https://doi.org/10.1351/goldbook.PT07441
Interaction of photons in a non-linear medium satisfying the energy and @M04007@ conservation laws. During the interaction the photon frequencies are mixed and photon(s) with different frequencies are created.
Note:
Optical parametric processes of three interacting beams, called frequency mixing processes, are: second-harmonic generation (SHG), sum-frequency generation (SFG), difference-frequency generation (DFG), optical parametric generation and amplification (OPG and OPA), and optical parametric oscillation (OPO).
Optical parametric processes of three interacting beams, called frequency mixing processes, are: second-harmonic generation (SHG), sum-frequency generation (SFG), difference-frequency generation (DFG), optical parametric generation and amplification (OPG and OPA), and optical parametric oscillation (OPO).