(
)
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
is referred to as frequency doubling,
is frequency tripling and
is frequency quadrupling. Even higher integer values of 
are possible.
Source:
PAC, 1996, 68, 2223
(Glossary of terms used in photochemistry (IUPAC Recommendations 1996))
on page 2245