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Tbe dynamics of coherent wavepacket oscillations in GaAsiAIxGal_xAs superlattices are investigated by transmittive electro-optic sampling (TEDS) with femtosecond time resolution. Tbe internal polarization dynamics of coherently superposed Wannier-Stark states in an externally biased superlattice, denoted as Bloch oscillations, can be monitored with high sensitivity in TEDS experiments. The amplitude, dephasing, and frequency of Bloch oscillations are studied as a function of applied electric field. . In femtosecond tirneresolved optical experiments petfonned on eiectro-optic materials the dynamies of coherent and incoherent polarization changes can be studied in the THz range.Here, we present filTIl data on the dynamies of Bloch oscillations obtained by time-resolved. transmittive electrooptic sampling (TEDS) [5].The unique feature of Bloch oscillations is their tunability by variation of the electric field strength. Tbe expected dependence of the osciHation frequency v on the electric field F is given by v = eFdJh, where d is the superlattice period and h Planck's constant. Tbis field dependence follows from both, the semi-classical Bloch theory as weIl as from the energy splitting of Wannier-Stark states, as long as excitonic comributions to the binding energy are neglected. In addition to the frequency, the Stark localization of the wavefunction involved depends strongly on the electric field and defmes the spatial amplitude of

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