Abstract

Here, we compare the speed, magnitude, and origin of the photorefractive responses of 3aTiO₃ and GaAs on picosecond time scales using two-beam mixing, three-beam transient grating and four-wave mixing techniques. We also measure the dark current decay time and the erasure fluence for picosecond pulses. These materials were chosen for their contrasting properties. BaTiO₃ has a relatively large electro-optic coefficient (r₁₃n³≃350, r₃₃n³≃1200), but exhibits a small mobility¹ (μ≃1 cm²/Vs) and is believed to have a subnanosecond recombination time.² By comparison, GaAs has a much smaller electro-optic coefficient (r₄₁n³≃60), a large mobility (μ≃6000 cm²/Vs), and a recombination time that can exceed a nanosecond. Our measurements differ from more conventional cw and nanosecond experiments, not only in providing enhanced time resolution, but in that the generation rate can significantly exceed the recombination rate, and large numbers of free photoionized carriers can be produced. The Drude-Lorentz (or band filling) change in the refractive index associated with these free carriers is the source of a second refractive grating. We monitor and separate the contributions of both grating sources in both two-beam mixing and three-pulse transient grating geometries.

© 1987 Optical Society of America