Abstract

Time-resolved terahertz (THz) emission spectroscopy is a powerful tool to investigate coherent processes in semiconductors. With this measurement technique, Bloch oscillations of optically excited electron wave-packets in GaAs/AlGaAs semiconductor superlattices have been observed [1]. All time-resolved experiments carried out up to now with this and other techniques [2,3] concentrated on wave-packets generated by resonant excitation of excitonic and free-carrier states of the Wannier-Stark (WS) ladder at the fundamental bandedge of the conduction band. Here, we present evidence for Bloch oscillations obtained by excitation of WS ladder states at photon energies as high as 70-80 meV above the fundamental bandgap. This surprising result is interpreted as evidence for carrier scattering processes that preserve the relative phase of the Wannier-Stark states forming the wave-packets. Our observation suggests that Esaki-Tsu emitters based on electrical injection of electron wave-packets into a superlattice may be realized.

© 1995 Optical Society of America