Lambda-enhanced grey molasses on the D2 transition of Rubidium-87 atoms

Laser cooling based on dark states, i.e. states decoupled from light, has proven to be effective to increase the phase-space density of cold trapped atoms. Dark-states cooling requires open atomic transitions, in contrast to the ordinary laser cooling used for example in magneto-optical traps (MOTs), which operate on closed atomic transitions. For alkali atoms, dark-states cooling is therefore commonly operated on the D-1 transition nS(1/2) -> nP(1/2). We show that, for Rb-87, thanks to the large hyperfine structure separations the use of this transition is not strictly necessary and that "quasi-dark state" cooling is efficient also on the D-2 line, 5S(1/2) -> 5P(3/2). We report temperatures as low as (4.0 +/- 0.3) mu K and an increase of almost an order of magnitude in the phase space density with respect to ordinary laser sub-Doppler cooling.