Laser studies of polaritons

Polaritons describe the quanta of coupled photon-phonon modes in a crystal. It was well-known that propagation of an electromagnetic wave in a medium near resonance is governed by anomalous dispersion, but treating the wave as an elementary excitation in the medium was a new and important concept. First, photons (can be extended outside a medium) and phonons (restricted to existing inside a medium) are now treated on equal footing. Second, the medium-dressed propagating electromagnetic modes are taken as characteristic modes of the medium. Third, the coupled mode idea can be extended to many other interesting problems in condensed matter physics. The advent of lasers has greatly facilitated the study of polaritons. This paper describes the important advances in the past quarter century