Lossless anomalous dispersion and an inversionless gain doublet via dressed interacting ground states (DIGS)

Transparent media exhibiting anomalous dispersion have been of considerable interest since Wang, Kuzmich, and Dogariu [Nature {\bf 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 DIGS system. The system consists of a $Λ$ atom prepared as in standard electromagnetically induced transparency (EIT), with two additional metastable ground states coupled to the $Λ$ 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.

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