Crystalline ${\mathrm{As}}_2$${\mathrm{Se}}_3$: Optical properties

Realistic ab initio total-energy calculations are performed to investigate the optical properties of crystalline ${\mathrm{As}}_2$${\mathrm{Se}}_3$. The complete dielectric tensor is calculated using interband transitions. This includes the off-diagonal element, which has not previously been considered in the literature. The presence of the off-diagonal element leads to optical axes which are not aligned with the crystal axes and whose directions change with frequency. Comparisons with experimental results on diagonal elements are made and a prediction of the off-diagonal reflectivity is given. The origin of the off-diagonal element is investigated, and the element is found to arise solely from interlayer interactions. Accordingly, experimental measurements of the off-diagonal element may reveal unique and interesting features of the environment in between the layers.