Laser cooling based on velocity-selective coherent population trapping is investigated theoretically. Quasiclassical treatment is used to identify the conditions for optimal cooling and to study the dynamics of the cooling process in detail. For different cooling schemes we give the analytical expressions for the temperature of the cold atoms, which can be much lower than the Doppler cooling limit. The effect of the degree of incompleteness of coherent population trapping on dynamics and the limits of laser cooling are revealed.
We report our experiment on sideband cooling with two Raman transitions in a $\mathrm{\ensuremath{\Lambda}}$ configuration that allows selective coherent population trapping (CPT) of the low-lying motional states. The cooling method is applied to
87Rb atoms in a circularly polarized one-dimensional optical lattice. Owing to the vector polarizability, the vibration frequency of a trapped atom depends on its Zeeman quantum number and the CPT resonance for a pair of bound states in the $\mathrm{\ensuremath{\Lambda}}$ configuration depends on their vibrational quantum numbers. We call this scheme motion-selective coherent population trapping (MSCPT) and it is a trapped-atom analog of the velocity-selective CPT developed for free He atoms. We observe a pronounced dip in temperature near a detuning for the Raman beams to satisfy the CPT resonance condition for the motional ground state. Although the lowest temperature we obtain is ten times the recoil limit owing to the heat load from motion along an axis, which is left uncooled, and the large Lamb-Dicke parameter of 2.3 in our apparatus, the experiment demonstrates that MSCPT enhances the effectiveness of Raman sideband cooling and enlarges the range of its application. Discussions of design parameters optimized for MSCPT on
87Rb atoms and opportunities provided by diatomic polar molecules, whose Stark shift shows a strong dependence on the rotational quantum number, are included.
We present a new laser-cooling scheme based on velocity-selective optical pumping of atoms into a nonabsorbing coherent superposition of states. This method has allowed us to achieve transverse cooling of metastable
4He atoms to a temperature of 2 \ensuremath{\mu}K, lower than both the usual Doppler cooling limit (23 \ensuremath{\mu}K) and the one-photon recoil energy (4 \ensuremath{\mu}K). The corresponding de Broglie wavelength (1.4 \ensuremath{\mu}m) is larger than the atomic-transition optical wavelength.