Nonlinear optical properties of quasilinear conjugated oligomers, polymers and organic molecules

Recent developments concerning the nonlinear optical (NLO) properties of quasilinear conjugated oligomers, polymers and organic molecules are reviewed. Although our approach is from a theoretical perspective, it is oriented towards the practical design of both second-and third-order NLO materials. Since the held is relatively new, we critically assess the current state of knowledge and indicate where further investigations are needed. Some of the topics covered, which have received limited attention in the past, are vibrational hyperpolarizabilities, the potential for enhancement of NLO response due to formation of charged structures, and the quantitative role of the solid-state medium. After the introduction there is a section on the theoretical background which presents the formal expressions that are needed, introduces the available computational methodology and points out its limitations especially when applied to large systems. Then we discuss the results that have been obtained, starting with some general elements and moving on to specifics for second-and third-order materials. There is an emphasis on ab initio calculations but semiempirical treatments are considered where appropriate. Finally, the concluding section deals with the important effect of intermolecular interactions.