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arXiv:physics/0607215 · Physical Review A

Suppression of reabsorption in ultracold gases via modulation of light

Anthony Gorges, Ansel Foxley, David M. French, Christopher M. Ryan, Jacob Roberts

Published May 3, 2007• 2 citations
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Abstract

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.

DOI
10.1103/physreva.75.053403

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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.
Resonant optical pumping in dense atomic media is discussed, where the absorption length is less than the smallest characteristic dimension of the sample. It is shown that reabsorption and multiple scattering of spontaneous photons (radiation trapping) can substantially slow down the rate of optical pumping. A very slow relaxation out of the target state of the pump process is then sufficient to make optical pumping impossible. As model systems an inhomogeneously and a radiatively broadened 3-level system resonantly driven with a strong broad-band pump field are considered.
Resonant optical pumping in dense atomic media is discussed, where the absorption length is less than the smallest characteristic dimension of the sample. It is shown that reabsorption and multiple scattering of spontaneous photons (radiation trapping) can substantially slow down the rate of optical pumping. A very slow relaxation out of the target state of the pump process is then sufficient to make optical pumping impossible. As model systems an inhomogeneously and a radiatively broadened 3-level system resonantly driven with a strong broad-band pump field are considered.
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 of spontaneously emitted photons, or radiation trapping, is a process that occurs when light interacts with optically thick media. It is shown, both theoretically and experimentally, that this effect in optically thick atomic vapour leads to a decrease in transmission of coherent laser radiation propagating under conditions of electromagnetically induced transparency (EIT). A simple theory is developed taking into account the radiation trapping, which is in a good agreement with the experimental observations and exact numerical simulation. This allows better understanding of the physics of EIT in general, and properties of dense coherent atomic media in particular.
There is a broad interest in enhancing the strength of light-atom interactions to the point where injecting a single photon induces a nonlinear material response. Here we show theoretically that sub-Doppler-cooled two-level atoms that are spatially organized by weak optical fields give rise to a nonlinear material response that is greatly enhanced beyond that attainable in a homogeneous gas. Specifically, in the regime where the intensity of the applied optical fields is much less than the off-resonance saturation intensity, we show that the third-order nonlinear susceptibility scales inversely with atomic temperature and, due to this scaling, can be two orders of magnitude larger than that of a homogeneous gas for typical experimental parameters. As a result, we predict that spatially bunched two-level atoms can exhibit single-photon nonlinearities. Our model is valid for all regimes of atomic bunching and simultaneously accounts for the backaction of the atoms on the optical fields. Our results agree with previous theoretical and experimental results for light-atom interactions that have considered only limited regimes of atomic bunching. For lattice beams tuned to the low-frequency side of the atomic transition, we find that the nonlinearity transitions from a self-focusing type to a self-defocusing type at a critical intensity. We also show that higher than third-order nonlinear optical susceptibilities are significant in the regime where the dipole potential energy is on the order of the atomic thermal energy. We therefore find that it is crucial to retain high-order nonlinearities to accurately predict interactions of laser fields with spatially organized ultracold atoms. The model presented here is a foundation for modeling low-light-level nonlinear optical processes for ultracold atoms in optical lattices.