Nonlinear optics (NLO) is the branch of
optics that describes the behavior of
light in
nonlinear media, that is, media in which the
dielectric polarization P responds 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 10
8 V/m) such as those provided by
lasers. Above the
Schwinger limit, the vacuum itself is expected to become nonlinear. In nonlinear optics, the
superposition principle no longer holds.