Collective name applied to techniques related to non-linear optical effects.
- Some of these spectroscopies are four-wave mixing, @H02834@, photon echoes, time-resolved coherent @A00381@-@S06027@ Raman (CARS), transient grating and stimulated pumping. These techniques differ in the nature of the pulse @ST06775@, the geometry and the choice of a spatial direction (phase matching), as well as the @M03958@ of detection.
- Low order non-linear techniques, such as three-wave mixing, are related to the second-order @P04712@. For a random @I03353@ medium with @I03146@ symmetry, the second-order @P04712@ vanishes and then the lowest order optical non-linear techniques, as well as the higher order non-linear techniques are related to the third-order @P04712@ and the corresponding 'hyper-susceptibility'.