Sum-frequency generation (SFG) is a nonlinear optical process. This phenomenon is based on the annihilation of two input photons at angular frequencies  and while, simultaneously, one photon at frequency is generated. As with any second order phenomenon in nonlinear optics, this can only occur under conditions where:

• The light is interacting with matter, which is asymmetric (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: