Frequency-dependent photorefractive response in the presence of applied ac electric fields

Abstract

We consider the process of nonstationary enhancement of the photorefractive effect through the application of time-varying (square-wave) electric fields. By examining the full second-order equation governing the photorefractive response, we show that this response is strongly dependent on the frequency of the applied ac field and exhibits a maximum when the period of oscillation is between the photorefractive response time, τP, and the charge-recombination time in the medium, tR. Furthermore, we show that the diffraction efficiency, the gain, and the phase of the response fluctuate in tandem with the oscillating applied field, and we examine the dependence of these fluctuations on the driving frequency. These frequency-dependent phenomena are described both by numerical integration of the charge-transport equations and through an analytic solution thereof. From this analysis we obtain simple expressions for the magnitude of the efficiency, the gain, and the phase fluctuations and the conditions for maximum average diffraction efficiency. In particular, we show that the oscillation period yielding the maximum response behaves as a weighted geometric average of the response and the recombination times of the form T = constant × (τP2tR)1/3. These results are compared with those of earlier analyses of ac-field enhancement and are then verified in a BSO crystal to which ac fields over a range of frequencies are applied.