experimentally the effect of a relat ively strong magnetic field on sub-Doppler laser cooling in a one-dime nsional optical molasses. We used the operator description of laser co oling 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 polariz ation vectors of the light beams. Analytical results for the force on the atom are obtained in two cases that illustrate the effect of the V SR on the force. These formulas are compared with numerical calculatio ns of the force. We also discovered a redistribution mechanism that re lies on the gradient of the eigenstates of the light-shift operator, w ith eigenvalues that are independent of position so that a ''Sisyphus cooling'' picture does not apply. The theory is compared with many exp erimental results and excellent agreement is found. We believe that al l essential features of laser cooling at law intensity are well descri bed by this operator theory.