The question is raised about whether the pulse delay in a saturable absorber has anything to do with the "slow light" and group velocity reduction. We also wonder why the 40-years-old trivial interpretation of the effect remains so far ignored.
The slow-light effect, which implies reduction of the group velocity of light by many orders of magnitude, is considered to be one of the most exciting discoveries in quantum optics of the last decade. The physics underlying the effect is related to an extremely steep dispersion of the medium in a very narrow region of its transparency, while all the manifestations of the effect are usually reduced just to a small pulse delay. Perhaps this is the reason why, after the discovery of the slow-light effect, some of the simplest nonlinear phenomena associated with a retarded optical response have been implicitly revised and assigned to reduction of the light group velocity. In this lecture, a few examples of common inconsistencies in the area of the slow-light physics will be presented.
A comparative analysis of two approaches to the description of the light modulation pulse in a saturable absorber is presented. According to the simplest model, the delay of the optical pulse is a result of the distortion of its shape due to the self-modulation of absorption in the nonlinear medium. The second model of the effect, arisen at the beginning of our century, relates the pulse delay to the so-called “slow light,” attributing this delay to the group velocity reduction under the condition of coherent oscillations of the population. It is shown that the data of all known experiments on the light pulse delay in saturable absorbers can be comprehensively described in terms of the simplest model of saturable absorber and do not require invoking the effect of population coherent oscillations with spectral hole-burning and anomalous modifications of the light group velocity. It is concluded that the effect of the group velocity reduction under the condition of population coherent oscillations has not received so far any experimental confirmation, and the assertions about real observation of the slow light based on this mechanism are unfounded.