The Polariton is a quasi-particle in condensed matter systems, where it describes the superposition of an excitation—typically an exciton (itself the quasi-particle that arises from binding a solid-state electron with a valence hole)—with a photon.
The concept was first proposed theoretically by John Hopfield[1] and observed experimentally by Claude Weisbuch.[2]
A historical review of the polariton concept, from Hopfield's lone-warrior paper[1] that gets all the credit despite fairly serious variations with the finally adopted results, along with other contributions.[3][4]