Nonlinear transport in superlattices: Bloch oscillations and Zener breakdown

Semiconductor superlattices have been intensively used as a model system for the investigation of high field transport in solids. Here, we discuss ultrafast optical experiments which allow us to monitor the electron wave packet motion in detail. First, we discuss a novel technique which directly traces 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 harmonic Bloch oscillations. A comparison with theory shows that this coherent analog to the Shapiro effect in superconductors is associated with gain at THz frequencies. In a second part, we discuss recent results which address the Zener breakdown due to tunneling to higher bands by linear spectroscopy, which directly shows the wave function delocalization. We also trace the dynamics of the Zener tunneling by time-resolved spectroscopy and show the damping of Bloch oscillations due to Zener tunneling.