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Physical Review A

Propagation and density effects in the coherent-population-trapping maser

Aldo Godone, Filippo Levi, Salvatore Micalizio

Published Feb 4, 2002• 22 citations
Publisher

Abstract

The coherent microwave emission from an optically thick atomic ensemble in a cavity under coherent population trapping is analyzed. Transient and continuous operations are theoretically examined within the frame of a closed three-level system in the Dicke regime. The effects related to the atomic density and to the propagation in the active medium are examined with particular reference to the subnatural linewidth, the low group velocity and the shifts of the maser emission profile from the unperturbed atomic transition. The case of alkali-metal atoms submitted to a \ensuremath{\Lambda} excitation scheme is addressed in view of applications in the atomic frequency standard field. Experimental observations in agreement with the theoretical predictions are reported for the case of rubidium in a buffer gas. Apparent superluminal propagation is also reported and briefly discussed.

DOI
10.1103/physreva.65.033802

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The coherent microwave emission from an optically thick atomic ensemble in a cavity under coherent population trapping is analyzed. Transient and continuous operations are theoretically examined within the frame of a closed three-level system in the Dicke regime. The effects related to the atomic density and to the propagation in the active medium are examined with particular reference to the subnatural linewidth, the low group velocity and the shifts of the maser emission profile from the unperturbed atomic transition. The case of alkali-metal atoms submitted to a \ensuremath{\Lambda} excitation scheme is addressed in view of applications in the atomic frequency standard field. Experimental observations in agreement with the theoretical predictions are reported for the case of rubidium in a buffer gas. Apparent superluminal propagation is also reported and briefly discussed.
The propagation of a band-limited light pulse through an atomic medium under a \ensuremath{\Lambda} excitation scheme is theoretically analyzed in this paper. We consider in particular the case where the light pulse is detected through the coherent microwave emission of the atomic ensemble (coherent population trapping maser). Significant differences are predicted with respect to the more usual optical detection (electromagnetically induced transparency signal) and found in agreement with the experimental results. Higher signal delays were observed in the microwave emission than in the optical signal with an equivalent group velocity of 6 m/s as well as highly superluminal propagation under a proper modulation scheme. The experiments were performed with a thermal 87Rb{}^{87}\mathrm{Rb}87Rb vapor in buffer gas.
The phenomenon of coherent microwave stimulated emission in a cavity is analyzed for the case of alkali atoms such as Cs and Rb under coherent population trapping (CPT) in a \ensuremath{\Lambda} scheme. The calculations are made in transient and in continuous operation within the framework of a three level model. The coupling of the atoms to the microwave field generated inside the cavity by the atomic ensemble, is fully taken into account; it introduces radiation damping and an imbalance in the populations of the ground-state hyperfine levels, decreasing the intrinsic symmetry of the \ensuremath{\Lambda} scheme. The effect leads also to a broadening and to a frequency shift of the microwave emission profile. A comparison is made with the optically pumped alkali atom maser, allowing a deeper understanding of the CPT stimulated emission phenomenon.
The propagation of a band-limited light pulse through an atomic medium under a Λ excitation scheme is theoretically analyzed in this paper. We consider in particular the case where the light pulse is detected through the coherent microwave emission of the atomic ensemble (coherent population trapping maser). Significant differences are predicted with respect to the more usual optical detection (electromagnetically induced transparency signal) and found in agreement with the experimental results. Higher signal delays were observed in the microwave emission than in the optical signal with an equivalent group velocity of 6 m/s as well as highly superluminal propagation under a proper modulation scheme. The experiments were performed with a thermal 87Rb vapor in buffer gas.
The phenomenon of coherent population trapping in alkali-metal atoms is analyzed by means of a perturbation approach. Closed form transparent solutions are obtained for the coherences existing within the system and the populations of the ground levels and of the excited state. The presence of dark lines and coherent microwave emission from the ground state are made explicit. Experimental results that confirm the theoretical calculations are reported for the case of cesium in a buffer gas. Conclusions are drawn in connection with the application of coherent population trapping to the field of atomic frequency standards.
We report the observation of the 87Rb{}^{87}\mathrm{Rb}87Rb ground-state hyperfine transition with a subcollisional linewidth performed in a cell with buffer gas. The transition is observed as a coherent-population-trapping maser-emission profile excited by means of a \ensuremath{\Lambda} interaction scheme in the high-atomic-density regime. A theoretical explanation of the phenomenon is given in good agreement with the observed linewidth behavior.
The paper gives an overview of our present theoretical understanding of coherent population trapping in alkali metal atoms as used in the implementation of an atomic frequency standard. Two approaches are examined: The passive approach in which the microwave hyperfine resonance is detected directly on the transmitted radiation and the maser approach in which the same resonance is observed by means of stimulated emission in a microwave cavity. The theoretical analysis reviewed is supported by experimental results.