Chirped Bloch oscillations in strain-balanced InGaAs/InGaAs superlattices

Bloch oscillations excited in a strain-balanced InxGa1-xAs/InyGa1-yAs super lattice by fs optical pulses at 1.55 mu m are investigated in time-resolved transmission spectroscopy. The transition from the coherent oscillatory mo tion to an incoherent drift transport of the electrons is observed via a tr ansient frequency shift of the Bloch oscillations due to the associated scr eening of the applied electric field. These electric field changes are anal yzed quantitatively as a function of the initial held strengths and excitat ion densities. The incoherent transport can be described by a drift-diffusi on model. As a result, the carrier mobility in the superlattice is obtained on a picosecond timescale. (C) 1999 Academic Press.