Polariton

A polariton is a quasiparticle that arises from the strong coupling between a photon and an electric or matter excitation in a solid or structured medium. In this mixed light–matter state, the electromagnetic field and the excitation exchange energy faster than they dissipate, creating new normal modes that propagate with properties different from either constituent.

Several polariton families are distinguished by the type of matter excitation involved. Exciton-polaritons form from the coupling of photons with excitons, bound electron–hole pairs in semiconductors, and are often studied in microcavities where quantum wells are placed between mirrors. Phonon-polaritons result from coupling photons with optical phonons in polar crystals. Plasmon-polaritons (or surface plasmon–polaritons) arise from coupling with collective electron oscillations in metals and graphene. Other hybrids, such as magnon-polaritons, involve coupling with magnetic excitations.

In a strongly coupled system, the photon and matter excitations hybridize into two polariton branches, typically labeled upper and lower, which exhibit anticrossing in the dispersion relation and a characteristic Rabi splitting. The effective mass of these polaritons is very small, especially for exciton-polaritons, enabling high coherence and macroscopic quantum effects at comparatively high temperatures.

Polaritons are realised in various devices, notably semiconductor microcavities embedding quantum wells, and in other engineered structures. Material platforms include GaAs, GaN, perovskites, and two-dimensional materials. They have motivated interest in low-threshold polariton lasing, nonlinear optics, and quantum fluids of light.

Research on polaritons explores fundamental light–matter interactions, polariton condensation and superfluidity, and potential applications in optical information processing and quantum simulation.