Sum frequency generation

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Sum frequency generation (SFG) is an example of a second order non-linear optical process, in which two beams of light of different frequencies ω₁ and ω₂ interact, creating light with frequency ω₃ = ω₁ + ω₂. It is a generalization of second harmonic generation. In the latter, ω₁ = ω₂, both of which can be provided by a single light source. From a quantum mechanical perspective, SFG can be seen as result of combining two photons to produce a third. Since the energy of a photon is given by

the frequency summing ω₃ = ω₁ + ω₂ is simply a statement that energy is conserved.

In a common SFG application, light from a tunable infrared laser is combined with light from a fixed frequency visible laser to produce an invisible sum frequency (SF) beam created by a wave mixing process.