  We investigate polarization-dependent ultrafast photocurrents in the Weyl
semimetal TaAs using terahertz (THz) emission spectroscopy. Our results reveal
that highly directional, transient photocurrents are generated along the
non-centrosymmetric c-axis regardless of incident light polarization, while
helicity-dependent photocurrents are excited within the ab-plane. This is
consistent with earlier static photocurrent experiments, and demonstrates on
the basis of both the physical constraints imposed by symmetry and the temporal
dynamics intrinsic to current generation and decay that optically induced
photocurrents in TaAs are inherent to the underlying crystal symmetry of the
transition metal monopnictide family of Weyl semimetals.
