Production of coherent radiation of frequency \(k\ \nu \) (\(k=2,3,...\)) 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.