Maximal Refraction and Superluminal Propagation in a Gaseous Nanolayer

We present an experimental measurement of the refractive index of high density Rb vapor in a gaseous atomic nanolayer. We use heterodyne interferometry to measure the relative phase shift between two copropagating laser beams as a function of the laser detuning and infer a peak index n = 1.26 +/- 0.02, close to the theoretical maximum of 1.31. The large index has a concomitant large index gradient creating a region with steep anomalous dispersion where a subnanosecond optical pulse is advanced by > 100 ps over a propagation distance of 390 nm, corresponding to a group index n(g) = -(1.0 +/- 0.1) x 10(5), the largest negative group index measured to date. DOI: 10.1103/PhysRevLett.109.233001