  In van der Waals (vdW) heterostructures formed by stacking two monolayer
semiconductors, lattice mismatch or rotational misalignment introduces an
in-plane moir\'e superlattice. While it is widely recognized that a moir\'e
superlattice can modulate the electronic band structure and lead to novel
transport properties including unconventional superconductivity and insulating
behavior driven by correlations, its influence on optical properties has not
been investigated experimentally. We present spectroscopic evidence that
interlayer excitons are confined by the moir\'e potential in a high-quality
MoSe2/WSe2 heterobilayer with small rotational twist. A series of interlayer
exciton resonances with either positive or negative circularly polarized
emission is observed in photoluminescence, consistent with multiple exciton
states confined within the moir\'e potential. The recombination dynamics and
temperature dependence of these interlayer exciton resonances are consistent
with this interpretation. These results demonstrate the feasibility of
engineering artificial excitonic crystals using vdW heterostructures for
nanophotonics and quantum information applications.
