Maximal refraction and superluminal propagation in a gaseous nanolayer

TL;DR

This paper experimentally measures a high refractive index in dense Rb vapor nanolayers, demonstrating superluminal pulse propagation with the largest negative group index recorded, revealing new optical phenomena in atomic nanolayers.

Contribution

It provides the first experimental measurement of maximal refraction and superluminal pulse propagation in a gaseous atomic nanolayer, with precise phase shift analysis.

Findings

Refractive index peak n = 1.26 ± 0.02 close to theoretical maximum.

Sub-nanosecond pulses advanced by >100 ps over 390 nm.

Largest negative group index (-1×10^5) measured to date.

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 \pm 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 sub-nanosecond optical pulse is advanced by >100 ps over a propagation distance of 390 nm, corresponding to a group index of -1x10^5, the largest negative group index measured to date.