Transient grating studies of excitonic optical nonlinearities in GaAs/AlGaAs multiple-quantum-well structures

We report time-resolved studies of transient gratings in room-temperature GaAs/AlGaAs multiple-quantum-well structures made using a subpicosecond laser system. Forward-traveling and counterpropagating four-wave mixing geometries produced carrier modulation either parallel or perpendicular to the quantum wells such that the highly anisotropic carrier diffusion could be monitored. Carrier heating and cross-well transport was monitored and geometrical control of the diffusion and resulting four-wave mixing time constant demonstrated. Measured diffraction efficiencies gave the refractive-index change of the order of Δn ~ 0.02 at the onset of saturation of the heavy-hole exciton nonlinearity. Implications for refractive optical bistability are discussed through a determination of the limiting conditions for switching.