COHERENT EXCITATIONS IN THE STARK LADDER - EXCITONIC BLOCH OSCILLATIONS

We investigate theoretically the time evolution of electron and exciton wave packets in semiconductor superlattices in a uniform along-axis electric field. We obtain analytic results showing that electron wave packets undergo Bloch oscillations as predicted by semiclassical theory, but with spatial amplitudes which are strongly dependent on the initial conditions. We examine the nature of exciton wave packets in the experimentally relevant problem of short-pulse photoexcited electron-hole pairs in undoped semiconductor superlattices. We find that, when we neglect the effects of the electron-hole Coulomb interaction on the wave function in the direction of the field, the motion of the electron-hole separation ranges from a ''breathing mode'' all the way to a semiclassical-type Bloch oscillation, depending on the laser pulse width and frequency. When the electron-hole Coulomb interaction is included, much of the character of the motion is unaffected. There are, however, some significant changes in the nature of the wave packet evolution; in particular a breathing mode motion no longer exists, and there is an enhancement in the total oscillating dipole over a wide range of laser pulse parameters.