POLING DYNAMICS AND RELAXATION OF POLAR ORDER IN GUEST/HOST POLYMERS BY 2ND-HARMONIC GENERATION

A detailed study of the decay of the second-harmonic generation (SHG) signals of several guest-host nonlinear optical polymer systems has been carried out. The decay of the SHG signal is found to be affected by surface and trapped space charges. A fast component is observed at a low poling field and is found to be mainly due to surface charges which orient the chromophores near the surface. A second component, associated with macroscopic polarization, is-induced at high field. The intensity of the second component, which decays considerably slower than the fast component, rapidly increases when the poling held is greater than a threshold voltage. Above the temperature of glass transition T-g, a single exponential function gives a good fit to the shape of the second component of the SHG intensity decay curve. The time constant of the slow component is found to increase with successive poling-decay cycles and reaches a steady-state value after several cycles have been performed. This lengthening effect is found both above and below T-g. Furthermore, the relaxation time of the slow component is also found to depend on the poling field strength: when the host is polymethylmethacrylate, it is found that the relaxation time increases with increasing poling field strength; on the other hand, it decreases with the poling field when polysulfone is used as the host. The temperature dependence of the steady-state relaxation time is not Arrhenius; the Vogel-Fulcher-Tammann equation gives a satisfactory fit to the temperature dependence data.