  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-principle calculations using the GW-BSE
formalisms. We show that the brightness of the exciton absorption peaks is
tunable with the polarization of the light. Due to the symmetry of the bands
under z-axis 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 s-like
excitons emerge at large angle of incidence, while the overall absorbance
reduces over the rest of the spectrum.
