Photorefractive effect in crystals with a nonlinear recombination of charge carriers: theory and observation in KNbO_3

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Abstract
Photorefractive and photoconductivity steady-state as well as time-resolved measurements have been performed in strongly reduced KNbO3. The recorded photorefractive gratings show a nonexponential dark decay and an intensity-dependent two-wave mixing gain. The measured photoconductivity depends sublinearly on light intensity. The observed behavior is interpreted in terms of a band transport model that uses a single donor level with only electrons as free charge carriers, taking the nonlinear recombination of free charge carriers (owing to variation of the trap density) and the time dependence of the free-charge-carrier density into account. An analytical solution for the nonexponential dark decay of the photorefractive grating is derived. As an application of this model the results of the photorefractive and the stationary and transient photoconductivity experiments are used to determine the model parameters (thermal excitation rate, photoexcitation cross section, recombination coefficient, acceptor and donor densities, and electron mobility).