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Europhysics Letters (EPL)

Operation of a “Cold-Atom Laser” in a Magneto-Optical Trap

Laurent Hilico, Claude Fabre, E. Giacobino

Published Apr 15, 1992• 48 citations
Publisher

Abstract

We have observed a laser oscillation when a cloud of magneto-optically trapped cesium atoms is inserted in an optical cavity. The laser oscillation occurs at a frequency which is detuned from the trapping beam frequency by 150 kHz. The gain mechanism is due to stimulated Raman processes recently observed in such media.
DOI
10.1209/0295-5075/18/8/004

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We demonstrate enhancement in the number of trapped cesium atoms in a magneto-optical trap (MOT) using a control laser that illuminates only a small faction of the capture region of the trap without interacting with the cold cloud of atoms. The enhancement is maximized when the laser is slightly blue detuned with respect to the cooling transition. Trap loading curves point to approximately a twofold increase in the capture rate, which as a consequence results in the increase in the steady state number of trapped atoms. Enhanced loading is confirmed by MOT loading and decay curves taken under the modulation of the control laser beam. Optical pumping of the inaccessible Zeeman states into the stretched states is suggested as a possible mechanism.
The results of characterization of cold cloud of cesium (Cs) atoms trapped in a magneto-optical trap (MOT) have been presented. The MOT is a part of the Physics package of the Cs Fountain Clock being developed at National Physical Laboratory (NPL), India. The number of atoms, size and temperature of the cloud have been measured and calculated. It is also been investigated how the number of trapped atoms changes with the trapping laser’s beam intensity, detuning and magnetic-field gradient of the trap.
Absorption spectra of cold Cs atoms in a magneto-optical trap under the influence of a strong external pump laser field with arbitrary detuning and intensity have been systematically investigated. The spectra featured the Rabi sidebands of each transition, and their behavior was nicely fitted by the dressed-atom theory. A pronounced dispersion-type profile with sub-natural linewidth at the pump frequency may be attributable to the stimulated Raman process by sublevels that are lightshifted and optically pumped by the pump laser.
A magneto-optical trap of cesium atoms was generated by applying a circularly polarized cooling laser beam onto a reflective two-dimensional diffraction grating with an aperture and by retroreflecting the incident beam passing through the aperture while reversing the circular polarization. The cooling laser beams comprised the incident, retroreflected, and four diagonally diffracted beams at an angle of 50° with respect to the normal direction of the grating surface. The intensity of the retroreflected beam was carefully adjusted to balance the radiation forces acting on the atoms. A significant number of cold atoms (7.0 × 106) were captured, in spite of the difficulty in a magneto-optical trap of cesium atoms, using nonorthogonal cooling beams owing to the high nuclear spin. The significance of the retroreflected beam in the trapping process was highlighted when the intensity of the retroreflected beam was reduced, resulting in the absence of trapped atoms. Notably, the cold atom cloud was generated near the edge of the region, where all the cooling beams were overlapped. The phenomenon is explained by the numerical calculations of the radiation forces considering all the Zeeman sublevels in the cooling transition and the Gaussian intensity profile of the incident beam.
We study optical gain in a gas of cold 39K^{39}\mathrm{K}39K atoms. The gain is observed during operation of a conventional magneto-optical trap without the need for additional fields. Measurements of transmission spectra from a weak probe show that the gain is due to stimulated Raman scattering between hyperfine ground states. The experimental results are reproduced by a simplified six-level model, which also helps explain why such gain is not observed in similar experiments with rubidium or cesium.