Dark decay of photorefractive gratings and persistent photocurrents in Bi12SiO20are interpreted using a model of the photorefractive effect which includes shallow traps as well as deep traps. The model explains the grating spacing dependence of the coasting and initial decay rate quite well. The intensity dependence of the initial decay rate and coasting, however, suggest large optical absorption contradicting the results of direct measurements. It is suggested that the recombination of holes to the shallow traps as a possible explanation of the discrepancy. It is demonstrated, for the first time to our knowledge, that the photorefractive grating dark decay experiments can be used to determine the densities of ionized donors in dark (NA) in addition to the effective trap density (NE). We also report, for the first time to our knowledge, the observation of oscillatory dark decays of photorefractive grating in three Bi12SiO20crystals and explain them in terms of the dynamics of coupled charge gratings.
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