Coherent terahertz emission with tunable ellipticity and optical chirality from the Weyl semimetal TaAs

Polarization control of ultrafast coherent light pulses through light-matter interactions in quantum materials holds the key to many technical innovations, including quantum communication, on-chip information processing and storage. Here, we discover strong coherent terahertz emission from the Weyl semimetal TaAs and demonstrate unprecedented manipulation over its polarization and chirality on a femtosecond timescale. Such controllability is achieved via polarization-dependent, colossal ultrafast photocurrents in TaAs generated using the circular and/or linear photogalvanic effect. The experimental results are well supported by theory. Our findings pave the way to design conceptually novel chiral photon sources using quantum materials and open up new opportunities for developing ultrafast opto-electronics in topological Weyl semimetals.