We observe a narrow, isolated, two-photon absorption resonance in <sup>87</sup>Rb for large one-photon detuning in the presence of a buffer gas. In the absence of a buffer gas, a standard Λ configuration of two laser frequencies gives rise to electromagnetically induced transparency (EIT) for all values of one-photon detuning throughout the inhomogeneously (Doppler) broadened line. However, when a buffer gas is added and the one-photon detuning is comparable to or greater than the Doppler width, an absorption resonance appears instead of the usual EIT resonance. We also observe a large negative group delay (≈−300 μs for a Gaussian pulse that propagates through the media with respect to a reference pulse not affected by the media), corresponding to a superluminal group velocity v<sub>g</sub>=−c/(3.6×10<sup>6</sup>)=−84 m/s.
© 2004 Optical Society of America
Eugeniy E. Mikhailov, Vladimir A. Sautenkov, Yuri V. Rostovtsev, and George R. Welch, "Absorption resonance and large negative delay in rubidium vapor with a buffer gas," J. Opt. Soc. Am. B 21, 425-428 (2004)