  We study the effect of point-defect chalcogen vacancies on the optical
properties of monolayer transition metal dichalcogenides using ab initio GW and
Bethe-Salpeter equation calculations. We find that chalcogen vacancies
introduce unoccupied in-gap states and occupied resonant defect states within
the quasiparticle continuum of the valence band. These defect states give rise
to a number of strongly-bound defect excitons and hybridize with excitons of
the pristine system, reducing the valley-selective circular dichroism. Our
results suggest a pathway to tune spin-valley polarization and other optical
properties through defect engineering.
