A nonlinear Absorption process where two Photons are simultaneously absorbed.

Two-photon absorption is a process where two photons are absorbed simultaneously, exciting e.g. an atom or ION to higher-lying state, with the Energy increase being equal to the sum of the photon energies. This is a nonlinear process, occurring with significant rates only at high optical intensities, because the two-photon Absorption Coefficient is proportional to the optical intensity. The absorbed power is thus proportional to the square of the optical input power.