Slow light by means of spectral hole burning in atomic vapor

In a Doppler-free spectroscopy setup a rubidium vapor cell is subjected to two counterpropagating laser beams. The saturating beam creates narrow spectral holes (of approximately the natural linewidth) in the weaker signal beam’s Doppler-broadened absorption spectrum. A strong dispersion leads to a large reduction of the signal pulse’s group velocity at the transmission peaks and negative group velocities at their edges. Group indices of order 103 with less than 70% absorption have been obtained.