FREQUENCY-DEPENDENT PHOTOREFRACTIVE RESPONSE IN THE PRESENCE OF APPLIED AC ELECTRIC-FIELDS

We consider the process of nonstationary enhancement of the photorefra ctive effect through the application of time-varying (square-wave) ele ctric fields. By examining the full second-order equation governing th e photorefractive response, we show that this response is strongly dep endent on the frequency of the applied ac field and exhibits a maximum when the period of oscillation is between the photorefractive respons e time, tau(P), and the charge-recombination time in the medium, t(R). Furthermore, we show that the diffraction efficiency, the gain, and t he phase of the response fluctuate in tandem with the oscillating appl ied field, and we examine the dependence of these fluctuations on the driving frequency. These frequency-dependent phenomena are described b oth by numerical integration of the charge-transport equations and thr ough an analytic solution thereof. From this analysis we obtain simple expressions for the magnitude of the efficiency, the gain, and the ph ase fluctuations and the conditions for maximum average diffraction ef ficiency. In particular, we show that the oscillation period yielding the maximum response behaves as a weighted geometric average of the re sponse and the recombination times of the form T = constant x (tau(P)2 t(R))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.