Abstract
The photorefractive properties of LiNbO3∶Fe and LiNbO3∶Cu have been studied in combination with optical absorption-, Mössbauer- and EPR-measurements. The charge states of Fe in successively reduced LiNbO3∶Fe have been investigated with respect to the influence on the photorefractive sensitivity and saturation value of the refractive index change. The results of this experiment demonstrate clearly the close correlation between the concentration of Fe2+ impurities and the optical absorption band around 2.6 eV in LiNbO3∶Fe, which is known to give rise to an anisotropic charge transport upon optical excitation. The resulting photocurrents determine the photorefractive sensitivity mainly in the initial state of halographic exposure. With increasing conversion from Fe3+ to Fe2+ the photorefractive sensitivity saturates and the saturation value of the refractive index change decreases remarkably.
In the case of LiNbO3∶Cu a similar behaviour of the photorefractive storage parameters after successive reduction treatments has been observed qualitatively. However, in contradiction to LiNbO3∶Fe the Cu2+ centers cannot be related to the photorefractive sensitivity of LiNbO3∶Cu.
These results are discussed with respect to the predictions of two models concerning the microscopic nature of the photorefractive process in doped LiNbO3.
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