Sub-Doppler laser cooling of potassium atoms

We investigate the sub-Doppler laser cooling of bosonic potassium isotopes, whose small hyperfine splitting has so far prevented cooling below the Doppler temperature. We find instead that the combination of a dark optical molasses scheme that naturally arises in this kind of system and an adiabatic ramping of the laser parameters allows us to reach sub-Doppler temperatures for small laser detunings. We demonstrate temperatures as low as 25 +/- 3 mu K and 47 +/- 5 mu K in high-density samples of the two isotopes K-39 and K-41, respectively. Our findings should find application to other atomic systems.