  As a fascinating topological phase of matter, Weyl semimetals host chiral
fermions with distinct chiralities and spin textures. Optical excitations
involving those chiral fermions can induce exotic carrier responses, and in
turn lead to novel optical phenomena. Here, we discover strong coherent chiral
terahertz emission from the Weyl semimetal TaAs and demonstrate unprecedented
manipulation over its polarization on a femtosecond timescale. Such
polarization control is achieved via the colossal ultrafast photocurrents in
TaAs arising from the circular or linear photogalvanic effect. We unravel that
the chiral ultrafast photocurrents are attributed to the large band velocity
changes when the Weyl fermions are excited from the Weyl bands to the
high-lying bands. The photocurrent generation is maximized at near-IR frequency
range close to 1.5 eV. Our findings provide an entirely new design concept for
creating chiral photon sources using quantum materials and open up new
opportunities for developing ultrafast opto-electronics using Weyl physics.
