Poling dynamics and effects of trapped charge in poled polymer films for nonlinear optical applications

The behavior of surface and trapped charge that originates during contact electric-field poling of thin films of optically nonlinear polymers has been investigated. Thin films of poly(methyl methacrylate) doped with two different tricyanothiophene chromophores have been studied during and after the poling process by simultaneous measurement of the current through the poling circuit and the second harmonic generation signal (SHG) from the polymer system. The poling current present in a fresh sample was found to differ in both magnitude and temporal dependence to that observed in the sample during subsequent polings. The magnitude of the steady-state current eventually reached during the poling process was found to depend on temperature and was related to the mobility of charge. A sharp drop off in SHG signal intensity occurring when the poling electrodes were grounded is considered to be due to 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 increased as the film was subjected to multiple polings. Finally, it is demonstrated that the release of charges trapped in the polymer matrix depends upon the heating rate and continues far above the glass transition temperature of the polymer.