Electromagnetic field energy in dispersive materials

A general expression for the electromagnetic energy density in a lossy dispersive medium, applicable for a field having a narrow temporal frequency bandwidth, is derived and compared with exact results for an example dielectric constant. Consequently, the possibility of negative time-averaged stored field energy is shown to have physical meaning. This observation is of interest in the study of dispersive metamaterials, such as those which can exhibit a negative refractive index. (C) 2010 Optical Society of America