Without using the usual assumption of free-carrier quasi-static distributio
n, we derive a forced-oscillation equation with damping to describe the bui
ldup of a fundamental harmonic space-charge field in inorganic photorefract
ive (PR) crystals. Thus the PR holographic records of static gratings and d
ynamic gratings can be understood in terms of the usual vibration theories.
The solution of the equation is biexponential and encompasses two time con
stants that had been observed in some PR materials. The ratio of the two ti
me constants is calculated as a function of experimental parameters, such a
s the total incident intensity and the applied electric field, to prove the
applicability of our model. Results show that the difference between the t
wo time constants may be so small that neither of the constants can be negl
ected. Additionally, the resonance frequencies obtained from the two models
for the moving grating are compared, and a figure of merit Q is defined to
describe the resonance properties. All the results show that our model can
be applied to general cases. (C) 2001 Optical Society of America.