Picosecond investigations of the photorefractive effect in GaAs

Abstract

We report the first observation of the photorefractive effect on time scales of <50 ps in a sample of semi-insulating, undoped GaAs. In this crystal, which is from the same boule as a sample previously investigated with a cw 1.06- μm laser,1 we observe beam coupling between two equal Intensity beams with the direction of energy transfer dependent on the orientation of the crystal, an unambiguous signature of the photorefractive effect, The magnitude of the energy transfer is consistent with theoretical calculations based on the set of equations (rate equations, continuity, and Poisson's equation) proposed by Vinetskii and Kukhtarev.2 On subnanosecond time scales the space charge grating that is responsible for the photorefractive effect is caused by charge separation between conduction band electrons and ionized traps. Unlike the photorefractive effect in GaAs on time scales greater than the electron recombination time in which the internal space charge field is limited by the trapped charge density,1 in GaAs on subnanosecond time scales recombination is unimportant and the space charge is limited by the donor number density (the number density of electrons available for photoionization).