Λ-enhanced sub-Doppler cooling of lithium atoms in D₁ gray molasses

Following the bichromatic sub-Doppler cooling scheme on the D₁ line of 40K recently demonstrated in  Fernandes, we introduce a similar technique for 7Li atoms and obtain temperatures of 60μK while capturing all of the 5×108 atoms present from the previous stage. We investigate the influence of the detuning between the the two cooling frequencies and observe a threefold decrease of the temperature when the Raman condition is fulfilled. We interpret this effect as arising from extra cooling due to long-lived coherences between hyperfine states. Solving the optical Bloch equations for a simplified Λ-type three-level system we identify the presence of an efficient cooling force near the Raman condition. After transfer into a quadrupole magnetic trap, we measure a phase space density of ∼10^-5. This laser cooling offers a promising route for fast evaporation of lithium atoms to quantum degeneracy in optical or magnetic traps.