Understanding subluminal and superluminal propagation through superposition of frequency components.

Propagation of a light pulse through a dielectric slab is discussed theoretically in this paper. It is exhibited via a multiple-scattering approach that the slab can modify the phase of the pulse's frequency components, so that, when the frequency components are superposed, they cause the peak in the output pulse to appear to travel either faster or slower than in vacuum, depending on whether the slab is absorbing or amplifying. The expressions of the corresponding advancement and delay of the peak are derived and argued to be limited in magnitude by the pulse's duration.