Simultaneous sub-Doppler laser cooling of fermionic 6Li and 40K on the D1 line: Theory and experiment

We report on simultaneous sub-Doppler laser cooling of fermionic Li-6 and K-40 using the D-1 optical transitions. We compare experimental results to a numerical simulation of the cooling process applying a semiclassical Monte Carlo wave-function method. The simulation takes into account the three-dimensional optical molasses setup and the dipole interaction between atoms and the bichromatic light field driving the D-1 transitions. We discuss the physical mechanisms at play, identify the important role of coherences between the ground-state hyperfine levels, and compare D-1 and D-2 sub-Doppler cooling. In 5 ms, the D-1 molasses phase greatly reduces the temperature for both Li-6 and K-40 at the same time, with final temperatures of 44 and 11 mu K, respectively. For both species this leads to a phase-space density close to 10-4. These conditions are well suited to direct loading of an optical or magnetic trap for efficient evaporative cooling to quantum degeneracy.