  In-plane optical anisotropy has been detected from monolayer MoS$_2$ grown on
a-plane (11-20) sapphire substrate in the ultraviolet-visible wavelength range.
Based on the measured optical anisotropy, the energy differences between the
optical transitions polarized along the ordinary and extraordinary directions
of the underlying sapphire substrate have been determined. The results
corroborate comprehensively with the dielectric environment induced
modification on the electronic band structure and exciton binding energy of
monolayer MoS$_2$ predicted recently by first principle calculations. The
output of this study proposes the symmetry as a new degree of freedom for
dielectric engineering of the two-dimensional materials.
