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 (for example, surfaces and interfaces);
the light has a very high intensity (typically from a pulsed laser).
Sum-frequency generation is a "parametric process", meaning that the photons satisfy energy conservation, leaving the matter unchanged: