Energy transport in linear dielectrics

We examine the energy exchanged between an electromagnetic pulse and a line ar dielectric medium in which it propagates. While group velocity indicates the presence of field energy ( the locus of which an move with arbitrary s peed), the velocity of energy transport maintains strict luminality. This i ndicates that the medium treats the leading and trailing portions of the pu lse differently. The principle of causality requires the medium to respond to the instantaneous spectrum, the spectrum of the pulse truncated at each new instant as a given lo ale in the medium experiences the pulse. (C) 2001 Optical Society of America.