  TaAs and NbAs are two of the earliest identified Weyl semimetals that possess
many intriguing optical properties, such as chirality-dependent optical
excitations and giant second harmonic generation (SHG). Linear and nonlinear
optics have been employed as tools to probe the Weyl physics in these crystals.
Here we extend these studies to address two important points: determining the
complete anisotropic dielectric response, and to explore if and how they can
reveal essential Weyl physics. For the first time, we determine the complete
anisotropic dielectric functions of TaAs and NbAs by combining spectroscopic
ellipsometry and density functional theory (DFT). Parameterized Lorentz
oscillators are reported from 1.2-6 eV (experiment) and 0-6 eV (DFT), and good
agreement is shown between them. Both linear and nonlinear optical properties
have been reported to reveal Weyl physics. We suggest that strong optical
resonances from trivial bands are the likely origin of the large optical second
harmonic generation previously reported at these energies. Furthermore, by
comparing the contribution of a small k-space centered around the Weyl cones to
the total linear dielectric function, we find that these contributions are
highly anisotropic and are <25% of the total dielectric function below 0.5 eV;
above 1eV, these contributions are negligible. Thus, the study of Weyl physics
using optical techniques requires very low energies and even there, a careful
assessment is required in distinguishing the much smaller contributions of the
Weyl bands from the dominant contributions of the trivial bands and Drude
response to the total dielectric function.
