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

We present a novel experimental technique which allows to precisely measure the spatial displacement of Blochoscillating electrons in semiconductor superlattices as a function of time: The dipole field caused by the motion of the electrons is traced by small shifts of the Wannier-Stark ladder states. The electron wave packet displacement can then be derived from these shifts with the excitation density as the only free parameter. Using this method, we show that the optically generated electron wave packets perform harmonic oscillations, as predicted by Zener for the semiclassical motion of electrons in 1934. The absolute amplitudes of the wave packets depend inversely on the static field and are close to the values expected from semiclassical theory.