Nonlinear transport in superlattices: Bloch oscillations and Zener breakdown

Semiconductor superlattices have been intensively used as a model system fo r the investigation of high field transport in solids. Here, we discuss ult rafast optical experiments which allow us to monitor the electron wave pack et motion in detail. First, we discuss a novel technique which directly tra ces the center of mass spatial motion. We use this method for the detection of a linear motion of the wave packets, which is superimposed on the harmo nic Bloch oscillations. A comparison with theory shows that this coherent a nalog to the Shapiro effect in superconductors is associated with gain at T Hz frequencies. In a second part, we discuss recent results which address t he Zener breakdown due to tunneling to higher bands by linear spectroscopy, which directly shows the wave function delocalization. We also trace the d ynamics of the Zener tunneling by time-resolved spectroscopy and show the d amping of Bloch oscillations due to Zener tunneling. (C) 2001 Elsevier Scie nce B.V. All rights reserved.