High speed absorption recovery in quantum well diodes by diffusive electrical conduction.

We present here picosecond time-resolved electro- absorption measurements in GaAs quantum well p-i-n diode structures. While the dynamics of the vertical transport is not completely understood at present, our data reveal the importance of the "lateral" propagation of the photoexcited voltage pulse over the area of the doped regions. We propose a two-dimensional "diffusive conduction" mechanism, which predicts a fast relaxation of the electrical pulse, with time constants ranging from 50fs to 500ps, determined by the size of the exciting spot, the resistivity of the doped regions and the capacitance of the intrinsic region.