The propagation of light in dispersive media is a subject of fundamental as well as practical importance. In recent years attention has focused in particular on media in which the refractive index can vary with frequency in such a way that the group velocities of optical pulses can be much greater or much smaller than the speed of light in vacuum, or in which the refractive index can be negative. This book treats these topics at an introductory to intermediate level, focusing on the basic theory but also describing the significant experimental progress made during the past decade.

Considerable attention is paid to the fact that superluminal group velocities are not in conflict with special relativity, and to the role of quantum effects in preventing superluminal communication and violations of Einstein causality. Some of the basic physics at the opposite extreme of very slow group velocities, and stopped and regenerated light, is discussed, including the concepts of electromagnetically induced transparency and dark-state polaritons.

Another very active aspect of the subject concerns the possibility of designing "metamaterials" in which the refractive index can be negative and propagating light is left-handed in the sense that the phase and group velocities are in opposite directions. The last two chapters of the book are an introduction to some of the basic theory and consequences of negative refractive index, with emphasis on the seminal work carried out since 2000. The possibility that "perfect" lenses can be made from negative-index metamaterials, which has been perhaps the most controversial aspect of the field, is introduced and discussed in some detail.