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arXiv:1810.03147 · Physical Review Letters

Directional Bistability and Nonreciprocal Lasing with Cold Atoms in a Ring Cavity

Balázs Megyeri, Graeme Harvie, Andreas Lampis, J. Goldwin

Published Oct 19, 2018• 25 citations
arXivHTMLPDF

Abstract

We demonstrate lasing into counterpropagating modes of a ring cavity using a gas of cold atoms as a gain medium. The laser operates under the usual conditions of magneto-optical trapping with no additional fields. We characterize the threshold behavior of the laser and measure the second-order optical coherence. The laser emission exhibits directional bistability, switching randomly between clockwise and counterclockwise modes, and a tunable nonreciprocity is observed as the atoms are displaced along the cavity axis.

DOI
10.1103/physrevlett.121.163603

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We demonstrate lasing into counterpropagating modes of a ring cavity using a gas of cold atoms as a gain medium. The laser operates under the usual conditions of magneto-optical trapping with no additional fields. We characterize the threshold behavior of the laser and measure the second-order optical coherence. The laser emission exhibits directional bistability, switching randomly between clockwise and counterclockwise modes, and a tunable nonreciprocity is observed as the atoms are displaced along the cavity axis.
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This thesis describes an experiment with a cloud of magneto-optically trapped Potassium-39 spatially overlapped with the modes of a high-finesse ring cavity. Optical gain was observed with the system in steady state, without any beams additional to the normal cooling and repump beams in a potassium-39 magneto-optical trap. This was found to operate on a Raman gain mechanism between atomic hyperfine ground states, which has not been seen under similar conditions with other atomic species. Lasing into the cavity modes was then studied, with a particular focus on aspects specific to the ring cavity geometry. This showed a directional bistability and non-reciprocity. The bistability can be described by existing theory, but the mechanism of the non-reciprocity is unknown, despite a detailed investigation. The second order coherence time of the system was explored below threshold, and demonstrates a strong dependence on gain, which may have applications in the detection of extremely small gains.
We experimentally study the coherence time of a below-threshold Raman laser in which the gain medium is a gas of magneto-optically trapped atoms. The second-order optical coherence exhibits photon bunching with a correlation time that is varied by two orders of magnitude by controlling the gain. Results are in good agreement with a simple analytic model that suggests the effect is dominated by gain, rather than dispersion, in this system. Cavity ring-down measurements show the photon lifetime, related to the first-order coherence time, is also increased.
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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The interaction of laser cooled atoms with resonant light is determined by the natural linewidth of the excited state. An optical cavity is another optically resonant system where the loss from the cavity determines the resonant optical response of the system. The near resonant combination of an optical Fabry-Pérot cavity with laser cooled and trapped atoms couples two distinct optical resonators via light and has great potential for precision measurements and the creation of versatile quantum optics systems. Here we show how driven magneto-optically trapped atoms in collective strong coupling regime with the cavity leads to lasing at a frequency red detuned from the atomic transition. Lasing is demonstrated experimentally by the observation of a lasing threshold accompanied by polarization and spatial mode purity, and line-narrowing in the outcoupled light. Spontaneous emission into the cavity mode by the driven atoms stimulates lasing action, which is capable of operating as a continuous wave laser in steady state, without a seed laser. The system is modeled theoretically, and qualitative agreement with experimentally observed lasing is seen. Our result opens up a range of new measurement possibilities with this system.