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Register a new userLoading and Cooling in an Optical Trap via Hyperfine Dark States
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We present a novel optical cooling scheme that relies on hyperfine dark states to enhance loading and cooling atoms inside deep optical dipole traps. We demonstrate a seven-fold increase in the number of atoms loaded in the conservative potential with strongly shifted excited states. In addition, we use the energy selective dark-state to efficiently cool the atoms trapped inside the conservative potential rapidly and without losses. Our findings open the door to optically assisted cooling of trapped atoms and molecules which lack the closed cycling transitions normally needed to achieve low temperatures and the high initial densities required for evaporative cooling.
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One important factor which determines efficiency of loading cold atoms into an optical dipole trap from a magneto-optical trap is the distance between the trap centers. By studying this efficiency for various optical trap depths (2--110 mK) we find that for optimum dipole trap loading, longitudinal displacements up to 15 mm are necessary. An explanation for this observation is presented and compared with other work and a simple analytical formula is derived for the optimum distance between the trap centers.
In recent years, cold atoms could prove their scientific impact not only on ground but in microgravity environments such as the drop tower in Bremen, sounding rockets and parabolic flights. We investigate the preparation of cold atoms in an optical dipole trap, with an emphasis on evaporative cooling under microgravity. Up to $ 1times10^{6} $ rubidium-87 atoms were optically trapped from a temporarily dark magneto optical trap during free fall in the droptower in Bremen. The efficiency of evaporation is determined to be equal with and without the effect of gravity. This is confirmed using numerical simulations that prove the dimension of evaporation to be three-dimensional in both cases due to the anharmonicity of optical potentials. These findings pave the way towards various experiments on ultra-cold atoms under microgravity and support other existing experiments based on atom chips but with plans for additional optical dipole traps such as the upcoming follow-up missions to current and past spaceborne experiments.
We simultaneously trap ultracold lithium and cesium atoms in an optical dipole trap formed by the focus of a CO$_2$ laser and study the exchange of thermal energy between the gases. The cesium gas, which is optically cooled to $20 mu$K, efficiently decreases the temperature of the lithium gas through sympathetic cooling. The measured cross section for thermalizing $^{133}$Cs-$^7$Li collisions is $8 times 10^{-12}$ cm$^2$, for both species in their lowest hyperfine ground state. Besides thermalization, we observe evaporation of lithium purely through elastic cesium-lithium collisions (sympathetic evaporation).
We have examined loading of 85Rb atoms into a shallow Far-Off-Resonance Trap (FORT) from an optical molasses and compared it to loading from a Magneto-Optical Trap (MOT). We found that substantially more atoms could be loaded into the FORT via an optical molasses as compared to loading from the MOT alone. To determine why this was the case, we measured the rate of atoms loaded into the FORT and the losses from the FORT during the loading process. For both MOT and molasses loading, we examined atom load rate and losses over a range of detunings as well as hyperfine pump powers. We found that the losses induced during MOT loading were essentially the same as the losses induced during molasses loading at the same MOT/molasses detuning. In contrast, load rate of the molasses was higher than that of a MOT at a given detuning. This caused the optical molasses to be able to load more atoms than the MOT. Optimization of FORT loading form an optical molasses improved the number of atoms we could trap by a factor of two over that of optimal loading from a MOT.
We study optical gain in a gas of cold 39K atoms. The gain is observed during operation of a conventional magneto-optical trap without the need for additional fields. Measurements of transmission spectra from a weak probe show that the gain is due to stimulated Raman scattering between hyperfine ground states. The experimental results are reproduced by a simplified six-level model, which also helps explain why such gain is not observed in similar experiments with rubidium or cesium.
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