Coupled Bloch-phonon oscillations in semiconductor superlattices

We investigate coherent Bloch oscillations in GaAs/AlxGa1-xAs superlattices with electronic miniband widths larger than the optical phonon energy. In these superlattices the Bloch frequency can be tuned into resonance with th e optical phonon. Close to resonance a direct coupling of Bloch oscillation s to LO phonons is observed which gives rise to the coherent excitation of LO phonons. The density necessary for driving coherent LO phonons via Bloch oscillations is about 2 orders of magnitude smaller than the density neces sary to drive coherent LO phonons in bulk GaAs. The experimental observatio ns are confirmed by the theoretical description of this phenomenon [A.W. Gh osh et al., Phys. Rev. Lett. 85, 1084 (2000)].