Polaritons

Polaritons are quasiparticles that arise when photons strongly couple with an excitation in a material, such as an exciton, a phonon, or a plasmon. In the strong-coupling regime the photon and the material excitation exchange energy faster than they decay, creating new hybrid light–matter eigenmodes known as polaritons. The resulting dispersion shows an avoided crossing and two branches, called the upper and lower polariton branches.

The most studied are exciton-polaritons, formed by coupling photons to excitons in semiconductors or organic crystals, typically realized in optical microcavities or thin films with quantum wells. Other well-known types include phonon-polaritons, produced by coupling photons to optical phonons in polar crystals, and plasmon-polaritons, which involve coupling to surface plasmons at metal–dielectric interfaces. There are also magnon-polaritons that involve coupling to magnetic excitations in some materials. Polaritons are thus inherently mixed light-matter states with properties inherited from both constituents.

Key properties of polaritons include relatively light effective mass due to their photonic component, and strong nonlinear interactions from their matter component. Their lifetimes are set by material losses and photon leakage, and their group velocity reflects the hybrid composition. Observations often rely on angle-resolved spectroscopy in microstructures to map the polariton dispersion and confirm the upper and lower branches, Rabi splitting, and strong coupling.

Applications focus on reducing device thresholds and enabling new quantum and nonlinear optical functionalities. Polaritons enable phenomena such as Bose-Einstein-like condensation at elevated temperatures, polariton lasers, ultrafast switches, and potential platforms for quantum simulation and information processing.