DIRECT MEASUREMENT OF THE SPATIAL DISPLACEMENT OF BLOCH-OSCILLATING ELECTRONS IN SEMICONDUCTOR SUPERLATTICES

Bloch oscillations in semiconductor superlattices have recently been o bserved using optical techniques. However, it remained to show that th e optically observed quantum beats of the Wannier-Stark ladder are con nected with a true spatial oscillation of the wave packet. We present an experimental technique which measures directly the displacement of the wave packet center-of-mass: The oscillating Bloch wave packets cre ate a microscopic dipole moment which can be determined using the shif t of the Wannier-Stark ladder transition energy as a sensitive dipole field detector. We show that the wave packet undergoes harmonic spatia l motion in accordance with the predictions of Bloch and Zener.