Saturable absorption and 'slow light'
  Quantitative evaluation of some recent 'slow light' experiments based on coherent population oscillations (CPO) shows that they can be more simply interpreted as saturable absorption phenomena. Therefore they do not provide an unambiguous demonstration of 'slow light'. Indeed a limiting condition on the spectral bandwidth is not generally satisfied, such that the requirements for burning a narrow spectral hole in the homogeneously broadened absorption line are not met. Some definitive tests of 'slow light' phenomena are suggested, derived from analysis of phase shift and pulse delay for a saturable absorber 
Adrian C Selden