Up- and Down-Conversion between Intra- and Intervalley Excitons in Waveguide Coupled Monolayer WSe2

We develop a waveguide coupled 1L tungsten diselenide (WSe2) device to probe its various exciton species. With TM and TE excitations, the optical fields can efficiently couple to the atomic layer's in-plane and out-of-plane dipole moments. Our work reveals several upconversion processes that bring out a coupling network linking spin-0 and spin-1 intra- and intervalley 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 promising platforms for manipulating versatile exciton species in TMD semiconductors.