BAND-EDGE PHOTOREFRACTIVITY IN SEMICONDUCTORS - THEORY AND EXPERIMENT

At wavelengths close to the band edge, strong photorefractive gratings using the Franz-Keldysh electrorefractive effect can be written in semiconductors. Two-beam-coupling exponential gain coefficient as high as GAMMA = 16.3 cm-1 have been obtained in GaAs by combining the electrorefractive photorefractive grating with the conventional electro-optic photorefractive grating and using the moving grating technique to enhance the photorefractive space-charge field. A method for calculation of the gain coefficient near the band edge of materials is presented. The method is applied to GaAs and the results are compared to the experimental data. Reasonable agreement with experiment has been achieved. An optimal spectral range (910 nm < lambda < 930 nm) for near-band-edge photorefractivity in GaAs has been found. Conventional theories of photorefractivity based on Kukhtarev's equations are found to be sufficient for calculation of the photorefractive space-charge field near the band edge. Predictions of the gain coefficient near the band edge using the moving grating technique are presented. Other methods of increasing the photorefractive gain such as the temperature-dependent resonance in InP:Fe are also discussed.