It is found that ultrashort pulses in a high-gain 3.51-μm xenon laser propagate through the amplifying medium at a velocity less than the vacuum speed of light by as much as a factor of 2.5. The pulse velocity is a function of the gain and agrees with the group velocity.

• Received 23 November 1970

DOI:https://doi.org/10.1103/PhysRevLett.26.293