A model-independent theorem demonstrates how a causal linear dielectric medium responds to the instantaneous spectrum, that is, the spectrum of the electric field pulse that is truncated at each new instant (as a given locale in the medium experiences the pulse). This process leads the medium to exchange energy with the front of a pulse differently than with the back as the instantaneous spectrum laps onto or off of nearby resonances. So-called superluminal pulse propagation in either absorbing or amplifying media as well as highly subluminal pulse propagation are understood qualitatively and quantitatively within this context.
© 2001 Optical Society of America
Justin Peatross, Michael Ware, and Scott A. Glasgow, "Role of the instantaneous spectrum on pulse propagation in causal linear dielectrics," J. Opt. Soc. Am. A 18, 1719-1725 (2001)