Bloch oscillations in semiconductor superlattices

More than 60 years ago, Zener has predicted that an electron fit a periodic potential subject to a static electric field will perform harmonic oscillations (so-called Bloch oscillations) in real space. We present here results which directly prove the existence of these spatial oscillations: By observing the field induced shift of the Wannier-Stark ladder, the macroscopic dipole moment associated with the Bloch oscillations of the electron wave packets can be traced. The displacement of the electron wave packet can then be deliverd as a function of time with the excitation density as the sole parameter. We discuss the dependence of Bloch oscillation dynamics oin the optical excitation conditions. In particular, we show that the motion can be continuously tuned between the harmonic spatial motion envisioned by Zener and a symmetric breathing-mode motion where no macroscopic dipole oscillation is present. The results further show that the dynamics of the wave packets can be significantly influenced by excitonic effects due to the photo excited holes.