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Journal of Physics B Atomic Molecular and Optical Physics

A phenomenological model for collisional coherence transfer in an optically pumped atomic system

K. Khanbekyan, G. Bevilaqua, Alen Khanbekyan, E. Mariotti, A. Papoyan, L. Moi

Published Feb 18, 2011• 0 citations
Publisher

Abstract

Abstract\nWe consider a dual ?-system under double laser excitation to investigate the possibility of indirect coherence transfer between atomic ground states through an excited state. The atomic system is excited by a frequency modulated pump laser and probed by a low-power cw laser. All the decoherence mechanisms are discussed and taken into account. Adjustment of parameters of the two radiations aimed at maximization of coherence transfer is addressed. The study can help to understand phenomena as collisional transfer of coherence and can find application in the experimental realization of atomic sensors.

DOI
10.1088/0953-4075/44/5/055502

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Abstract\nWe consider a dual ?-system under double laser excitation to investigate the possibility of indirect coherence transfer between atomic ground states through an excited state. The atomic system is excited by a frequency modulated pump laser and probed by a low-power cw laser. All the decoherence mechanisms are discussed and taken into account. Adjustment of parameters of the two radiations aimed at maximization of coherence transfer is addressed. The study can help to understand phenomena as collisional transfer of coherence and can find application in the experimental realization of atomic sensors.
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The emergence of laser action from an atomic system without population inversion is beginning to attract theoretical and experimental attention as a possible way of generating useful coherent radiation at unusual wavelengths. The goal of this communication is to clarify the physical origin of this phenomenon using a four-level system whose ground state consists of two quasidegenerate levels that are coupled to a higher-lying state by a Raman-like transition. In the presence of the ground-state coherence induced by the Raman field, we calculate the absorption spectrum of a weak probe whose tunable frequency scans across the transition frequencies between the two ground-state levels and another higher-lying state that may or may not be weakly excited by an incoherent pump mechanism. Our calculation, based on a dressed atomic state approach, clarifies the conditions that must be satisfied to produce gain, identifies the frequency (or frequencies) at which the gain is maximum, and elucidates the functions of the incoherent pump, the Raman field, and the coherence between the ground-state levels in this process. An appropriate set of Maxwell- Bloch equations for a ring-cavity model of this system confirms the appearance of laser action when the small-signal gain conditions are satisfied.
In real atoms, levels connected by a dipole transition contain a number of degenerate Zeeman states. Thus in practice, the phenomena of optical pumping and collisional transfer amongst states severely restrict the application of a simple two-state model. There are a number of cases, however, where the macroscopic dipole induced in an atomic gas has the same general form as predicted by the two-state model, and these are presented here. The effects of optical pumping and collisional transfer, and the implications for the description of optical bistability, are discussed. In particular, ground-state optical pumping can, under certain conditions of low perturber density, considerably reduce the power necessary to achieve bistability.
We analyze a model where a closed VVV system is excited by two uncorrelated partially coherent fields. We use a collisionally broadened cw laser, which is a good model for an experimentally realizable partially coherent field, and show that it is possible to generate excited-state coherences even if the two fields are uncorrelated. This transient coherence can be increased if splitting between the excited states is reduced relative to the radiation coherence time ${\ensuremath{\tau}}_{d}$. For small excited-state splitting, one can use this scheme to generate a long-lived coherent response in the system.
We have investigated atomic coherence changes caused by optical pumping applied to electromagnetically induced absorption (EIA) occurring at the open system, which consists of the transition between the F g = 2 ground state and the F e = 3 excited state in the 85 Rb D1-line. For optical pumping, another laser resonantly tuned to the transition between the F g = 3 ground state and the F e = 2 excited state was used. We found that the atomic coherence for EIA can be changed not by increasing the pumping rate but by pumping differently populations into the Zeeman sublevels in the F g = 2 ground state, which is realized by changing the polarization of the pumping laser. The experimental results were able to be analysed by density matrix equations in the simple model system.