The interaction of electromagnetic waves with the excitations of the medium resulting in their coupling becomes particularly strong when their frequency ω and wave vectors k are the same (resonance). In this area, coupled waves, i.e., polaritons, are formed, which have a characteristic dispersion law ω (k).

Contributions of polariton energy are made by both the electromagnetic energy and the energy of the medium's own excitations. Polaritons resulting from the interaction of photons with different excitations of the medium - optical phonons, excitons, plasmons, magnons and so on - are called, respectively, phonon polaritons, exciton polaritons, plasma polaritons, magnon polaritons, etc. Description of, for example, phonon polaritons requires one to solve the lattice vibration equation together with Maxwell's equations.