This paper is concerned with the spectral profile of spontaneously emitted radiation from Λ and cascade models of driven three-level atoms, and with the absorption spectra of a weak probe. The atoms are excited by a pair of coherent external fields that are resonant or nearly resonant with the two dipole-allowed transitions of each of these two models. The main aim of this work is to extend earlier studies of the V-model configuration of three-level atoms and to present a comprehensive survey of the emission and absorption features of these systems. In addition to a derivation of exact formulas for the spectra and explicit analytic approximations in the high-intensity limit, we provide an explanation for the existence of simultaneous stationary population inversion between pairs of atomic levels and explore the effect of Doppler broadening on the absorption profile of the weak probe. In view of the latter analysis, in particular, we conclude that gain features persist even in the presence of inhomogeneous broadening. This suggests the possibility of experimental tests in a cell rather than an atomic-beam environment.

• Received 21 September 1990

DOI:https://doi.org/10.1103/PhysRevA.43.3748