Bloch wave packets in semiconductor superlattices: Composition and spatial displacement

We investigate the temporal and spatial displacement dynamics of optically excited wave packets in a semiconductor superlattice. We present an experimental technique which measures directly the displacement of the wave packet center-of-mass: the oscillating Bloch wave packets create a microscopic dipole moment which can be detected using the shift of the Wannier-Stark ladder transition energy as a sensitive field detector. A true spatial oscillation of the photo-excited wave packet identifies quantum beats of the Wannier-Stark states as Bloch oscillations. We show that the Bloch wave packet undergoes harmonic spatial motion in accordance with the predictions of Bloch and Zener. The influence of initial experimental conditions on evolution and composition of the wave packet is discussed.