DFG
Difference Frequency Generation (DFG)
is a nonlinear process where two photons of two different frequencies (pump and signal), generate a third photon with a frequency which is the difference between the initial two (idler). In this process the lower frequency incident beam (signal) is amplified and not depleted(parametric amplification).![DFG Fourth Harmonic Generation](https://raicol.com/wp-content/uploads/2023/09/dfg.jpg)