Dependence of light pulse propagation on its temporal width: Transition from group velocity to c-propagation

We show how thevelocity of an optical pulse propagating through a dispersive medium depends on the pulse duration. A transition from the group velocity for long pulses to the in-vacuum velocity for short pulses is shown both in experimental results and in theoretical predictions.The temporal duration of the experimental pulses are 150ps and 3.5ns. A description of the pulse propagation in terms of the time “center of mass” of the energy flow allows an intuitive overview of the results