Electro-optic detection of Bloch oscillations.

The coherent motion of electronic wave packets in the Wannier-Stark regime of extemally biased GaAs/AIxGal _xAs superlattices is investigated by a time-resolved electro-optic technique. As predicted by the semiclassical theory, electronic wave packets undergo Bloch oscillations. They are observed via anisotropie changes in the refractive index associated with a intraband polarization, which is set up by the coherent spatial motion of electronic wave packets relative to localized holes. The oscillation amplitude depends strongly on the initial excitation conditions. Frequency, amplitude, and dephasing of Bloch oscillations are investigated over a wide range of electric fields. Complementary results to four-wave mixing and THz emission experiments are obtained.