Sum Frequency Generation

From Qwiki

Also called frequency up-conversion, it is a second-order nonlinear process involving the creation of a photon at frequency $ω 3$ and the annihilation of photons at frequencies $ω 1$ and $ω 2$ , such that $ω 3 = ω 1 + ω 2$ . To obtain an appreciable efficiency of sum frequency photons, momentum conservation (phase matching, i.e., $\mathbf{k}_3=\mathbf{k}_{1}+\mathbf{k}_{2}$ ) must also be satisfied. This is typically achieved through a precise orientation of the nonlinear crystal used.