Terahertz Radiation from Coherent Electron Oscillations in a Double-Quantum-Well Structure

Abstract

We report the direct observation of coherent charge oscillations in an asymmetric semiconductor double-well potential with a thin tunneling barrier between the wells. Such a structure allows tuning of the energy levels of the two wells by an external electric bias field. At a certain resonance bias (here: -0.4 V), the conduction band levels are aligned and the electron wave functions extend over both wells. A 100 fs optical pulse resonant in energy to the lowest interband transition generates an exciton wave packet in a single well only.[1] From this nonstationary state the electrons begin to oscillate between the two wells at a frequency that is determined by the tunneling probability through the barrier between the two wells. The holes remain localized because the valence band energy levels are not aligned at this bias. Electrons and holes together form an oscillating dipole that emits electromagnetic radiation in the THz frequency range until the phase coherence of the oscillating wave packets is destroyed.