Saturable absorption or slow light?

TL;DR

This paper argues that many observed slow light effects in saturable media can be explained by simple saturable absorption phenomena, challenging the interpretation of these effects as true slow light.

Contribution

It demonstrates that slow light effects in various media can be fully explained by a basic saturable absorber model, questioning the need for more complex explanations.

Findings

Saturable absorption explains phase shifts and modulation gains

Spectral hole burning is a key indicator of true slow light

Superluminal effects can be explained by reverse saturable absorption

Abstract

Quantitative analysis of slow light experiments utilising coherent population oscillation (CPO) in a range of saturably absorbing media, including ruby and alexandrite, Er3+:Y2SiO5, bacteriorhodopsin, semi-conductor quantum devices and erbium doped optical fibres, shows that the observations may be more simply interpreted as saturable absorption phenomena. A basic two-level model of a saturable absorber displays all the effects normally associated with slow light, namely phase shift and modulation gain of the transmitted signal, hole burning in the modulation frequency spectrum and power broadening of the spectral hole, arising from the finite response time of the non-linear absorption. Only where hole-burning in the optical spectrum is observed (using independent pump and probe beams), or pulse delays exceeding the limits set by saturable absorption are obtained, can reasonable confidence be placed in the observation of slow light in such experiments. Superluminal ('fast light') phenomena in media with reverse saturable absorption (RSA) may be similarly explained.