Relaxation of Polar Orientational Order in Nonlinear Optical Polymers by Second Harmonic Generation

An investigation of the decay of the second harmonic generation SHG signals from a main chain accordion and a guesthost nonlinear optical NLO polymer has been carried out. The shape of the decay of the SHG signal from the main chain polymer is stretched exponential with the width parameter, beta, increasing with increasing temperature. The increase in beta indicates a narrowing of the distribution of relaxation times. On the other hand, the SHG decay signal from the guesthost system polarized with contact electrode poling is found to be better described by the sum of two single exponential functions with the first short time component affected strongly by the surface charge effect and the second slow component associated with the relaxation of the macroscopic polarization. The time constant of the slow component is found to be successively increasing with the polingdecay cycle and reaches a steady state value after several cycles. The relaxation time lengthening effect is obtained both above and below Tg. The effect is considered to be due to trapped charges that enhance the polar orientation correlation.