Sub-Doppler cooling of sodium atoms in gray molasses

We report on the realization of sub-Doppler laser cooling of sodium atoms in gray molasses using the D₁ optical transition (3 s ^{1/2 2}S →3 p ^{1/2 2}P) at 589.8 nm. The technique is applied to samples containing 3 ×10⁹ atoms, previously cooled to 350 μ K in a magneto-optical trap, and it leads to temperatures as low as 9 μ K and phase-space densities in the range of 10^-4. The capture efficiency of the gray molasses is larger than 2/3, and we observe no density-dependent heating for densities up to 10¹¹cm^-3 .