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Physical Review A

Suppression of photon rescattering due to spatial anisotropy in a cold atomic gas

Mukund Vengalattore, Wilbert Rooijakkers, Richard Conroy, Mara Goff Prentiss

Published Jun 30, 2003• 16 citations
Publisher

Abstract

Photon rescattering in anisotropic clouds of laser cooled atoms is investigated by studying the amplification of a weak probe beam propagating through the atoms. In the case where the atomic ensemble is optically thick in all directions, the amplification of the probe is rapidly saturated due to multiple scattering of spontaneous photons. As the aspect ratio of the cloud is increased, the amplification converges to the values predicted by Mollow's theory indicating a suppression of light reabsorption.

DOI
10.1103/physreva.67.063412

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Photon rescattering in anisotropic clouds of laser cooled atoms is investigated by studying the amplification of a weak probe beam propagating through the atoms. In the case where the atomic ensemble is optically thick in all directions, the amplification of the probe is rapidly saturated due to multiple scattering of spontaneous photons. As the aspect ratio of the cloud is increased, the amplification converges to the values predicted by Mollow's theory indicating a suppression of light reabsorption.
Reabsorption, the multiple scattering of spontaneously emitted photons in optically thick gases, is a major limitation to efficient optical pumping and laser cooling in ultracold gases. We report mitigation of reabsorption using spatial and frequency modulation of laser light illuminating such gases. We developed a semiclassical model that successfully describes the reabsorption process when frequency-modulated light is present. It was necessary to extend the treatment in the model beyond a simple two-atom picture in order to reproduce our experimental results.
We measure the coherent scattering of light by a cloud of laser-cooled atoms with a size comparable to the wavelength of light. By interfering a laser beam tuned near an atomic resonance with the field scattered by the atoms, we observe a resonance with a redshift, a broadening, and a saturation of the extinction for increasing atom numbers. We attribute these features to enhanced light-induced dipole-dipole interactions in a cold, dense atomic ensemble that result in a failure of standard predictions such as the "cooperative Lamb shift". The description of the atomic cloud by a mean-field model based on the Lorentz-Lorenz formula that ignores scattering events where light is scattered recurrently by the same atom and by a microscopic discrete dipole model that incorporates these effects lead to progressively closer agreement with the observations, despite remaining differences.
The phase space density of dense, cylindrical clouds of atoms in a 2D magneto-optic trap is investigated. For a large number of trapped atoms (>10(8)), the density of a spherical cloud is limited by photon reabsorption. However, as the atom cloud is deformed to reduce the radial optical density, the temperature of the atoms decreases due to the suppression of multiple scattering leading to an increase in the phase space density. A density of 2 x 10(-4) has been achieved in a magneto-optic trap containing 2 x 10(8) atoms.
We study the emergence of collective scattering in the presence of dipole-dipole interactions when we illuminate a cold cloud of rubidium atoms with a near-resonant and weak intensity laser. The size of the atomic sample is comparable to the wavelength of light. When we gradually increase the number of atoms from 1 to ~450, we observe a broadening of the line, a small redshift and, consistently with these, a strong suppression of the scattered light with respect to the noninteracting atom case. We compare our data to numerical simulations of the optical response, which include the internal level structure of the atoms.