Picosecond polarization rotation by photorefractive and free-carrier index gratings

Abstract

Recently, we reported extensive measurements of the energy transfer in picosecond two-beam-coupling experiments in semi-insulating GaAs in which the photorefractive gain competed with (and was obscured by) nonlinear loss and transient energy transfer from the Drude-Lorentz free-carrier index grating.1 Here, by using a crossed polarizer-analyzer pair in a similar geometry, we measure the pump-induced rotation of the probe polarization as a function of pump fluence, time delay between the two pulses, pump polarization, analyzer angle, and sample orientation. The photorefractive and free-carrier index gratings are expected to have dramatically different dependences on these experimental parameters. We illustrate these dependences and use them to identify, separate, and contrast the contributions of the two gratings. One advantage of this technique, which we demonstrate, is that the contribution of the photorefractive grating to the cross-polarized two-beam coupling can be separated from the contribution of the free-carrier grating by controlling the pump polarization. Thus we separate and measure the photorefractive gain using this technique, and we compare it to the gain deduced from our conventional scalar two-beam coupling measurements and to a modified version of our original theory.