Transparent media exhibiting anomalous dispersion have been of considerable interest since Wang, Kuzmich, and Dogariu [Nature 406, 277 (2000)] first observed light propagate with superluminal and negative group velocities without absorption. Here, we propose an atomic model exhibiting these properties, based on a generalization of amplification without inversion in a five-level dressed interacting ground-state system. The system consists of a $\Lambda$ atom prepared as in standard electromagnetically induced transparency (EIT), with two additional metastable ground states coupled to the $\Lambda$ atom ground states by two rf-microwave fields. We consider two configurations by which population is incoherently pumped into the ground states of the atom. Under appropriate circumstances, we predict a pair of new gain lines with tunable width, separation, and height. Between these lines, absorption vanishes but dispersion is large and anomalous. The system described here is a significant improvement over other proposals in the anomalous dispersion literature in that it permits additional coherent control over the spectral properties of the anomalous region, including a possible ${10}^4$-fold increase over the group delay observed by Wang, Kuzmich, and Dogariu.

• Received 14 May 2009

DOI:https://doi.org/10.1103/PhysRevA.81.023806