Anisotropic optical response in quasi-one-dimensional topological insulators Bi₄Br₄ and Bi₄I₄

Quasi-two-dimensional materials like graphite exhibit an anisotripic optical response that depends on the direction of polarization of the incident light. This anisotropy manifests as a relatively large in-plane optical conductivity, and a relatively small out-of-plane optical conductivity. In a similar way, the recently predicted topological insulators Bi₄Br₄ and Bi₄I₄ (which can be both first and second order) are quasi-one-dimensional; atomic chains with relatively large optical conductivity stack to form sheets with moderate transverse optical conductivity which pile into a three-dimensional material with relatively small out-of-plane optical conductivity. By using tight-binding models derived from first-principles calculations, we provide quantitative predictions for the optical properties. Additionally, we report refractive indices, absorbance coefficients, and the optical properties of surface states and of finite-layer systems.