Anisotropic optical response of 3D Dirac semiemtals. (a) Schematic depiction of a 3D Dirac cone band structure with anisotropic Fermi velocities in each momentum direction. Longitudinal conductivities of anisotropic 3D DSMs Cd₃As₂ (b) and Na₃Bi (e) in each direction as a function of angular frequency ω. Due to the extreme anisotropy of the Fermi velocities between the in-plane (p_x, p_y) and out-of-plane (p_z) directions in both 3D DSMs, σ_zz ≪ σ_xx,σ_yy. The polar plots in (c), (d), (f), and (g) show the directional dependence of the optical conductivity σ as a function of angles. For all cases considered here, we considered 3D DSMs doped to ℰF=150 meV at temperature T = 4 K, a scattering time of 450 fs, and a combined spin and valley degeneracy g = 4. We consider experimentally determined values of the Fermi velocities, as listed in Table 1.