Sum-frequency generation (SFG) is a second order nonlinear optical process based on the mixing of two input photons at frequencies
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",[2] meaning that the photons satisfy energy conservation, leaving the matter unchanged: A special case of sum-frequency generation is second-harmonic generation, in which
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
, two 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 in which
For sum-frequency generation to occur efficiently, phase-matching conditions must be satisfied:[3] where
are the angular wavenumbers of the three waves as they travel through the medium.
As this condition is satisfied more and more accurately, the sum-frequency generation becomes more and more efficient.
Sum frequency generation spectroscopy uses two laser beams mixed at an interface to generate an output beam with a frequency equal to the sum of the two input frequencies.
Sum frequency generation spectroscopy is used to analyze surfaces and interfaces, carrying complementary information to infrared and Raman spectroscopy.