Bloch oscillations of excitonic wave packets in semiconductor superlattices

We present a detailed investigation of the coherent dynamics of excitonic wave packets composed of heavy/light-hole, electron miniband, and Wannier-Stark states in GaAs/${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As superlattices. Using transient degenerate four-wave mixing, we study the dependence of Bloch oscillations and heavy/light-hole beats on the applied field, miniband width, lattice temperature, and excitation conditions. Bloch oscillations are observed in samples with minibandwidths varying from 13 to 46 meV and at lattice temperatures up to 200 K. Under certain excitation conditions, we observe higher harmonics of the Bloch oscillation frequency. Spectrally resolved transient four-wave mixing experiments show unambiguously that quantum-mechanical interference is the origin of the oscillations. The experimental four-wave mixing traces are compared with a theoretical model based on many-level third-order density-matrix theory.