Continuous loading of 1S0 calcium atoms into an optical dipole trap

We demonstrate an efficient scheme for continuous trap loading based upon spatially selective optical pumping. We discuss the case of 1S0 calcium atoms in an optical dipole trap (ODT), however, similar strategies should be applicable to a wide range of atomic species, which do not permit efficient conventional trap loading. Our starting point is a moderately cold (≅300 μK) and dense (≅9x109 cm-3) reservoir of metastable 3P2 atoms in a magneto-optic trap (triplet MOT). A focused 532 nm laser beam produces a strongly elongated optical potential for 1S0 atoms with up to 350 μK well depth. A weak focused laser beam at 430 nm, carefully superimposed upon the ODT beam, selectively pumps the 3P2 atoms inside the capture volume to the singlet state, where they are confined by the ODT. The triplet MOT perpetually refills the capture volume with 3P2 atoms, thus providing a continuous stream of cold atoms into the ODT at a rate of 107 s-1. Limited by evaporation loss, in 200 ms we typically load 5x105 atoms with 3.5x1010 cm-3 peak density and axial and radial temperatures of 300 and 85 μK, respectively. After terminating the loading we observe evaporation during 50 ms, typically leaving us with 105 atoms with nearly equal radial and axial temperatures close to 40 μK and a peak density of 5x1011 cm-3. A simple model is presented, which identifies the relevant physical mechanisms of the loading and decay dynamics. We point out that a comparable scheme could be employed to load a dipole trap with 3P0 atoms