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Quantum Electronics and Laser Science Conference

Optical gain and loss spectra of driven degenerate two-level transitions

Thomas H. Loftus, Jason R. Bochinski, THOMAS W. MOSSBERG

Published Nov 13, 2002• 0 citations
Publisher

Abstract

Summary form only given. Studies of the classic problem of two non-degenerate atomic energy states exposed to an intense monochromatic driving field have enabled fundamental discoveries in quantum optics including inversionless optical gain, multiphoton laser action, and electromagnetically induced transparency. Nevertheless, the level degeneracy present in many atomic systems can and has been shown to play key roles in a variety of unique driven atom dynamics ranging from light-shift mediated stimulated Raman optical gain and lasing to electromagnetically induced absorption. To date, however, the potentially rich dynamics of these novel systems has received relatively little attention. Atomic barium (Ba) possesses a diversity of naturally abundant isotopes with varying degrees of level degeneracy, making it ideally suited to studies of degenerate two-level driven atom dynamics.

DOI
10.1109/qels.2001.961802

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Summary form only given. Studies of the classic problem of two non-degenerate atomic energy states exposed to an intense monochromatic driving field have enabled fundamental discoveries in quantum optics including inversionless optical gain, multiphoton laser action, and electromagnetically induced transparency. Nevertheless, the level degeneracy present in many atomic systems can and has been shown to play key roles in a variety of unique driven atom dynamics ranging from light-shift mediated stimulated Raman optical gain and lasing to electromagnetically induced absorption. To date, however, the potentially rich dynamics of these novel systems has received relatively little attention. Atomic barium (Ba) possesses a diversity of naturally abundant isotopes with varying degrees of level degeneracy, making it ideally suited to studies of degenerate two-level driven atom dynamics.
We show that continuous-wave, two colour excitation of three-level atoms opens new channels of irreversible population transfer between atomic levels. This process, which we describe as dynamically induced irreversibility, leads to unanticipated phenomena including the inversion of atomic transitions whose upper atomic level has no conventional source of irreversible population pumping. We also study the effect of excitation-field bandwidth on the gain exhibited by driven, three-level, V-type atomic systems. Gain, which may or may not be inversionless in the atomic-state basis, is produced through strong monochromatic driving of one atomic transition and weaker pumping of a coupled transition. It is found that monochromatic pumping frequently yields greater optical gain than incoherent (wide bandwidth) pumping. This is particularly true when system gain can be attributed to hidden inversions among the eigenstates (semi-classical dressed states) of the coupled atom+driving field quantum system. Last, we report the observation of continuous-wave lasing in a strongly driven barium cascade system and interpret its properties in terms of the models presented.
The radiative emission of strongly driven, two-level-like barium atoms into the modes of a confocal optical cavity is studied as a function of barium density. As the barium density is increased, the spatial profile, intensity, and spectrum of light emitted out the ends of the confocal cavity display dramatic changes indicative of a transition from fluorescence to lasing. In this novel two-level system, laser action can be explained in terms of an inversion between the dressed atom-field states or in terms of a type of stimulated hyper-Raman scattering in which atoms lase from the ground to the excited atomic state. A simple dressed-atom rate-equation analysis of this two-level-atom laser is presented. It is pointed out that this same system may provide a means of realizing two-photon-like laser action.
Summary form only given. Barium (Ba) is a versatile atom for basic quantum optical measurements because it possesses multiple transitions at conveniently accessible wavelengths which allow the creation of a variety of useful systems. This atom has been used as a three-level /spl Lambda/, cascade, and V system for driven atom and lasing without inversion experiments. Other work relied on the (6s/sup 2/)/sup 1/S/sub 0/-(6s6p)/sup 1/P/sub 1/ 553.5 nm transition as a two-level system for cavity quantum electrodynamics, two-photon laser, and single-atom laser experiments. However, these observations, which utilize natural Ba, are complicated by the presence of multiple isotopes with small frequency shifts. These isotopes introduce additional features and under certain conditions mask small structures, making interpretation of the results non-trivial.
It is well known that driven two-level atoms can exhibit optical amplification.1 This fact has led to the recent demonstration of single-photon lasing in cell and atomic-beam type ensembles of two-level atoms.2 We show that the same simple two-level system can also be utilized as a two- or more-photon amplifying medium. The underlying physics can be understood in terms of the atom field dressed-state picture. When driven by a field detuned from the atomic resonance, significant population inversion between the dressed-state doublets can be created. In the dressed picture, single-photon lasing occurs between inverted sublevels of adjacent dressed-state doublets. Transitions between nonadjacent doublets are still inverted, but require two or more photons. In the degenerate case, the multiphoton emission frequencies differ from the single-photon emission frequency and can hence be selectively enhanced using an optical cavity. Due to the peculiar structure of dressed levels, the multiphoton emissions are all connected through near resonant intermediate states. The multiphoton emissions can also be viewed as a stimulated hyper-Raman scattering of successively higher order.
We experimentally study the production of laser gain (phase-insensitive optical amplification) with and without atomic population inversion in a doubly driven three-level system in atomic barium. The external fields are found to create inversionless gain and to enhance gain available in the presence of inversion. Laser oscillation based on enhanced gain is observed. In analysis of the driven system, we identify an irreversible multiphoton process that is intrinsically supportive of gain in the absence of population inversion. \textcopyright{} 1996 The American Physical Society.
We consider a four-level atomic system with a degenerate or quasidegenerate ground-state doublet and two excited states. One of the upper levels is coupled to the ground doublet by a coherent field of variable strength, which we refer to as the Raman field, whose role is to create atomic coherence between the driven atomic states. The other higher lying state is coupled to the ground states when laser action develops. This model includes an incoherent pump mechanism to create some population in the top lasing state. In appropriate conditions, for the incoherent pump rate and the Raman field strength, the absorption profile of the laser transition develops gain features. We discuss the physical origin of this gain which we find to be the consequence of a population inversion between dressed atomic states. It is interesting to compare the results of this analysis with a similar study of a three-level lambda system which was previously recognized to yield lasing without inversion.1 In this case the appearance of gain can be attributed to the induced atomic coherence.