Poling dynamics and relaxation of polar order in guest/host polymers by second-harmonic generation

Abstract

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 field is greater than a threshold voltage. Above the temperature of glass transition Tg, 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 Tg. 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.