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.

Second harmonic generation studies of intrinsic and extrinsic relaxation dynamics in poly(methy1 methacrylate)

“…Our results are in agreement with previous results [33][34][35]. Charge injected by the electrodes into the matrix could be responsible for the memory effect in a number of ways.…”

“…The scans showed a single glass transition at 85°C for pure PMMA and 80°C for DR1/PMMA mixture. All the SHG measurements were carried out above T g. We note that the present study uses atactic PMMA with much lower molecular weight than other studies [33][34][35]. The molecular weight dependence on the structural relaxation and the glass transition of atactic PMMA is well documented [37].…”

“…It must be pointed out that in most works on SHG, e.g. see [33][34][35], very limited attention was paid to the SHG onset, most probably due to both experimental complications [28] and the theoretical conclusion [36], with experimental support [28], that the onset and the decay curves exhibit equal relaxation times.…”

“…Next, the effect of thermal conditions above T g on the characteristic time of both the onset and the decay mode was examined. The effect of the poling process on the SHG signal has been widely studied [32][33][34][35]. Wang et al [33][34][35] found that in a sequence isothermal poling experiment the relaxation time is lengthening after every poling process.…”

“…To understand and interpret our data it is essential to take into account the charge effects [33]. So we conducted isothermal poling process above T g to investigate charge effect on relaxation time.…”