harmonic frequency generation

Production of coherent radiation of frequency kν(k = 2, 3, ...) from coherent radiation of frequency. In general, this effect is obtained through the interaction of laser light with a suitable optical medium with non-linear polarizability. The case k = 2 is referred to as frequency doubling, k = 3 is frequency tripling and k = 4 is frequency quadrupling. Even higher integer values of k are possible.
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2245 (https://doi.org/10.1351/pac199668122223)