We present a detailed experimental study of the physics involved in transferring atoms from a magneto-optical trap (MOT) to an optical dipole trap. The loading is a dynamical process determined by a loading rate and a density dependent loss rate. The loading rate depends on cooling and the flux of atoms into the trapping volume, and the loss rate is due to excited state collisions induced by the MOT light fields. From this study we found ways to optimize the loading of the optical dipole trap. Key ingredients for maximum loading are found to be a reduction of the hyperfine repump intensity, increased detuning of the MOT light, and a displacement of the optical dipole trap center with respect to the MOT. A factor of 2 increase in the number of loaded atoms is demonstrated by using a hyperfine repump beam with a shadow in it. In this way we load $8\ifmmode\times\else\texttimes\fi{}{10}^{6}{}^{85}\mathrm{Rb}$ atoms into a 1 mK deep optical dipole trap with a waist of $58 \ensuremath{\mu}\mathrm{m},$ which is 40% of the atoms initially trapped in the MOT.
We report on an investigation of loading characteristics of deep microscopic dipole traps. The dipole trap is loaded from a low density magneto optical trap (MOT) containing
≈5×106 atoms. We determine the loading parameters that maximize the trapped atom number for a trap of waist 5 μm with trap depths ranging from 3.5 mK to 10 mK. We determine the optimal trap loading conditions and the loading rates, loss coefficients and temperature of the trapped atoms under these conditions. We show that it is possible to load a few hundred to thousand atoms in dipole traps of depth 3.5 mK to 8.5 mK under the optimal loading conditions.
We have investigated the behavior of an optical dipole force trap realized using a mode-locked Nd:YAG laser and have compared performance to a continuous-wave (cw) trap built using the same laser but running in a cw mode. The traps are used to confine ultracold
85Rb atoms which are loaded from a magneto-optical trap (MOT). In most respects, the two traps behave similarly over a wide range of laser parameters provided that the average potential well depth is the same; however, there is a notable difference in the dipole trap loading efficiency dependence on the detuning of the MOT trap laser frequency during the loading stage.
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 $(\ensuremath{\approx}300\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K})$ and dense $(\ensuremath{\approx}9\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3})$ reservoir of metastable
3P2 atoms in a magneto-optic trap (triplet MOT). A focused
532nm laser beam produces a strongly elongated optical potential for
1S0 atoms with up to $350\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ well depth. A weak focused laser beam at
430nm, 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 ${10}^{7}\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. Limited by evaporation loss, in
200ms we typically load $5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms with $3.5\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ peak density and axial and radial temperatures of 300 and $85\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$, respectively. After terminating the loading we observe evaporation during
50ms, typically leaving us with
105 atoms with nearly equal radial and axial temperatures close to $40\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ and a peak density of $5\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. 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.
We report a method for loading cold atoms into an optical trap with high initial phase-space density (PSD). When the trap beam is overlapped with atoms in optical molasses of optimized parameters including large cooling beam detuning compared with conventional detuning used for a magneto-optical trap (MOT), more than $3\ifmmode\times\else\texttimes\fi{}{10}^{6}$ rubidium atoms with an initial temperature less than 20 $\ensuremath{\mu}\mathrm{K}$ are loaded into a single beam trap. The obtained maximum initial PSD is estimated to be $1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, which is one or two orders of magnitude greater than that achieved with the conventional loading into an optical trap from atoms in a MOT. The proposed method is promising for creating a quantum gas with a large number of atoms in a short evaporation time.
Here, we present our studies on intra-trap dynamics of an optical dipole trap (ODT) loaded from a Magneto-optical trap (MOT) of
87Rb atoms. A spatio-temporal Monte Carlo (MC) simulation approach has been employed in conjunction with the semi-classical theory based calculations for estimation of various intra-trap two-body loss parameters in the ODT. The temporal MC method based on estimated parameters from spatial MC, gives a close estimation of the experimentally observed atom-number evolution in ODT. We also find that, the decay rate of atoms in an ODT is dominated by momentum-transfer elastic collisions in the absence of MOT beams. However, in presence of MOT beams, the radiative escape process is the dominant two-body loss mechanism in the trap, which surpasses the fine-structure changing and hyperfine changing collisional losses by nearly an order of magnitude.