We report enhanced sub-Doppler cooling of the bosonic atoms of ${}^{39}$K facilitated by formation of dark states with the cooling and repumping lasers tuned to the Raman resonance in $\Lambda$ configuration near the $D_1$ transition. A temperature of about 12 $\mu$K and phase-space density $>$$2\times {10}^{-5}$ is achieved in the two-stage $D_2$-$D_1$ molasses and spans a very large parameter region where quantum interference persists robustly. We also present results on enhanced radiation heating with a subnatural linewidth (0.07$\Gamma$) and a signature Fano-like profile of a coherently driven three-level atomic system. The optical Bloch equations relevant for the three-level atom in a bichromatic light field are solved with the method of continued fractions to show that cooling occurs only for a small velocity class of atoms, emphasizing the need for precooling in the $D_2$ molasses stage.

• Received 24 May 2013

DOI:https://doi.org/10.1103/PhysRevA.88.053407