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arXiv:physics/0306040 · Physical Review A

Observation of caustics in the trajectories of cold atoms in a linear magnetic potential

Wilbert Rooijakkers, Saijun Wu, Pierre Striehl, Mukund Vengalattore, Mara Goff Prentiss

Published Dec 19, 2003• 16 citations
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Abstract

We have studied the spatial and temporal dynamics of a cold atom cloud in the conservative force field of a ferromagnetic guide, after laser cooling has been switched off suddenly. We observe outgoing ``waves'' that correspond to caustics of individual trajectories of trapped atoms. This provides detailed information on the magnetic field, the energy distribution, and the spin states.

DOI
10.1103/physreva.68.063412

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We have studied the spatial and temporal dynamics of a cold atom cloud in the conservative force field of a ferromagnetic guide, after laser cooling has been switched off suddenly. We observe outgoing ``waves'' that correspond to caustics of individual trajectories of trapped atoms. This provides detailed information on the magnetic field, the energy distribution, and the spin states.
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We report the observation of a very high flux of ultra-cold bosonic chromium atoms in a magnetic guide. The beam is created by operating a magneto-optical trap/moving optical molasses within the magnetic field of the guide. A relative detuning between two pairs of the cooling lasers cools the atoms into a frame moving along the axes of the guide. When the atoms are cooled into a moving frame with a velocity of 6 m s −1 we observe a maximum of the flux of 6 × 10 9 atoms s −1 . For these parameters the transversal temperature of the atoms after a 25 fold increase of the confining magnetic potential is about 1.2 mK. The longitudinal temperature is 400 µK.
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We experimentally and numerically study the temporal dynamics of light scattered by large clouds of cold atoms after the exciting laser is switched off, in the low intensity (linear-optics) regime. Radiation trapping due to multiple scattering as well as subradiance lead to decay much slower than the single atom fluorescence decay. These two effects have already been observed separately, but the interplay between them remained to be understood. Here, we show that with well chosen parameters of the driving field, the two effects can occur at the same time, but follow different scaling behaviors. The subradiant decay is observed at late time and its rate is independent of the detuning, while the radiation trapping decay is observed at intermediate time and depends on the detuning through the optical depth of the sample. Numerical simulations based on random walk process and coupled-dipole equations support our interpretations. Our study clarifies the different interpretations and physical mechanisms at the origin of slow temporal dynamics of light in cold atoms.
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