A branch of optics concerned with the optical properties of matter subjected to intense electromagnetic fields. For nonlinearity to manifest itself, the external field should not be negligible compared to the internal fields of the atoms and molecules of which the matter consists. Lasers are capable of generating external fields sufficiently intense for nonlinearity to occur. Indeed, the subject of nonlinear optics has been largely developed as a result of the invention of the laser. In nonlinear optics the induced electric polarization (see dielectric) of the medium is not a linear function of the strength of the external electromagnetic radiation. This leads to more complicated phenomena than can occur in linear optics, in which the induced polarization is proportional to the strength of the external electromagnetic radiation.