[en] 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)]
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[en] 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
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