Envelope evolution of a laser pulse in an active medium

We show that the envelope velocity, v_env, of a short laser pulse can, via propagation in an active medium, be made less than, equal to, or even greater than c, the vacuum phase velocity of light. Simulation results, based on moving frame propagation equations coupling the laser pulse, active medium, and plasma, are presented, as well as equations that determine the design of superluminous and subluminous v_env values. In this simulation the laser pulse evolves in time in a moving frame as opposed to earlier work [D. L. Fisher and T. Tajima, Phys. Rev. Lett. 71, 4338 (1993)], where the profile was fixed. The elimination of phase slippage and pump depletion effects in a laser wake field accelerator is discussed as a particular application. Finally, we discuss media properties necessary for an experimental realization of this technique.