  Chiral pumping from optical electric fields oscillating at THz frequencies is
observed in the Weyl material TaAs with electric and magnetic fields aligned
along both the a- and c-axes. Free carrier spectral weight enhancement is
measured directly for the first time, confirming theoretical expectations of
chiral pumping. A departure from linear field-dependence of the Drude weight is
observed at the highest fields in the quantum limit, providing direct evidence
of field-dependent Fermi velocity of the chiral Landau level. Implications for
the chiral magnetic effect in Weyl semimetals from the optical f-sum rule are
discussed.
