Editing Nonlinear optics (section)

{{Short description|Branch of physics}}
[[File:EntryWithCollCode173233downbaxis.png|thumb|right|Structure of [[Potassium titanyl phosphate|KTP crystal]], viewed down b axis, used in second harmonic generation.]]
'''Nonlinear optics''' ('''NLO''') is the branch of [[optics]] that describes the behaviour of [[light]] in [[Nonlinearity|nonlinear]] media, that is, media in which the [[polarization density]] '''P''' responds non-linearly to the [[electric field]] '''E''' of the light. The non-linearity is typically observed only at very high light intensities (when the electric field of the light is >10<sup>8</sup> V/m and thus comparable to the atomic electric field of ~10<sup>11</sup> V/m) such as those provided by [[laser]]s. Above the [[Schwinger limit]], the vacuum itself is expected to become nonlinear. In nonlinear optics, the [[superposition principle]] no longer holds.<ref name=boyd2008/><ref name=shen2002/><ref name=agra2006/>