Two-field electromagnetically induced transparency and switching between ultranarrow absorption and gain features in rubidium atoms

We study the generation of an ultranarrow gain spectrum in an atomic transi tion when the upper transition level is coupled to the lower transition lev el by one laser and to a metastable state by another laser. It is shown tha t this spectrum switches abruptly into an ultranarrow absorption line with an increase in the first laser intensity. These results are explained by in vestigating the electromagnetically induced transparency (EIT) in a system where the upper transition level is coupled to two metastable states by two laser fields (ir; system). We discuss various features of such a transpare ncy and show how it can be described as a structuring process, similar to t he laser-induced continuum structure. Utilizing such a description of EIT, we explain the ultranarrow gain generation as a stimulated Rayleigh scatter ing process. [S1050-2947(99)04805-2].