Energy balance and energy transport velocity in dispersive media

Abstract

The energy balance equation is considered for an electromagnetic wave in a medium with frequency dispersion. It is shown that the energy density consists of two parts representing the field energy and the energy stored in the medium, respectively. The energy transport is carried solely by the field energy. A corresponding transport velocity is introduced. It differs from the group velocity and is always smaller than (or equal to) the vacuum speed of light, even in the case of superluminal group velocities.