Abstract
The propagation of ultrashort light pulses in optical media is one of the most interesting problems of optics. The group velocity is generally used as a quantity which gives the speed of a wave packet. In dispersive media the group velocity can behave abnormally exceeding c, the velocity of light in vacuum, or even become negative1. Superluminous pulse propagation has also been observed upon propagation of pulses in deeply saturated amplifiers2, in a resonant medium with inverted population3–4 and in quantum-mechanical tunneling of single photons through dielectric mirrors5. It should be noted that the effect discussed below has a completely different origin.
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