Four-wave mixing
= Four-wave mixing
{title2=$\omega_4=\omega_1+\omega_2-\omega_3$}
A third-order <nonlinear optical susceptibility> produces a polarization term proportional to $E_1E_2E_3^*$. Its temporal and spatial phases give the displayed output <frequency> and the source <wave vector> $\mathbf k_1+\mathbf k_2-\mathbf k_3$. Efficient emission also requires that this source match a propagating optical mode. The pump waves supply the generated <energy>.