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Physical Review A

Multiphoton amplification processes and quantum-path interferences in a coherently driven atomic vapor

J. J. Fernández-Soler, J. L. Font, Ramon A. Vilaseca, Daniel J. Gauthier, Alexander M. Kul'minskii

Published Oct 24, 2003• 3 citations
Publisher

Abstract

We develop a theoretical model of two-photon amplification in laser-driven potassium atoms and use it to analyze the recent experiments reported by Pfister et al. [Phys. Rev. A 60, R4249 (1999)]. The model takes into account most of the essential factors influencing the amplification process, including the atomic hyperfine structure (which makes multiphoton emission possible) and the simultaneous interaction with intense drive and probe beams with arbitrary detunings. We determine the origin and analyze the properties of different multiphoton gain resonances that appear in the light-matter interaction. In particular, the influence of the drive and probe field amplitudes and detunings on the strength and frequency of the two-photon amplification resonance is studied in detail, showing clearly the differences with respect to the behavior of single-photon or other multiphoton amplification processes. In addition, we investigate interferences between different quantum pathways originating from the hyperfine structure and determine the conditions under which they can enhance or suppress multiphoton resonances. The predictions of the model are in good agreement with the observations, indicating that it can be used to understand recent experiments on two-photon lasing reported by Pfister et al. [Phys. Rev. Lett. 86, 4512 (2001)].

DOI
10.1103/physreva.68.043824

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Annual Meeting Optical Society of America•Jan 1, 1989•0 citations
We develop a theoretical model of two-photon amplification in laser-driven potassium atoms and use it to analyze the recent experiments reported by Pfister et al. [Phys. Rev. A 60, R4249 (1999)]. The model takes into account most of the essential factors influencing the amplification process, including the atomic hyperfine structure (which makes multiphoton emission possible) and the simultaneous interaction with intense drive and probe beams with arbitrary detunings. We determine the origin and analyze the properties of different multiphoton gain resonances that appear in the light-matter interaction. In particular, the influence of the drive and probe field amplitudes and detunings on the strength and frequency of the two-photon amplification resonance is studied in detail, showing clearly the differences with respect to the behavior of single-photon or other multiphoton amplification processes. In addition, we investigate interferences between different quantum pathways originating from the hyperfine structure and determine the conditions under which they can enhance or suppress multiphoton resonances. The predictions of the model are in good agreement with the observations, indicating that it can be used to understand recent experiments on two-photon lasing reported by Pfister et al. [Phys. Rev. Lett. 86, 4512 (2001)].
Two-photon amplification and lasing in laser-driven potassium atoms is investigated theoretically using a rigorous model that takes into account the hyperfine level structure of the atoms for arbitrary intensity drive and lasing fields. The model correctly predicts most features of the recent experiments, such as the characteristics of the multiphoton gain resonances and the two-photon laser emission intensity and line shape. In addition, it constitutes a new tool for exploring behaviors that are difficult to address experimentally, such as interference effects between different quantum pathways. In contrast to single-photon lasers, the laser emission line shape is frequency pushed and has the form of a closed curve composed of a stable and an unstable branch.
We show theoretically that it is possible to generate laser light based on two-photon and other high-order multiphoton processes when an atomic beam of optically driven potassium atoms crosses a high-finesse optical cavity. We use a rigorous model that takes into account all the atomic substates involved in the optical interactions and is valid for any drive and lasing field intensities. The polarizations of the drive and lasing fields are assumed to be fixed. Stable and unstable laser emission branches are obtained, which are represented as a function of cavity detuning and are analyzed in terms of the fundamental quantum processes yielding them. Closed-curve laser-emission profiles are obtained for multiphoton lasing based on processes involving more than one lasing photon. Two-photon laser emission branches show relatively long segments of stationary emission, combined in general with some segments of nonstationary emission, or with segments of mixture with three-photon emission processes. Rayleigh and hyper-Rayleigh processes can become simultaneously resonant, entailing in such case a large and fast transfer of population from the atomic initial ground sublevel to other ground sublevels with different zzz components of the total angular momentum. They could be useful in generating multiphoton correlated field states. In all cases the largest laser emission intensities are obtained from the highest-order processes, rather than the lowest. These results open the way to the understanding of experiments performed in the past years and suggest possibilities for more efficient and varied types of multiphoton laser operation.
This study presents a more complete and wider ranging theoretical study of the amplification and lasing possibilities in alkaline atoms that takes advantage of the multiplicity of hyperfine and magnetic sublevels involved in the /sup 39/K 4S/sub 1/2/-4P/sub 1/2/ transition. Using drive and probe fields of appropriate polarizations and propagation directions, and optically pumped atoms, (n+m) multi-photon resonances involving absorption of n drive photons and simultaneous emission of m probe photons can be generated. Theoretical predictions are in good agreement with experimental findings.
Summary form only given. The two-photon laser is a new type of quantum oscillator based on the two-photon stimulated emission process. The laser is intrinsically nonlinear due to the nature of the stimulated emission and is thus expected to display novel nonlinear phenomena. While there has been continued interest in two-photon lasers since the original conception significant progress has been hampered by the difficulty in achieving two-photon lasing. In this contribution we summarize a model of the interaction between two intense fields and a collection of potassium atoms and use it to explain features of the recent two-photon experiments and to make predictions for future experiments.
Atomic systems that display a linear or weakly nonlinear interaction with light are well known and well understood. However, when the interaction between light and matter becomes highly nonlinear and the light and matter strongly couple, the systems become much more difficult to understand both theoretically and experimentally. One example of a strongly coupled, highly nonlinear system is the two-photon laser that is based on the two-photon stimulated emission process. This laser has intrigued theorists and experimentalists alike. Research has been hindered, however, by the difficulties in constructing such a laser. Most two-photon gain media prove unsuitable due to small gain and the occurrence of destructive competing nonlinear effects. I have developed a new two-photon gain medium that overcomes these difficulties. It consists of a laser-driven potassium vapor in which the origin of the gain is due to the two-photon Raman scattering process. The two-photon gain feature is identified by performing spectroscopy of the laser-driven potassium vapor. To complement the experimental observations, I have developed a theoretical model of the two-photon Raman gain medium using the semi-classical density-matrix formalism. The predictions of the model are in qualitative agreement with the experimentally observed frequency- and intensity-dependence of the two-photon gain. I also describe a simplified rate-equation model of two-photon lasers through which I explore their steady-state and transient behavior. The model highlights the novel threshold behavior of two-photon lasers and the need to inject an external field to initiate lasing. This work, both theoretical and experimental, provides the first step toward a robust experimental realization of a two-photon laser.
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.