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

We show how the velocity 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. •Experimental investigation on dependence of the propagation velocity on the pulse temporal width.•Transition from propagation at c to in matter group velocity.•Numerical simulation and theoretical picture regarding the modification of the pulse velocity.