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 transition when the upper transition level is coupled to the lower transition level by one laser and to a metastable state by another laser. It is shown that this spectrum switches abruptly into an ultranarrow absorption line with an increase in the first laser intensity. These results are explained by investigating the electromagnetically induced transparency (EIT) in a system where the upper transition level is coupled to two metastable states by two laser fields (K system). We discuss various features of such a transparency and show how it can be described as a structuring process, similar to the laser-induced continuum structure. Utilizing such a description of EIT, we explain the ultranarrow gain generation as a stimulated Rayleigh scattering process.