Mobility-lifetime product of photoexcited electrons in GaAs

We show that photorefractive beam coupling gain as a function of grating period measured in semi-insulating GaAs with an external ac electric field or with a dc field and moving fringes can be explained theoretically by a mobility-lifetime product about four orders of magnitude smaller than that obtained in low-field conditions. This reduction is caused by enhanced occupation of the L band at high fields coupled with the low L band mobility and by the increase in recombination due to the cascade capture process that occurs for electric fields above a few kV/cm.