Poling dynamics and investigation into the behavior of trapped charge in poled polymer films for nonlinear optical applications

Abstract

The behavior of surface and trapped charge that originate during contact electric-field poling of a model guest–host nonlinearly active polymer has been investigated. Charge effects in thin 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 second harmonic generation (SHG) signal 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 is related to the mobility of charge. A sharp drop-off in SHG signal intensity that occurred when the poling electrodes were 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 increased as the film was subjected to multiple polings until a steady-state value was reached. Finally, it is demonstrated that the charges trapped in the polymer matrix are released only after the polymer is heated to high above its temperature of glass transition and the SHG signal has completely decayed away.

Enhanced Photorefractivity of Poly(N‐vinylcarbazole)‐Based Composites through Electric‐Field Treatments and Ionic Liquid Doping

“…Other researchers have attributed the large dark currents observed in ''fresh'' samples (non-exposed to either electric field or illumination) to mobile ionic impurities. [9,12] Thus, as the samples are kept under an electric field, mobile ions move towards the oppositely charged electrodes and are neutralized. As a consequence, dark currents decay with time until they reach steady state.…”

Birefringence and dielectric relaxation in strong electric fields

“…The study of these hyperpolarizabilities has received a continuing interest in development of nonlinear optics for useful practical applications. This is the case, for example, of the second harmonic generation of laser beams obtained by using new organic and polymeric materials having high nonlinear optical coefficient rather than standard inorganic crystals [93]. This effect is quadratic and based on first-order hyperpolarizability.…”

All Organic NLO Sol-Gel Material Containing a One-Dimensional Carbazole Chromophore

“…Similar phenomena have been observed in literature. [8,52] Yet, thermal treatments does not seem to provide enough information for optimizing the poling conditions. Overshooting poling temperatures (T p A 150 8C) leads to the drastic reduction of SH signal because of dielectric breakdown.…”