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Ellipsometric investigation of the mechanism of hologram storage in lithium niobate Wong, William K. Y.


A computer-controlled ellipsometer was used to obtain data on the birefringence changes induced in lithium niobate crystals by irradiation with an argon-ion laser. The instrument was set to take readings on a rectangular grid of points before and after irradiation. Irradiation was performed with circular and with narrow rectangular light beams. Results are compared with the prediction of the model of Chen in which he postulated the presence of an internal field Eo in the direction of the c-axis of the crystal. According to his theory, electrons, photoexcited from traps by the laser light, drift under the influence of Eo along the c-axis before being retrapped. The resulting space-charge field set between positive ionised centres and trapped electrons gives rise to the observed birefringence changes via the linear electro-optic effect of the crystal. Approximate mathematical models based on Chen's theory are used to solve the problem with narrow rectangular light beams. Results are compared with experiments.

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