Direct measurement of the spatial displacement of bloch-oscillating electrons in semiconductor superlattices

Bloch oscillations in semiconductor superlattices have recently been observed using optical techniques. However, it remained to show that the optically observed quantum beats of the Wannier-Stark ladder are connected with a true spatial oscillation of the wave packet. We present an experimental technique which measures directly the displacment of the wave packet center-of-mass: The oscillating Bloch wave packets create a microscopic dipole moment which can he determined using the shift of the Wannier-Stark ladder transition energy as a sensitive dipole field detector. We show that the wave packet undergoes harmonic spatial motion in accordance with the predictions of Bloch and Zener.