Photorefractive phase shift induced by nonlinear electronic transport

A photorefractive phase shift can be generated under dc applied fields if the dominant photocarriers have a nonlinear velocity-field dependence with a vanishing differential mobility. Phase shifts as large as pi/2 are possible when velocity saturation disables dielectric relaxation while still permitting large drift rates. The inability of the space-charge field to relax leads to a saturated trap density that mimics trap-limited behavior. All direct-gap photorefractive semiconductors have strong velocity saturation from hot-electron transport effects, most widely known for the origin of the Gunn effect. Previous photorefractive trap-limited-field studies may have to be reevaluated in the context of transport nonlinearity.