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arXiv:0811.0410 · Physical Review A

Influence of optical molasses in loading a shallow optical trap

Mathew Hamilton, Anthony Gorges, Jacob Roberts

Published Jan 21, 2009• 6 citations
arXivHTMLPDF

Abstract

We have examined loading of $^{85}\mathrm{Rb}$ atoms into a shallow far-off-resonance trap (FORT) from an optical molasses stage following a magneto-optical trap (MOT) stage and compared it to loading from a MOT stage only. Substantially more atoms could be loaded into the FORT using an optical molasses. To determine why this was the case, we measured the rate of atoms loaded into the FORT and the losses from the FORT during both loading processes over a range of detunings and hyperfine pump powers. We found that the losses induced during MOT loading were essentially the same as the losses induced during molasses loading and decreased with increased red detuning; however, we found that the differences in load rate caused an optical molasses to optimize at a greater detuning, improving the number of atoms we could trap by a factor of 2 over that of optimal loading from a MOT.

DOI
10.1103/physreva.79.013418

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We have examined loading of 85Rb^{85}\mathrm{Rb}85Rb atoms into a shallow far-off-resonance trap (FORT) from an optical molasses stage following a magneto-optical trap (MOT) stage and compared it to loading from a MOT stage only. Substantially more atoms could be loaded into the FORT using an optical molasses. To determine why this was the case, we measured the rate of atoms loaded into the FORT and the losses from the FORT during both loading processes over a range of detunings and hyperfine pump powers. We found that the losses induced during MOT loading were essentially the same as the losses induced during molasses loading and decreased with increased red detuning; however, we found that the differences in load rate caused an optical molasses to optimize at a greater detuning, improving the number of atoms we could trap by a factor of 2 over that of optimal loading from a MOT.
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.
Examination of loading the isotopes 85^{85}85Rb and 87^{87}87Rb simultaneously into a shallow far-off-resonance trap (FORT) has revealed an unexpected decrease in maximum atom number loaded as compared to loading either isotope alone. The simultaneous loading of the FORT will be affected by additional homonuclear and heteronuclear light-assisted collisional losses. However, these losses are measured and found to be insufficient to explain the observed drop in total number of atoms loaded into the FORT. We find that our observations are consistent with a decrease in loading rate caused by inter-isotope disruptions of the efficient laser cooling required to load atoms into the optical trap.
We investigate the properties of the focused-beam magneto-optical trap (FBT) of Rb 85 atoms, which provides more optical accessibility in contrast to typical magneto-optical trap (MOT). We have characterised the relations between temperature, number and density of trapped atoms versus trap parameters. In this paper, we discuss some distinctive features of the FBT comparing to those of standard MOTs, and determine a suitable condition in which loading atoms into a far-off-resonance optical dipole trap (FORT) is possible. Although the trap can capture fewer atoms comparing to the standard MOTs, due to its asymmetric geometry, it may offer an alternative trap configuration for preparing single atom.
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.
We have studied the effects of loading 87^{87}87Rb into a far off resonant trap (FORT) in the presence of an ultracold cloud of 85^{85}85Rb. The presence of the 85^{85}85Rb resulted in a marked decrease of the 87^{87}87Rb load rate. This decrease is consistent with a decrease in the laser cooling efficiency needed for effective loading. While many dynamics which disrupt loading efficency arise when cooling in a dense cloud of atoms (reabsorption, adverse optical pumping, etc.), the large detuning between the transitions of 85^{85}85Rb and 87^{87}87Rb should isolate the isotopes from these effects. For our optical molasses conditions we calculate that our cooling efficiencies require induced ground-state coherences. We present data and estimates which are consistent with heteronuclear long-ranged induced dipole-dipole collisions disrupting these ground state coherences, leading to a loss of optical trap loading efficiency.
We report the studies on the effect of Zeeman slower beam power on the loading rate and collision loss rate in an atomic beam loaded krypton magneto-optical trap (MOT). The results show that an increase in Zeeman slower beam power initially increases the MOT loading rate and reduces the background collision loss rate to increase the number of cold atoms in the MOT to an optimum value. With further increase in the Zeeman slower beam power, the number of cold atoms in the MOT decreases due to increased background collision loss rate and decrease in the trap loading rate. However, the cold collision loss rate is observed to remain unaffected by the variation in the Zeeman slower beam power. Therefore, the study emphasizes the need to optimize the Zeeman slower beam power to trap maximum number of cold atoms in an atomic beam loaded MOT.