  The presence of two spin-split valleys in monolayer (1L) transition metal
dichalcogenide (TMD) semiconductors supports versatile exciton species
classified by their spin and valley quantum numbers. While the spin-0
intravalley exciton, known as the "bright" exciton, is readily observable,
other types of excitons, such as the spin-1 intravalley (spin-dark) and spin-0
intervalley (momentum-dark) excitons, are more difficult to access. Here we
develop a waveguide coupled 1L tungsten diselenide (WSe2) device to probe these
exciton species. In particular, TM coupling to the atomic layer's out-of-plane
dipole moments enabled us to not only efficiently collect, but also resonantly
populate the spin-1 dark excitons, promising for developing devices with long
valley lifetimes. Our work reveals several upconversion processes that bring
out an intricate coupling network linking spin-0 and spin-1 intra- and
inter-valley excitons, demonstrating that intervalley scattering and spin-flip
are very common processes in the atomic layer. These experimental results
deepen our understanding of tungsten diselenide exciton physics and illustrate
that planar photonic devices are capable of harnessing versatile exciton
species in TMD semiconductors.
