In physics, polaritons /pɵˈlærɪtɒnz/ 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.

Peter Littlewood (born 1955) is a British physicist and former head of the Cavendish Laboratory. He previously headed the Theory of Condensed Matter group and the Theoretical Physics Research department at Bell Laboratories.

He is a long-standing fellow and alumnus of Trinity College, Cambridge, and he was awarded a Kennedy Scholarship to attend MIT in 1976. He was elected as a member of the Royal Society in 2007 for his contributions to the theory of collective phenomena in condensed matter physics, the discovery of amplitude collective mode in superconductors, the invention of marginal Fermi liquid phenomenology for cuprate superconductors, and other work involving friction and force flow.