Interband optical investigation of Bloch oscillations in semiconductor superlattices

The experimental observation of Bloch oscillations in solids was long thought to be out of experimental reach. The invention of the semiconductor superlattice has changed this situation decisively. Transport experiments performed in the past two decades have given indirect evidence for the existence of Bloch oscillations. Recently, it has been possible to directly observe Bloch oscillations in semiconductor superlattices by optical experiments: interband laser excitation of several Wannier-Stark ladder states creates a wave packet which oscillates in time with a characteristic dependence on the static electric field. These oscillations have been observed by several optical methods, both using detection of the interband polarization (four-wave mixing and photorefractive sampling) and intraband polarization (THz emission spectroscopy). The latter experimental technique directly proves the emission of radiation at the Bloch oscillator frequency. Recent experiments have directly measured the displacement of the Bloch wave packet as a function of time, proving the harmonic dependence as predicted by Zener. A number of experiments have addressed other key physical parameters of Bloch oscillations like the damping mechanisms. This article gives a brief review of recent investigations of Bloch oscillations in superlattices, focused on the interband optical experiments performed in the last few years.