Interaction of photons in a non-linear medium satisfying the energy and momentum
conservation laws. During the interaction the photon frequencies are mixed and photon(s)
with different frequencies are created.
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).
PAC, 2007, 79, 293
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
on page 381
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook