In physics, polaritons /pəˈlærɪtɒnz, poʊ-/[1] are quasiparticles resulting from strong coupling of electromagnetic waves with an electric or magnetic dipole-carrying excitation.[example needed] They are an expression of the common quantumphenomenon known as level repulsion, also known as the avoided crossing principle. Polaritons describe the crossing of the dispersion of light with any interacting resonance. To this extent polaritons can also be thought as the new normal modes of a given material or structure arising from the strong coupling of the bare modes, which are the photon and the dipolar oscillation. The polariton is a bosonic quasiparticle, and should not be confused with the polaron (a fermionicone), which is an electron plus an attached phonon cloud.
Whenever the polariton picture is valid (i.e. when the weak coupling limit is an invalid approximation), 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 of the photon. For exciton-polaritons, rich experimental results on various aspects have been gained in copper(I) oxide.
https://en.wikipedia.org/wiki/Polariton
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