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Electric-field domains in semi-insulating GaAs:Cr are investigated both by imaging the locally induced birefringence using a transverse electro-optic configuration and by measuring the dc current flowing through the crystal. It is shown that the formation of domains may be controlled by optical means.

The application of a voltage, Vo, across a photorefractive crystal in the presence of detuning is well known as a means of enhancing twowave gain. By mounting a crystal of bismuth silicate, with faces cut parallel to the (001), (110) and (110) planes, in a rotating holder, measurements of two. wave gain as a function of 0, the angle between the electric field vector Eo (in the [001] direction) and the grating vector, K (in the (1 TO) plane), have been made. Our results show that by having a nonzero value of 0 the signal gain can be improved by a factor of two. Furthermore the experimental results showed that the 0 dependent gain peaks differ in magnitude according to whether the crystal is rotated in an anti-clockwise or clockwise direction. In order to model the experimental results the steady-state expression for the space-charge field is extended to two dimensions and the usually neglected piezoelectric effect is included in the analysis of the modulation of the refractive index by the presence of this field inside the crystal. The asymmetry of the gain peaks is found to be caused by the optical activity of bismuth silicate and is predicted by the theory.

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