An effect brought about by electromagnetic radiation the magnitude of which is not
proportional to the irradiance. Non-linear optical effects of importance to
photochemistry are
harmonic frequency generation, lasers, Raman shifting,
upconversion, and others.
Source:
PAC, 1996, 68, 2223
(Glossary of terms used in photochemistry (IUPAC Recommendations 1996))
on page 2256