Investigation of third-order nonlinearity of substituted-polybenzonitriles by heterodyned femtosecond optical Kerr gate

摘要

The femtosecond optical heterodyne detected otpical Kerr effect is described. The real and the imaginary parts of complex third-order optical nonlinearity can be effectively speared and their values and signs can be determined. Using this method, we have thoroughly investigated the substitution effect by adding side-groups on the -(-C $EQ N- )-conjugated system, a new class of nonlinear optical material, and find that the polybenzonitrile is an order of magnitude superior to poly-amino-nitrile. But when we further add substituents on benzoid rings, the enhancement is limited. We can say that for the series of polybenzonitrile, the benzoid ring can improve the $chi$+(3)$/ value more effectively than other substitutions. We also find the value of second-order hyperpolarizability depends on the polymerization degree, and have discussed the saturation effect when the polymerization increases.