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CPM Seminar
Marc Dignam
Department of Physics When a static electric field is applied to a semiconductor superlattice along the growth axis, the electronic states are localized about superlattice sites and have discrete energy levels with equal spacing -- the so-called Wannier-Stark Ladder (WSL). If this system is excited by a 100 fs optical pulse near the bandgap, the optical pulse creates an electronic state which is a coherent superposition of WSL states. This coherent wavepacket oscillates periodically, undergoing what are commonly called Bloch Oscillations (BO). If a Terahertz electric field is applied, which is nearly resonant with the BO frequency, then the wavepacket can be driven either up or down the WSL. Due to the symmetry in the WSL, for non-interacting electrons the wavepackets are only driven over a time given by the intraband dephasing time (~ 1 ps). However, when the electron-hole Coulomb interaction is taken into account in the formation of excitons, the symmetry is broken and the wavepackets can be driven for times much greater than the dephasing time. Under the correct conditions, this leads to long-time gain for Terahertz radiation, which opens the possibility of forming a THz laser from the excitonic WSL. In this talk, I will present recent theoretical results which demonstrate this and other related phenomena in the excitonic WSL.
Wednesday, February 28th 2001, 15:30 |