Experimental Investigation of the Effect of AC and DC Fields on Photorefractive Phenomena.

In this dissertation, we have investigated the effect of an applied DC and AC fields on the enhancement of the photorefractive phenomena, both theoretically and experimentally. The numerical solutions predict that the photorefractive gain and the response time can be enhanced when a field is applied to a crystal. Experimental results for both beam fanning and two beam coupling carried out on several BSKNN crystals with different dopant concentrations support the prediction. In addition, the experimental results showed that an applied AC field resulted in a decrease in the photorefractive response time while avoiding the field screening effect associated with an applied DC field. A direct comparison of the numerical solutions with the experimental results indicates good agreement between theory and experiment on the temporal behavior. Based on numerical and experimental results we are able to predict the optimum parameters for producing a faster response time with an applied electric field. In addition to studying the effect of an electric field on crystal properties, we also demonstrated a new technique for measuring the effective trap density in a crystal. The technique is based on measuring the phase shift between the fringe pattern and the grating while applying a DC field to the crystal. We derived an equation for determination of the phase shift between the fringe pattern and the grating. By substituting the experimental data into this equation we determined the trap density in the crystal. Based on the same experimental data we were also able to determine the dependence of both the dielectric constant and the electro-optic coefficient on the applied field. Our results were compared with the traditional method of measuring the trap density and good agreement between both measurements is achieved. Finally, we investigated the dependence of grating formation and erasure times on an applied DC field, both theoretically and experimentally. The results showed that the grating erasure time was longer than the formation time as a field was applied. A good agreement between theory and experiment was obtained. With the same experimental geometry we also measured the dependence of the grating erasure rate on optical intensity.