Nonlinear optical properties of the main-chain type liquid crystalline polymers

The second harmonic generations (SHG) of liquid crystalline main-chain type polymers were investigated. The sample polymers are copolymers of 2-hydroxy-6-naphthoic acid (HNA) with 4-hydroxybenzoic acid (PHB), in various composition ratios. The SHG of the film samples was greatly affected not only by the polymerization conditions, but also by preparation conditions of the films. SHGs for the samples processed by mechanical stretching and for electrically poled samples were compared. The nonlinear optical coefficients d(exp) evaluated by Maker Fringe method of the electrically poled samples are 2-3 times larger than that of the mechanically oriented sample whose d(exp) is about 5pm/V. The high SHG of the main-chain type NLO polymers mainly originates from the additivity of hyperpolarizeability (beta) of the individual unit due to head to tail bonding.