non-linear optical effect 

https://doi.org/10.1351/goldbook.N04197
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 [Terms] [Paper]
Citation: 'non-linear optical effect' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.N04197 RIS BibTex EndNote
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Last revised: February 24, 2014