The evolution of the electric field in an optically excited semiconductor superlattice

We report on time-resolved photocurrent spectroscopy of an intrinsic GaAs/Al(0.3)Ga(0.7)As superlattice subsequent to femtosecond optical excitation. Information on the spatio-temporal evolution of the densities of electrons and holes and on the internal electric field is obtained by tracing Wannier-Stark photocurrent spectra as a function of delay time for various bias fields and pump excitation intensities. The experimental results are supplemented by simulations. We employ the combined information to define the conditions to be met for successful pump-probe Bloch gain experiments. In particular, we find that field screening sets on upper limit for the carrier density of 10(exp 16) cm-3, and that the time window during which gain should be found is defined by the duration of the sweep-out of the optically injected electrons from the superlattice which occurs within about 10 ps after excitation.