Maximal Refraction and Superluminal Propagation in a Gaseous Nanolayer

Abstract

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 $n=1.26\ifmmode\pm\else\textpm\fi{}0.02$, 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 $>100\text{ }\text{ }\mathrm{ps}$ over a propagation distance of 390 nm, corresponding to a group index ${n}_{g}=\ensuremath{-}(1.0\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}{10}^{5}$, the largest negative group index measured to date.