Harmonic generation
Harmonic generation (HG, also called multiple harmonic generation) is a nonlinear optical process in which n {\displaystyle n} photons with the same frequency interact with a nonlinear material, are "combined", and generate a new photon with n {\displaystyle n} times the energy of the initial photons (equivalently, n {\displaystyle n} times the frequency and the wavelength divided by n {\displaystyle n} ). == General process == In a medium having a substantial nonlinear susceptibility, harmonic generation is possible.