Bloch-phonon coupling and tunneling-induced coherent phonon excitation in semiconductor superlattices

The coupling of Bloch oscillations to longitudinal optical phonons is investigated in a narrow-well InGaAs/InAlAs superlattice. A strong increase of coherent phonon amplitudes is observed when the Bloch oscillations are subsequently tuned into resonance with different optical phonon modes. The rapid dephasing of the Bloch oscillations due to field induced tunneling from the weakly bound miniband into above-barrier continuum states leads to an additional and new excitation process of coherent phonons in superlattices at high electric fields. Polarization dependent measurements confirm their generation via Coulomb interaction excluding any contribution through impulsively stimulated Raman scattering.