Sub-Doppler Laser Cooling of ^23Na in Gray Molasses on the D_2 Line

We report on the efficient gray molasses cooling of sodium atoms using the D_2 optical transition at 589.1 nm. Thanks to the hyperfine split about 6\it \Gamma between |F'=2\rangle and |F'=3\rangle in the excited state 3^2P_3/2, this atomic transition is effective for the gray molasses cooling mechanism. Using this cooling technique, the atomic sample in F=2 ground manifold is cooled from 700 \muK to 56 \muK in 3.5 ms. We observe that the loading efficiency into magnetic trap is increased due to the lower temperature and high phase space density of atomic cloud after gray molasses. This technique offers a promising route for the fast cooling of the sodium atoms in the F=2 state.