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arXiv:physics/0201068 · New Journal of Physics

Guiding of cold atoms by a red-detuned laser beam of moderate power

B. T. Wolschrijn, R A Cornelussen, R. J. C. Spreeuw, H. B. van Linden van den Heuvell

Published Sep 27, 2002• 14 citations
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Abstract

We report measurements on the guiding of cold 87 Rb atoms from a magneto-optical trap by a continuous light beam over a vertical distance of 6.5 mm. For moderate laser power (<85 mW) we are able to capture around 40% of the cold atoms. We systematically study the guided fraction as a function of laser power and detuning, and give an analytical expression that agrees well with our results. Although the guide is red detuned, the optical scattering rate at this detuning (≈70 GHz) is acceptably low. For lower detuning (<30 GHz) a larger fraction is guided but radiation pressure starts to push the atoms upward, effectively lowering the acceleration due to gravity.

DOI
10.1088/1367-2630/4/1/369

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We report measurements on the guiding of cold 87 Rb atoms from a magneto-optical trap by a continuous light beam over a vertical distance of 6.5 mm. For moderate laser power (<85 mW) we are able to capture around 40% of the cold atoms. We systematically study the guided fraction as a function of laser power and detuning, and give an analytical expression that agrees well with our results. Although the guide is red detuned, the optical scattering rate at this detuning (≈70 GHz) is acceptably low. For lower detuning (<30 GHz) a larger fraction is guided but radiation pressure starts to push the atoms upward, effectively lowering the acceleration due to gravity.
We report measurements on the guiding of cold 87Rb atoms from a magneto-optical trap by a continuous light beam over a vertical distance of 6.5 mm. For moderate laser power (<85 mW) we are able to capture around 40% of the cold atoms. We systematically study the guided fraction as a function of laser power and detuning, and give an analytical expression that agrees well with our results. Although the guide is red detuned, the optical scattering rate at this detuning (≈70 GHz) is acceptably low. For lower detuning (<30 GHz) a larger fraction is guided but radiation pressure starts to push the atoms upward, effectively lowering the acceleration due to gravity.
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.
We demonstrate an experimental setup for the production of a beam source of cold 87Rb atoms. The atoms are extracted from a trapped cold atomic cloud in an unbalanced three-dimensional magneto—optical trap. Via a radiation pressure difference generated by a specially designed leak tunnel along one trapping laser beam, the atoms are pushed out continuously with low velocities and a high flux. The most-probable velocity in the beam is varied from 9 m/s to 19 m/s by varying the detuning of the trapping laser beams in the magneto—optical trap and the flux can be tuned up to 4×109 s−1 by increasing the intensity of the trapping beams. We also present a simple model for describing the dependence of the beam performance on the magneto—optical trap trapping laser intensity and the detuning.
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.
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.
We have guided cold (85)Rb atoms in a blue-detuned, high-order hollow laser beam generated by a binary spatial light modulator. The binary holograms written to the modulator produce smooth hollow laser beams with steep intensity walls that can be updated with a 1.6 kHz refresh rate. We analyze the guiding laser beams numerically and experimentally, and show that the atoms are exposed to an average intensity that is ~2% of the maximum intensity of the guide at a laser detuning of 1 GHz and 2 mW of optical power.