Polariton
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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 anti-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 a phonon; exciton-polaritons, resulting from coupling of visible light with an exciton; and surface plasmon-polaritons, resulting from coupling of surface plasmons with light (the wavelength depends on the substance).
Whenever the polariton picture is valid, the model of photons 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.
References
- U. Fano: Atomic Theory of Electromagnetic Interactions in Dense Materials, Phys. Rev. 103, 1202 (1956).
- J.J. Hopfield: Theory of the Contribution of Excitons to the Complex Dielectric Constant of Crystals, Phys. Rev. 112, 1555 (1958).
