Orbital Symmetry and the Optical Response of Single-Layer MX Monochalcogenides

We show that the absorption spectra of single-layer GaSe and GaTe in the hexagonal phase feature exciton peaks with distinct polarization selectivity. We investigate these distinct features from first-principles calculations using the GW-BSE formalism. We show that, because of the symmetry of the bands under in-plane mirror symmetry, the bound exciton states selectively couple to either in-plane or out-of-plane polarization of the light. In particular, for a p-polarized light absorption experiment, the absorption peaks of the hydrogenic s-like excitons emerge at large angle of incidence, while the overall absorbance reduces over the rest of the spectrum.