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Chinese Physics Letters

Enhancement of Transfer Efficiency of Cold Atoms Using an Optical Guiding Laser Beam

Ke Li, Xiaorui Wang, Lingxiang He, Mingsheng Zhan, Baolong Lü

Published Aug 23, 2007• 3 citations
Publisher

Abstract

We transfer cold 87 Rb atoms from a vapour cell chamber to a spatially separated UHV magneto-optical trap (MOT) with the assistance of a red-detuned optical guiding beam and a normal push beam. Efficient optical guiding of the cold atoms is observed within a small detuning window. A pulsed optical guiding beam enhances the transfer efficiency and hence allows us to collect more atoms in UHV MOT in a shorter time, which is favourable for our experiment of achieving Bose–Einstein condensates (BEC). Besides the easy operation, another advantage of this optical guiding technique is also demonstrated such that slower atomic beams may be efficiently transferred along horizontal direction. This study is a direct application of the optical guiding technique as a powerful tool.

DOI
10.1088/0256-307x/24/9/027

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We transfer cold 87 Rb atoms from a vapour cell chamber to a spatially separated UHV magneto-optical trap (MOT) with the assistance of a red-detuned optical guiding beam and a normal push beam. Efficient optical guiding of the cold atoms is observed within a small detuning window. A pulsed optical guiding beam enhances the transfer efficiency and hence allows us to collect more atoms in UHV MOT in a shorter time, which is favourable for our experiment of achieving Bose–Einstein condensates (BEC). Besides the easy operation, another advantage of this optical guiding technique is also demonstrated such that slower atomic beams may be efficiently transferred along horizontal direction. This study is a direct application of the optical guiding technique as a powerful tool.
A novel scheme of transferring cold atoms in a double magneto-optical trap (MOT) system has been experimentally demonstrated. Cold cesium atoms trapped in a vapor-cell MOT are efficiently transferred to an ultrahigh-vacuum (UHV) MOT by a continuous-wave divergent Gaussian transfer laser beam. When large red detuning and moderate intensity are adopted for the transfer laser beam, enhancement of the recapturing of atoms in the UHV MOT is clearly observed. Using the divergent transfer laser beam (diameter of approximately 1.60 mm in the vapor-cell MOT region) with typical power of approximately 20.2 mW, up to approximately 85% of transfer efficiency is obtained when the frequency detuning is set to around -1.2 GHz, and it is not sensitive to small detuning variation. This transfer is much efficient compared with that in the case of continuous-wave near-resonance weak transfer laser beam (typical power of order of approximately 100 microW and typical frequency detuning of approximately-10 MHz) which is normally used in double-MOT experiment. The enhancement is ascribed to the guiding effect on cold atomic flux by transverse dipole potential of the large red-detuned transfer laser beam.
In our double magneto-optical trap (MOT) setup containing a vapor chamber MOT (VC-MOT) and an ultra high vacuum MOT (UHV-MOT) for 87^{87}87Rb atoms, we find that transfer of atoms from VC-MOT to UHV-MOT can be enhanced by employing a pulsed VC-MOT loading followed by a pulsed push beam, as compared to that obtained by focusing a continuous wave (CW) push beam on a continuously loaded VC-MOT. By choosing appropriately the VC-MOT duration and push beam duration, the number of atoms in UHV-MOT was $\sim$3-times the number obtained with a continuous VC-MOT and a CW push beam of optimized power. The processes affecting the pulsed transfer have been studied.
Summary form only given. We report optical guiding of cold atoms in a free-propagating far-detuned laser beam. The atoms are initially cooled and collected in a magneto-optical trap (MOT), which typically contains over 10/sup 8/ atoms in a cigar shaped cloud with an aspect ratio of 4. Approximately 20% of these atoms are loaded into a far-detuned optical dipole beam aligned vertically through the MOT. The number loaded is limited primarily by geometrical overlap. Both guided and unguided atoms are launched with a velocity of 1.25 m/s and a temperature of 20 /spl mu/K. The guided beam is focused through a 1 mm aperture, 6 cm above the MOT, which connects the vapour cell to a UHV chamber. Atoms can be launched within the guide without heating and guided into a UHV chamber with 100% efficiency. In continuous operation, the scheme allows extraction of atoms from a MOT without loss of phase space density.
The primary purpose of this study is to investigate the influence of the vertical guiding laser beam waist on cold atom guiding efficiency. In this study, a double magneto-optical trap (MOT) apparatus is used. With an unbalanced force in the horizontal direction, a cold atomic beam is generated by the first MOT. The cold atoms enter the second chamber and are then re-trapped and cooled by the second MOT. By releasing a second atom cloud, the process of transferring the cold atoms from MOT to the dipole trap, which is formed by a red-detuned converged 1064-nm laser, is experimentally demonstrated. And after releasing for 20 ms, the atom cloud is guided to a distance of approximately 3 mm. As indicated by the results, the guiding efficiency depends strongly on the laser beam waist; the efficiency reaches a maximum when the waist radius (w0) of the laser is in the range of 15 to 25 μm, while the initial atom cloud has a radius of 133 μm. Additionally, the properties of the atoms inside the dipole potential trap, such as the distribution profile and lifetime, are deduced from the fluorescence images.
We have established a cesium double magneto-optical trap(double MOT) system,and realized transferring cold atoms from the vapor-cell MOT to the ultra-high-vacuum(UHV) MOT by continuous transfer laser.On the base of our former work on transferring atoms by weak resonance laser,we extended red detuning of transfer beam to 2.0 GHz,and increased the laser power.We have studied on the transportation of cold atoms.It is demonstrated that the stronger laser power would make the transportation much more efficient if the detuning is far off-resonance.It is induced by the confinement and guidance of cold atoms due to the transverse dipole force of the transfer beam.