Polariton
Polaritons are quasi-particles in crystals. They are an expression of the common quantum phenomenon known as level repulsion also known as the anti-crossing principle. Polaritons describe the crossing of the dispersion of light with any interacting resonance or quasi-particle.Thus, a polariton is the result of the mixing of a photon with an excitation of a material. The most discussed types of polaritons are phonon-polaritons resulting from coupling of a photon with a phonon and exciton-polaritons (with exciton).
Whenever the polariton picture is valid, the model of photons in crystals should be rejected. A major feature of polaritons is a strong dependency of the propagation speed of light through the crystal on the frequency. For exciton-polaritons rich experimental results on various aspects have been gained in Cuprous Oxide.