Sum-frequency generation

Sum-frequency generation (SFG) is a second order nonlinear optical process. This phenomenon is based on the annihilation of two input photons at angular frequencies ${\displaystyle \omega _{1}}$ and ${\displaystyle \omega _{2}}$ while, simultaneously, one photon at frequency ${\displaystyle \omega _{3}}$ is generated. As with any second order ${\displaystyle \chi ^{(2)}}$ phenomenon in nonlinear optics, this can only occur under conditions where:

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