A model of the photorefractive effect is presented that includes a shallow-trap level (hole or electron trap) as well as a species of deep traps with electron and hole excitation. It is shown that, if the shallow traps can trap a significant density of charge carriers, the electron—hole competition factor ξ and the photorefractive grating can switch signs as a function of intensity. This effect may be realized in BaTiO₃ crystals, in which the photorefractive effect is strongly affected by electron—hole competition as well as by shallow traps. As in the case of the single-charge-carrier model, it is shown that each of the charge gratings in the shallow and deep levels consists of in-phase gratings and a screening grating. It is shown that part of the screening gratings remains nonzero at infinitly large grating spacings.

© 1992 Optical Society of America

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