'''Second harmonic generation''' ('''SHG''') is a [[nonlinear optics|nonlinear optical]] process, in which [[photon]]s interacting with a nonlinear material are effectively "combined" to form new photons with twice the energy, and therefore twice the [[frequency]] and half the [[wavelength]] of the initial photons. 

In ordinary circumstances, the rate of conversion of photons to the higher-energy photons is quite low, so the SHG light produced is very weak. Weak SHG signals generated in a sample can be measured with a [[photomultiplier]]. The SHG process, being nonlinear, is very sensitive to the intensity of the input light, however. With careful design, an intense [[laser]] beam can be converted to its second harmonic with greater than 80% efficiency. This is widely used, for example to generate green light at 532 nm from the [[near infrared]] output of a [[Nd:YAG laser]] at 1064 nm.

Some common materials used for second harmonic generation are [[potassium titanyl phosphate]] (KTP), [[CsLiB6O10|cesium lithium borate]] (CLBO), [[lithium tantalate]], and [[lithium niobate]].

To achieve high conversion efficiency, the input light and the second harmonic light must be kept in [[phase]]. This is difficult, because the [[speed of light]] in a material generally varies with wavelength, due to [[Dispersion (optics)|dispersion]] of the [[index of refraction]]. In some nonlinear crystals, a particular crystal orientation can be found where, due to [[birefringence]], the fundamental and second harmonic light see the same index of refraction, and so remain in phase as they propagate. In other nonlinear materials, where this is not possible, [[periodic poling]] is used to keep the waves ''approximately'' in phase. This is commonly used for lithium niobate, which is very efficient for SHG conversion, but which does not have a crystal orientation where the two wavelengths of light see the same index of refraction.

Second harmonic generation crystals are used in some commercially available green [[laser pointer]]s.

[[Category:Nonlinear optics|Second harmonic generation]]
[[de:Frequenzverdopplung]]