Time-resolved photocurrent spectroscopy of optically excited superlattices and the prospects for Bloch gain

We report on experiment and theory of the evolution of the electric field in undoped GaAs/AlGaAs semiconductor superlattices subjected to femtosecond optical excitation. We performed time-resolved pump-probe experiments and measured the photocurrent generated by spectrally narrowed and wavelength-tuned probe pulses as a function of delay time, pump power and bias field. The drift of the charge carriers, subsequent to the optical excitation, leads to the build-up of an inhomogeneity of the electric field which was traced via the temporal changes of the Wannier-Stark spectra. Although the photocurrent spectra by themselves only yield information on the absorption integrated spatially over the superlattice, we extract information on the local electric fields and the charge-carrier densities by a comparison of the measured data with the results of Monte-Carlo simulations. We find that at moderate excitation densities (10(exp 16) -cm-3 range) the superlattice within a few picoseconds splits into two moving field regions, one with strong field gradient and low electron density, the other with partially screened field at low gradient and high electron density. The largest field differences are found just when the last electrons are swept out after 10-30 ps, the exact time depending on the superlattice parameters and excitation conditions. The initial homogeneous field is restored on a much longer time scale of hundreds of picoseconds which is defined basically by the drift of the heavy holes. Our calculations show that Bloch gain in optically excited semiconductor superlattice is expected in spite of the inhomogeneous field if the field in the electron-rich region is not heavily screened. The time window during which Bloch gain exists is determined by the sweep-out of the electrons.