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.The optical excitations of coherent excitonic wave packets and the study of their dephasing in semiconductor heterostructures using femtosecond time-resolved optical techniques is a subject of great current interest.1 Recently, the excitation of coherent wave packets performing Bloch oscillations (BO's) in GaAs/AIxGal-xAs superlattices (SL's) has been demonstrated. [2][3][4] The optically controlled coherent superposition of several Wannier-Stark (WS) transitions 5 creates a nonstationary wave packet in a biased SL. In close analogy to semiclassical BO's in solid state,6,7 the electronic wave packets perform oscillations in real and k space under the combined influence of a periodic potential of the SL and a static potential applied. 8 The first evidence for the existence of BO's in four-wave mixing (FWM) experiments is the fact that the frequency v can be tuned by the applied electric field F according to v=eFd/h, where d is the SL period and h Planck's constant. Evidence for the oscillation of the wave packet in space is drawn from the observation of eoherent THz emission. Here, we demonstrate an experimental method, which enables the direct detection of the internal polarization in the SL setup by the spatial wave-packet oscillations. Transmittive electro-optic sampling (TEOS) is based on the detection of the optical anisotropy induced by a macroscopic polarization P z( t) via the linear electro-optic effect. TEOS probes directly the amplitude and phase of BO's and with that the source of the THz emission.In the present paper, we would like to emphasize the specific difference between the modulation of the interband and intraband polarizations arising from the nonstationary wave packets initially generated. In FWM experiments, a modulation of the interband polarization with the Bloch frequency is detected. In THz and TEOS experiments, however, the dipole moment set up by the spatial oscillations of the electronic wave packets alone is detected, when the holes can be assumed as localized in weak electric fields. From this point of view, we also expect distinct differences in the dephasing times determined in FWM and TEOS experiments. The dephasing of the interband polarization in FWM depends on the loss of coherence of excitons, while in experiments sensi...

Vfe 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/Al Gai 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.