Editing Nonlinear optics (section)

==Nonlinear optical processes==
Nonlinear optics explains nonlinear response of properties such as [[Frequency#Light|frequency]], polarization, phase or path of incident light.<ref name=Rigamonti2010/> These nonlinear interactions give rise to a host of optical phenomena:

===Frequency-mixing processes===
* [[Second-harmonic generation]] (SHG), or ''frequency doubling'', generation of light with a doubled frequency (half the wavelength), two photons are destroyed, creating a single photon at two times the frequency.
* [[Third-harmonic generation]] (THG), generation of light with a tripled frequency (one-third the wavelength), three photons are destroyed, creating a single photon at three times the frequency.
* [[High-harmonic generation]] (HHG), generation of light with frequencies much greater than the original (typically 100 to 1000 times greater).
* [[Sum-frequency generation]] (SFG), generation of light with a frequency that is the sum of two other frequencies (SHG is a special case of this).
* [[Difference-frequency generation]] (DFG), generation of light with a frequency that is the difference between two other frequencies.
* [[Optical parametric amplification]] (OPA), amplification of a signal input in the presence of a higher-frequency pump wave, at the same time generating an ''idler'' wave (can be considered as DFG).
* [[Optical parametric oscillation]] (OPO), generation of a signal and idler wave using a parametric amplifier in a resonator (with no signal input).
* [[Optical parametric generation]] (OPG), like parametric oscillation but without a resonator, using a very high gain instead.
* [[Half-harmonic generation]], the special case of OPO or OPG when the signal and idler degenerate in one single frequency, 
* [[Spontaneous parametric down-conversion]] (SPDC), the amplification of the vacuum fluctuations in the low-gain regime.
* [[Optical rectification]] (OR), generation of quasi-static electric fields.
* [[Nonlinear light-matter interaction with free electrons and plasmas]].<ref>{{cite journal|title=Experimental observation of relativistic nonlinear Thomson scattering|first1=Szu-yuan|last1=Chen|first2=Anatoly|last2=Maksimchuk|first3=Donald|last3=Umstadter|date=17 December 1998|journal=Nature|volume=396|issue=6712|pages=653–655|doi=10.1038/25303|arxiv=physics/9810036|bibcode=1998Natur.396..653C|s2cid=16080209}}</ref><ref>{{cite journal|title=Observation of Nonlinear Effects in Compton Scattering|first1=C.|last1=Bula|first2=K. T.|last2=McDonald|first3=E. J.|last3=Prebys|first4=C.|last4=Bamber|first5=S.|last5=Boege|first6=T.|last6=Kotseroglou|first7=A. C.|last7=Melissinos|first8=D. D.|last8=Meyerhofer|first9=W.|last9=Ragg|first10=D. L.|last10=Burke|first11=R. C.|last11=Field|first12=G.|last12=Horton-Smith|first13=A. C.|last13=Odian|first14=J. E.|last14=Spencer|first15=D.|last15=Walz|first16=S. C.|last16=Berridge|first17=W. M.|last17=Bugg|first18=K.|last18=Shmakov|first19=A. W.|last19=Weidemann|date=22 April 1996|journal=Phys. Rev. Lett.|volume=76|issue=17|pages=3116–3119|doi=10.1103/PhysRevLett.76.3116|pmid=10060879|bibcode=1996PhRvL..76.3116B|url=https://cds.cern.ch/record/311275|type=Submitted manuscript|access-date=6 September 2018|archive-url=https://web.archive.org/web/20190621224531/https://cds.cern.ch/record/311275|archive-date=21 June 2019|url-status=dead}}</ref><ref>{{cite journal |last1=Koga |first1=J. |last2=Esirkepov |first2=T.Z. |last3=Bulanov |first3=S.V. |title=Nonlinear Thomson scattering in the strong radiation damping regime |journal=Physics of Plasmas |volume=12 |issue=9 |pages= |date= 2005|doi=10.1063/1.2013067 }}</ref><ref>{{cite journal|title=Plasma mirrors for ultrahigh-intensity optics|first1=C.|last1=Thaury|first2=F.|last2=Quéré|first3=J.-P.|last3=Geindre|first4=A.|last4=Levy|first5=T.|last5=Ceccotti|first6=P.|last6=Monot|first7=M.|last7=Bougeard|first8=F.|last8=Réau|first9=P.|last9=d’Oliveira|first10=P.|last10=Audebert|first11=R.|last11=Marjoribanks|first12=Ph|last12=Martin|date=1 June 2007|journal=Nat Phys|volume=3|issue=6|pages=424–429|doi=10.1038/nphys595|bibcode=2007NatPh...3..424T}}</ref>

===Other nonlinear processes===
* Optical [[Kerr effect]], intensity-dependent refractive index (a <math>\chi^{(3)}</math> effect).
* [[Self-focusing]], an effect due to the optical [[Kerr effect]] (and possibly higher-order nonlinearities) caused by the [[Spatial mode|spatial variation in the intensity]] creating a spatial variation in the refractive index.
* [[Kerr-lens modelocking]] (KLM), the use of [[self-focusing]] as a mechanism to [[Mode-locking|mode-lock]] laser.
* [[Self-phase modulation]] (SPM), an effect due to the optical [[Kerr effect]] (and possibly higher-order nonlinearities) caused by the [[Wavepacket|temporal variation in the intensity]] creating a temporal variation in the refractive index.
* [[Soliton (optics)|Optical solitons]], an equilibrium solution for either an [[Optical pulses|optical pulse]] (temporal soliton) or [[spatial mode]] (spatial soliton) that does not change during propagation due to a balance between [[dispersion (optics)|dispersion]] and the [[Kerr effect]] (e.g. [[self-phase modulation]] for temporal and [[self-focusing]] for spatial solitons).
* Self-diffraction, splitting of beams in a multi-wave mixing process with potential energy transfer.<ref>{{Cite journal|last1=Hernández-Acosta|first1=M A|last2=Soto-Ruvalcaba|first2=L|last3=Martínez-González|first3=C L|last4=Trejo-Valdez|first4=M|last5=Torres-Torres|first5=C|date=2019-09-17|title=Optical phase-change in plasmonic nanoparticles by a two-wave mixing|url=https://doi.org/10.1088/1402-4896/ab3ae9|journal=Physica Scripta|volume=94|issue=12|pages=125802|doi=10.1088/1402-4896/ab3ae9|bibcode=2019PhyS...94l5802H|s2cid=202145209|issn=0031-8949}}</ref>
* [[Cross-phase modulation]] (XPM), where one wavelength of light can affect the phase of another wavelength of light through the optical Kerr effect.
* [[Four-wave mixing]] (FWM), can also arise from other nonlinearities.
* [[Cross-polarized wave generation]] (XPW), a <math>\chi^{(3)}</math> effect in which a wave with polarization vector perpendicular to the input one is generated.
* [[Modulational instability]].<ref name="Zakharov 540–548">{{Cite journal|last1=Zakharov|first1=V. E.|last2=Ostrovsky|first2=L. A.|date=2009-03-15|title=Modulation instability: The beginning|journal=Physica D: Nonlinear Phenomena|volume=238|issue=5|pages=540–548|doi=10.1016/j.physd.2008.12.002|bibcode=2009PhyD..238..540Z}}</ref>
* [[Raman amplification]]<ref>{{cite journal |first1=A.P. |last1=Kouzov |first2=N.I. |last2=Egorova |first3=M. |last3=Chrysos |first4=F. |last4=Rachet |title=Non-linear optical channels of the polarizability induction in a pair of interacting molecules |journal=Nanosystems: Physics, Chemistry, Mathematics |volume=3 |issue=2 |page=55 |date=2012 |url=http://nanojournal.ifmo.ru/en/articles-2/volume3/3-2/physics/paper05/ |access-date=2015-10-31 |archive-date=2017-06-13 |archive-url=https://web.archive.org/web/20170613130115/http://nanojournal.ifmo.ru/en/articles-2/volume3/3-2/physics/paper05/ |url-status=live }}</ref>
* [[Optical phase conjugation]].
* [[Brillouin scattering#Stimulated Brillouin scattering|Stimulated Brillouin scattering]], interaction of photons with acoustic phonons
* [[Two-photon absorption|Multi-photon absorption]], simultaneous absorption of two or more photons, transferring the [[energy]] to a single electron.
* Multiple [[photoionisation]], near-simultaneous removal of many bound electrons by one photon.
* [[Optical chaos|Chaos in optical systems]].

===Related processes===
In these processes, the medium has a linear response to the light, but the properties of the medium are affected by other causes:
* [[Pockels effect]], the refractive index is affected by a static electric field; used in [[electro-optic modulator]]s.
* [[Acousto-optics]], the refractive index is affected by acoustic waves (ultrasound); used in [[acousto-optic modulator]]s.
* [[Raman scattering]], interaction of photons with optical [[phonon]]s.