  The strong light-matter interaction in monolayer transition metal
dichalcogenides (TMDs) is promising for nanoscale optoelectronics with their
direct band gap nature and the ultra-fast radiative decay of the strongly bound
excitons these materials host. However, the impeded amount of light absorption
imposed by the ultra-thin nature of the monolayers impairs their viability in
photonic applications. Using a layered heterostructure of a monolayer TMD
stacked on top of strongly absorbing, non-luminescent, multi-layer SnSe2, we
show that both single-photon and two-photon luminescence from the TMD monolayer
can be enhanced by a factor of 14 and 7.5, respectively. This is enabled
through inter-layer dipole-dipole coupling induced non-radiative Forster
resonance energy transfer (FRET) from SnSe2 underneath which acts as a
scavenger of the light unabsorbed by the monolayer TMD. The design strategy
exploits the near-resonance between the direct energy gap of SnSe2 and the
excitonic gap of monolayer TMD, the smallest possible separation between donor
and acceptor facilitated by van der Waals heterojunction, and the in-plane
orientation of dipoles in these layered materials. The FRET driven uniform
single- and twophoton luminescence enhancement over the entire junction area is
advantageous over the local enhancement in quantum dot or plasmonic structure
integrated 2D layers, and is promising for improving quantum efficiency in
imaging, optoelectronic, and photonic applications.
