“…In this section some specific examples of applications of thirdorder optical characterization using FWM, including Phase Conjugation and optical Kerr gate (OKG), will be presented. FWM has found applications in practically any media from gases, liquids, and solids [17,28,[33][34][35], including semiconductors [36], nanostructured materials [37], photorefractive crystals [38], molecules and polymers [39], and graphene [40]. Phase Conjugation, which is one of the most frequently applied varieties of FWM, has been exploited in many different fields [17] due to its ability to correct for wavefront distortion, which is very important for biological applications [41].…”