Induced transparency in an ensemble of Λ atoms

We consider the effect of induced transparency in an ensemble of Λ atoms controlled by high-power radiation. We study the case of an optically dense medium where nonlinear interaction between probe radiation and a Stokes satellite of the drive wave is important. It is shown that the probe radiation and the Stokes satellite form normal biharmonic modes. Each normal mode is characterized by a fixed ratio between the amplitudes of different-frequency partial components and that both partial waves have a common absorption coefficient and a common group velocity. Transparency of an optically dense medium is ensured in the regime of amplification of one of the normal modes. An exact solution (without considering inhomogeneous broadening) is constructed for an arbitrary nonuniform distribution of Λ atoms along the probe radiation path and uniform drive-wave intensity. A geometric-optical approach is developed for the case of a spatially varied intensity of the drive field. The obtained solution contains both new results and a generalization of the results of the previous papers.