Harmonic generation

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