Exciton-polariton

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Exciton-polaritons are hybrid light–matter particles created when photons in a semiconductor cavity strongly couple with excitons (bound electron-hole pairs). Because photons are massless and excitons have mass, their strong mixing forms a new quasiparticle with a very small effective mass and the ability to interact.

This coupling splits the system’s energy into two new modes: the upper polariton and the lower polariton. The size of the split grows with how strongly the light and matter mix, and it depends on the light’s polarization. In the upper polariton, the light and matter parts oscillate in phase; in the lower polariton, they oscillate out of phase.

Polaritons inherit the best of both worlds: light-like mobility from the photons and matter-like interactions from the excitons. They also interact with their environment (phonons) and lose energy by emitting light. Polaritons typically experience repulsive interactions between particles of the same spin, so increasing their density raises their energy a bit (a blueshift).

Polaritons can travel over several micrometers in organic and other microcavity materials, thanks to their coherent light component. Their motion can show a transition from diffusive to more ballistic behavior under different conditions.

Their energy–momentum relation is not a simple parabola because of their mixed nature, and they have a spin degree of freedom, making them spinor fluids that can show various polarization patterns. They can form Bose–Einstein condensates, exhibit superfluidity and quantum vortices, and are being explored for new technologies.

Researchers study devices based on exciton-polaritons such as polariton lasers and optically controlled transistors, as well as nonlinear states like solitons and complex coherence phenomena. Modeling often uses Gross–Pitaevskii equations, a type of nonlinear Schrödinger equation.