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arXiv:0804.0109 · Physical Review Letters

Mechanisms for Lasing with Cold Atoms as the Gain Medium

William Guerin, Franck Michaud, Robin Kaiser

Published Aug 29, 2008• 55 citations
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Abstract

We realize a laser with a cloud of cold rubidium atoms as gain medium, placed in a low-finesse cavity. Three different regimes of laser emission are observed corresponding, respectively, to Mollow, Raman, and four-wave mixing mechanisms. We measure an output power of up to 300 microW and present the main properties of these different lasers in each regime.

DOI
10.1103/physrevlett.101.093002

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We realize a laser with a cloud of cold rubidium atoms as gain medium, placed in a low-finesse cavity. Three different regimes of laser emission are observed corresponding, respectively, to Mollow, Raman, and four-wave mixing mechanisms. We measure an output power of up to 300 microW and present the main properties of these different lasers in each regime.
We demonstrate a Raman laser using cold (87)Rb atoms as the gain medium in a high-finesse optical cavity. We observe robust continuous wave lasing in the atypical regime where single atoms can considerably affect the cavity field. Consequently, we discover unusual lasing threshold behavior in the system causing jumps in lasing power, and propose a model to explain the effect. We also measure the intermode laser linewidth, and observe values as low as 80 Hz. The tunable gain properties of this laser suggest multiple directions for future research.
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While conventional lasers are based on gain media with three or four real levels, unconventional lasers including virtual levels and two-photon processes offer new opportunities. We study lasing that involves a two-photon process through a virtual lower level, which we realize in a cloud of cold ytterbium atoms that are magneto-optically trapped inside a cavity. We pump the atoms on the narrow $^{1}S_{0}\ensuremath{\leftrightarrow}^{3}P_{1}$ line and generate laser emission on the same transition. Lasing is verified by a threshold behavior of output power vs pump power and atom number, a flat g(2){g}^{(2)}g(2)-correlation function above threshold, and the polarization properties of the output. In the proposed lasing mechanism the trapping beams create the virtual lower level of the lasing transition. The laser process runs continuously, needs no further repumping, and might be adapted to other atoms or transitions such as the ultranarrow $^{1}S_{0}\ensuremath{\leftrightarrow}^{3}P_{0}$ clock transition in ytterbium.
Atoms can scatter light and they can also amplify it by stimulated emission. From this simple starting point, we examine the possibility of realizing a random laser in a cloud of laser-cooled atoms. The answer is not obvious as both processes (elastic scattering and stimulated emission) seem to exclude one another: pumping atoms to make them behave as amplifier reduces drastically their scattering cross-section. However, we show that even the simplest atom model allows the efficient combination of gain and scattering. Moreover, supplementary degrees of freedom that atoms offer allow the use of several gain mechanisms, depending on the pumping scheme. We thus first study these different gain mechanisms and show experimentally that they can induce (standard) lasing. We then present how the constraint of combining scattering and gain can be quantified, which leads to an evaluation of the random laser threshold. The results are promising and we draw some prospects for a practical realization of a random laser with cold atoms.