Quantum Mechanics_Nonlinear optics

Nonlinear optics (NLO) is the branch of optics that describes the behavior oflight in nonlinear media, that is, media in which the dielectric polarization Presponds nonlinearly to the electric field E of the light. This nonlinearity is typically only observed at very high light intensities (values of the electric field comparable to interatomic electric fields, typically 108 V/m) such as those provided by pulsed lasers. Above the Schwinger limit, the vacuum itself is expected to become nonlinear. In nonlinear optics, the superposition principle no longer holds.