Amplification without population inversion in a V three-level system: A physical interpretation

Amplification in a V system is studied both in the framework of the density matrix formalism and using the S matrix in the usual Hilbert space of the dressed-state vector. We show that it is possible to achieve amplification without inversion both in the bare- and in the dressed-state basis. In this case there is an asymmetry between amplification and absorption because of the occurrence of interferences between Feynman diagrams that can reduce the absorption. The comparison with the two-level atom shows a close analogy with the gain observed about the central resonance of the Mollow transmission spectrum. {} 1996 The American Physical Society.