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Physical Review A

Velocity diffusion and radiation trapping force in a one-dimensional expansion of cold atomic clouds in a magneto-optical trap

Swarupananda Pradhan, Y. S. Mayya, B. N. Jagatap

Published Sep 18, 2007• 3 citations
Publisher

Abstract

We experimentally investigate one-dimensional (1D) expansion of a cold cloud of cesium atoms in orthogonal 2D configuration of near resonant laser beams by temporally modulating a pair of counterpropagating trapping beams of a magneto-optical trap (MOT). The cloud is observed to undergo ballistic expansion followed by superballistic explosive growth due to the fluctuations of the 2D radiation force. A model based on the theory of Brownian motion is developed and a comparison of experiments with theory is shown to provide a direct measure of the velocity diffusion coefficient. We also observe sudden contraction of the cloud immediately after switching off the pair of trapping beams, which provides direct evidence for the existence of the radiation trapping force in a MOT.

DOI
10.1103/physreva.76.033407

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We experimentally investigate one-dimensional (1D) expansion of a cold cloud of cesium atoms in orthogonal 2D configuration of near resonant laser beams by temporally modulating a pair of counterpropagating trapping beams of a magneto-optical trap (MOT). The cloud is observed to undergo ballistic expansion followed by superballistic explosive growth due to the fluctuations of the 2D radiation force. A model based on the theory of Brownian motion is developed and a comparison of experiments with theory is shown to provide a direct measure of the velocity diffusion coefficient. We also observe sudden contraction of the cloud immediately after switching off the pair of trapping beams, which provides direct evidence for the existence of the radiation trapping force in a MOT.
Expansion of a cold cloud of cesium atoms trapped in a magneto-optical trap is studied by switching off a pair of counter propagating trapping beams. This corresponds to 1-D expansion of the cold cloud in presence of near resonant laser beams that are orthogonal to the expansion direction. Fluorescence imaging of the expanding cloud shows that the dynamics of expansion is complex and includes three distinctive temporal features- initial contraction followed by ballistic expansion and eventually sputtering of the cloud to leave a core that exhibits damped expansion. These are respectively related to the reduction in the repulsive radiation trapping force, initial temperature of the cold cloud and the stochastic heating arising from the random nature of the photon absorption and emission processes.
We discuss a simple time of flight technique for measurement of temperature of a cold cloud in a magneto-optical trap (MOT). The technique is based on spatiotemporal fluorescence imaging of the cloud that is allowed to undergo one-dimensional expansion in the presence of the orthogonal two-dimensional configuration of laser beams by temporal modulation of a pair of counterpropagating trapping beams in the MOT. We show that, in the time scale 0< or =t<5 ms, the expansion of the cloud is ballistic and the temperature can be extracted from the time variation of the rms size of the cloud in the expansion direction. The reliability of the technique has been established by comparing the results with release and recapture method, and also by fitting them to the known temperature scaling law.
A single-particle kinetic theory of the magneto-optical trap (MOT) for cold atoms is developed for a multilevel dipole interaction scheme. The theory analyzes the one-dimensional motion of (3+5)(3+5)(3+5)-level atoms in an inhomogeneous magnetic field and in the field of two counterpropagating circularly polarized laser waves. It is shown that in multilevel interaction schemes two-photon and generally even-order multiphoton processes are responsible for the formation of the narrow velocity and spatial structures in the radiation pressure force. The structures are located at zero velocity and near the origin of the trap. It is found that in the case of a (3+5)(3+5)(3+5)-level atom, single- and two-photon processes produce the two-component structure of the MOT potential well. Near the bottom of a conventional spatially broad potential well caused by one-photon processes, there is a much narrower well caused by two-photon processes. The double-structure MOT potential well is shown to be responsible for the two-component spatial distribution of the trapped atomic cloud. It is proposed that tight confinement of atoms near the narrow bottom of the MOT potential well can be used for the creation of a pointlike source of cold atoms for various applications in atom optics.
We present the observation of double stable clouds of cold trapped atoms in a magneto-optical trap and its explanation with a simple model based on radiative forces. This structure occurs for a specific misalignment of the trapping laser beams whose Gaussian profiles introduce an extra term in the equation of motion for the trapped atom. We were able to obtain simple analytical expressions for the trap potential and for the separation of the clouds, and carried out a numerical calculation which agrees well with the experimental observations.
We have successfully trapped sp7\\sp7sp7Li in a vapor cell magneto-optical trap (MOT). The trap is studied in detail and the parameters are measured from the experiment. The trapping mechanism has been understood in terms of the radiation force and a simple one dimensional model is used in interpreting and predicting the experimental results. The cold-cold atom collision rate coefficient is measured in a beam MOT. The two collision mechanisms, fine structure changing and radiative escape, are discussed in a molecular picture. One of the channels, the fine structure changing channel, can be turned on and off by the experimental conditions, namely the laser intensity in this experiment. The rate coefficient for each mechanism has been determined from the experiment.