A flexible high-repetition-rate source of sub-half-cycle terahertz pulses based on spatially indirect interband transitions

We present a flexible and scalable source of strong phase-locked THz pulses based on shift currents in type-II-aligned epitaxial semiconductor heterostructures. The THz waveforms are extremely asymmetric and exhibit only 0.45 optical cycles within the FWHM of their intensity envelope, peak fields above 1.1 kV cm
^-1
and spectral components up to the mid-infrared, at a repetition rate of 4 MHz. Our detailed quantum mechanical analysis reveals a novel emission process, where local charging dynamics pulls spatially separated electrons and holes back together, resulting in emission of almost unipolar THz waveforms.