Steep Anomalous Dispersion in Coherently Prepared Rb Vapor

Steep dispersion of opposite signs in driven degenerate two-level atomic transitions have been predicted and observed on the $D_2$ line of ${}^{87}\mathrm{Rb}$ in an optically thin vapor cell. The intensity dependence of the anomalous dispersion has been studied. The maximum observed value of anomalous dispersion ( $\mathrm{dn}/d\nu \simeq -6\mathrm{}\times {10}^{-11\mathrm{}}{\mathrm{Hz}}^{-1\mathrm{}}$) corresponds to a negative group velocity $V_g\simeq -c/23000$.