Linear dichroism of 3D DSMs Cd₃As₂ and Na₃Bi. We see from panels (a) and (b) that there is a large linear dichroism exhibited in the z-direction (black solid lines) due to the strong out-of-plane Fermi velocity anisotropy in p_z. We show in panels (c) and (d) that the anisotropy of the absorption coefficient persists over a range of realistic Fermi levels. We find that the anisotropy ratio of the absorption coefficient, α_i/α_j ∝ v_i/v_j. Unless otherwise stated, we consider the same parameters as those in Figs. 1 and 2. We compute the absorption coefficient using Eq. (14). We schematically illustrate in panel (e) a possible experimental geometry which could potentially enable the detection of the strong optical anisotropy of 3D DSM thin films. By changing the polarization of a linearly polarized laser normally incident on the x–z plane of the 3D DSM thin film (since v_x exceeds v_z by a significant amount), the strength of the transmitted field detected will change.