Nonlinear optical studies of Bloch oscillations

We have investigated the coherent dynamics of wave packets in semiconductor superlattices with an applied electric field. In particular, we have performed four-wave mixing experiments with time-resolving of the diffracted beam. The temporal behaviour of the signal shows the oscillation dynamics of the Bloch wave packet. Additionally, the delayed rise of the signals clearly demonstrates the influence of many-body effects due to Coulomb coupling in the nonlinear optical response of the system. We show that the influence of the many-body effects can be tuned by changing the static applied field: for low field (small exciton binding energy), the FWM signal rises fast; for high field (large exciton binding energy), the signal rises slowly. The experimental results are well described by theory.