Production of @[email protected] of frequency \(k\ \nu \)(\(k=2,3,...\)) from @[email protected] of frequency. In general, this effect is obtained through the interaction of @[email protected] light with a suitable optical medium with non-linear @[email protected] The case \(k=2\) is referred to as @[email protected], \(k=3\) is frequency tripling and \(k=4\) is frequency quadrupling. Even higher integer values of \(k\) are possible.