Sum-frequency generation (SFG) is a second order nonlinear optical process based on the mixing of two input photons at frequencies and to generate a third photon at frequency . As with any optical phenomenon in nonlinear optics, this can only occur under conditions where: the light is interacting with matter, that lacks centrosymmetry ; the light has a very high intensity . Sum-frequency generation is a "parametric process", meaning that the photons satisfy energy conservation, leaving the matter unchanged, . It is the basis of experimental techniques such as sum frequency generation spectroscopy.