Poling Dynamics and Investigation into the Behavior of Trapped Charges in PoledPolymer Films for Nonlinear Optical Applications

摘要

The behaviour of surface and trapped charges that originate during contactelectric-field poling of a model guest-host nonlinearly active polymer has been investigated. Charge effects in this films of para-nitroaniline doped polymethylmethacrylate were studied during and after the poling process by simultaneous measurement of the current through the poling circuit and the SHG signal from the polymer system. The poling current present in a fresh sample is found to differ in both magnitude and temporal dependence to that observed in the sample during subsequent polings. These effects are interpreted by the initial presence of ionic impurities in the sample and the injection of charge into the polymer matrix during poling. The magnitude of the steady-state current eventually reached during the poling process is found to depend on temperature and is related to the mobility of charges. A sharp drop-off in SHG signal intensity that occurs when the poling electrodes are grounded is shown to result from the removal of surface charge that orients chromophore dipoles near the surface of the film; the decay in the macroscopic polarization that then occurs is prolonged by charges trapped in the polymer matrix. The build-up of trapped charge explains a memory effect in which the SHG signal relaxation time gradually increases as the polymer is subjected to multiple polings until a steady-state value is reached. Finally, it is demonstrated that the charges trapped in the polymer matrix are released only after the polymer is heated to high above it temperature of glass transition and the SHG signal has completely decayed away. (MM).