Sum-frequency generation (SFG) is a nonlinear optical process. This phenomenon is based on the annihilation of two input photons at angular frequencies
Sum-frequency generation is a "parametric process", meaning that the photons satisfy energy conservation, leaving the matter unchanged:
A special case of sum-frequency generation is second-harmonic generation, in which ω1=ω2. In fact, in experimental physics, this is the most common type of sum-frequency generation. This is because in second-harmonic generation, only one input light beam is required, but if ω1≠ω2, 2 simultaneous beams are required, which can be more difficult to arrange. In practice, the term "sum-frequency generation" usually refers to the less common case where ω1≠ω2.
For sum-frequency generation to occur efficiently, a condition called phase-matching must be satisfied:
where
Some common SFG applications are described in the article sum frequency generation spectroscopy.