  By investigating the optoelectronic properties of prototypical
graphene/hexagonal boron nitride (h-BN) heterostructures, we demonstrate how a
nanostructured combination of these materials can lead to a dramatic
enhancement of light-matter interaction and give rise to unique excitations. In
the framework of ab initio many-body perturbation theory, we show that such
heterostructures absorb light over a broad frequency range, from the
near-infrared to the ultraviolet (UV), and that each spectral region is
characterized by a specific type of excitations. Delocalized electron-hole
pairs in graphene dominate the low-energy part of the spectrum, while strongly
bound electron-hole pairs in h-BN are preserved in the near-UV. Besides these
features, characteristic of the pristine constituents, charge-transfer
excitations appear across the visible region. Remarkably, the spatial
distribution of the electron and the hole can be selectively tuned by
modulating the stacking arrangement of the individual building blocks. Our
results open up unprecedented perspectives in view of designing van der Waals
heterostructures with tailored optoelectronic features.
