F. Vachss, FREQUENCY-DEPENDENT PHOTOREFRACTIVE RESPONSE IN THE PRESENCE OF APPLIED AC ELECTRIC-FIELDS, Journal of the Optical Society of America. B, Optical physics, 11(6), 1994, pp. 1045-1058
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.