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Physical Review A

Subcollisional linewidth observation in the coherent-population-trapping Rb maser

Aldo Godone, Filippo Levi, Salvatore Micalizio

Published Feb 28, 2002• 12 citations
Publisher

Abstract

We report the observation of the ${}^{87}\mathrm{Rb}$ 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.

DOI
10.1103/physreva.65.031804

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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 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.
Microwave emission has been observed at the ground-state hyperfine transition frequency of a cesium atomic vapor driven into a nonabsorbing state by means of coherent population trapping. The coherent emission observed is due to the oscillating magnetization generated by the coherence, which is induced between the ground-state hyperfine levels when they are coupled to an excited state by means of two laser radiations via a \ensuremath{\Lambda} scheme. The experiments described were done in a quartz cell containing buffer gases such as neon and nitrogen, reducing the linewidth through the Dicke effect. The cell was placed inside a microwave cavity tuned at 9.2 GHz, the ground-state hyperfine frequency of cesium, and a power output of the order of 100 fW was measured in the case where nitrogen was used as the buffer gas.
Coherent population trapping is studied in 87Rb{}^{87}\mathrm{Rb}87Rb in a buffer gas. Experimental results are reported on the contrast of the field independent hyperfine resonance line and on its linewidth. A physical model, which includes an optical pumping process that traps atoms in a Zeeman sublevel of the ground state, is developed. The model explains the general behavior of the experimental observations.
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.
We present the transformation in the Hanle configuration of the transmission that results from coherent population trapping (CPT) into the level-crossing absorption (LCA) that results from the single-photon optical pumping in the 87Rb^{87}\mathrm{Rb}87Rb D1{D}_{1}D1​ line of a Rb vapor cell with a Ne buffer gas when the polarization of the laser field is changed from linear to circular. The LCA spectrum, with a narrow spectral width of 2.42.42.4 mG (1.71.71.7 kHz), was observed in the ${F}_{g}\ensuremath{\rightarrow}{F}_{e}\ensuremath{\leqslant}{F}_{g}$ transition with the circularly polarized laser. This may be because the LCA is both related to the transverse magnetic field and the atom-laser interaction time resulting from diffusive atomic motion in the cell with the buffer gas. The CPT and LCA spectra were calculated numerically using the full density matrix equations for the relevant magnetic sublevels of the hyperfine levels, considering the residual magnetic fields perpendicular to laser propagation and the collision effects resulting from the buffer gas. There was good qualitative agreement between theoretical and experimental results.
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.