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In physics, polaritons // are quasiparticles resulting from strong coupling of electromagnetic waves with an electric or magnetic dipole-carrying excitation. They are an expression of the common quantum phenomenon known as level repulsion, also known as the avoided crossing principle. Polaritons describe the crossing of the dispersion of light with any interacting resonance.
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 an infrared photon with an optic phonon; exciton-polaritons, resulting from coupling of visible light with an exciton; intersubband-polaritons, resulting from coupling of an infrared or terahertz photon with an intersubband excitation; and surface plasmon-polaritons, resulting from coupling of surface plasmons with light (the wavelength depends on the substance and its geometry). Recently, special kinds of polaritons, called Bragg-polaritons or Braggoritons, have been observed  and studied theoretically.
Whenever the polariton picture is valid, the model of photons propagating freely in crystals is insufficient. 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 copper (I) oxide.
The polariton is a bosonic quasiparticle, and should not be confused with the polaron, a fermionic one, e.g. an electron plus attached phonon cloud. Polaritons were first considered theoretically by Kirill Borisovich Tolpygo, a Soviet physicist, and were initially termed light-excitons in Russian scientific literature.
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