Coherent emission of electromagnetic pulses from bloch oscillations in semiconductor superlattices

In the last few years, terahertz-wave-emission spectroscopy has emerged as a powerful spectroscopic tool for the time-resolved investigation of coherent processes. We have employed this technique to study the spatial dynamics of coherent charge carriers in semiconductor superlattices. The observation of Bloch oscillations, i.e. the periodic motion of wave packets in a homogeneous electric field, in GaAs/AlGaAs superlattices is reported. The charge oscillations are detected via their emission of electromagnetic transients. Fundamental aspects of Bloch oscillations are discussed. We then address the proposal of Esaki and Tsu from 1970 to employ charge carrier oscillations in superlattices as a source for bias-tunable ultrahigh-frequency electromagnetic signals. A quasi-optical approach in contrast to an all-electrical scheme for generation and amplification of terahertz electromagnetic transients is presented.