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Physical Review A

Coherent microwave emission in cesium under coherent population trapping

Aldo Godone, Filippo Levi, Jacques Vanier

Published Jan 1, 1999• 37 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.59.r12

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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.
According to the Λ -type three-level system consisting of cesium 6S1/2 (F=3 and F=4) long-lived ground states and 6P3/2 (F'=4) excited state, we experimentally investigate and theoretically analyze the parameters of coherent population trapping (CPT) spectra in a cesium atomic vapor cell with or without neon as buffer gas. The CPT of the full width at half maximum (FWHM), the signal amplitude as a function of optical intensity of the two phased-locked laser beams with a frequency difference of 9.19263177 GHz (the hyperfine splitting in the cesium ground state 6S1/2), relative intensity ratio, beam spot size, residual magnetic field of the magnetic shielding, and the influence of with and without the cesium vapor cell containing neon as buffer gas are investigated experimentally. With the optimized parameters, we obtain the CPT signal FWHM to be as narrow as ～ 340 Hz.
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 description of a new type of frequency standard based on coherent population trapping (CPT) in cesium is given. The phenomenon creates a magnetization oscillating at the hyperfine frequency. Microwave radiation emitted by such a magnetization is detected by means of a microwave cavity. The process takes place without population inversion. Results are given for continuous and pulsed operation. The possibility of using the CPT coherent microwave emission as the basis of a frequency standard is analyzed.
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.
Coherent population trapping (CPT) has been demonstrated as an interesting technique for miniature atomic frequency references [1,2] and quantum information. It is based on the coupling of the two hyperfine ground states of an alkali atom – namely cesium (133Cs) for atomic clocks – through excitation to a common atomic level by two phase-coherent laser fields nearly resonant with the atomic transitions. The frequency difference between the two laser fields is tuned at the atomic frequency splitting in the microwave range, equal to 9.192 GHz for 133Cs atoms. Outputs powers in the mW range and narrow-linewidth emission (<500 kHz) are required for the two laser beams.