Recent Advances in Nonlinear Optical and Electro-Optical Materials

The discovery of optical second harmonic generation in crystals by Franken and his co-workers ( 1 ) in 1961 spurred rapid progress in nonlinear optics. Over the past two decades, this field has developed into a new branch of science (2). The rapid development was mainly due to simultaneous progress in laser technology. Only a laser can provide sufficiently strong, coherent radiation that can be efficiently converted in frequency and modulated in amplitude by nonlinear optical (NLO) and electro-optical (EO) effects. After the discovery of the NLO effect, scientists immediately recognized that any practical applications of NLO would depend on the development of new materials. Today some scientists are even inclined to think of NLO crystals as the future optical semiconductors, and the search for new materials is still very active. Historically, the development of NLO can be divided into three stages. The first stage was from 196 1 to the mid 1960s. During this period, the nonlinear response of matter was recognized in theory to be dependent upon susceptibilities X(n) and the applied optical electric field as shown in