Second-harmonic generation

Summary

Second-harmonic generation is the process in which light of frequency $\omega$ passing through a nonlinear medium produces light of frequency $2\omega$. It comes from the second-order nonlinear susceptibility $\chi^{(2)}$, so it needs a medium without a centre of inversion. In the photon picture, two photons of energy $\hbar\omega$ merge into one photon of energy $2\hbar\omega$. It was the first effect of nonlinear optics to be observed, by Franken and coworkers in 1961. It is used to make green laser light from infrared lasers.