  We report on the first realization of sub-Doppler laser cooling of 39K atoms
using degenerate 3D Raman sideband cooling. We take advantage of the
well-resolved excited hyperfine states on the D1 optical transition to produce
spin polarized samples with $1.4 \times 10^8$ atoms at temperatures of 1.8
$\mu$K. The phase-space densities are $\geq 10^{-4}$, which significantly
improves the initial conditions for a subsequent evaporative cooling step. The
presented cooling technique using the D1 line can be adapted to other atomic
species and is applicable to high-resolution imaging schemes in far
off-resonant optical lattices.
