Ultrafast Optical Control of Exciton Diffusion in WSe2/Graphene Heterostructures Revealed by Heterodyne Transient Grating Spectroscopy

Using heterodyne transient grating spectroscopy, we observe a significant enhancement of exciton diffusion in a monolayer WSe₂ stacked on graphene. The diffusion dynamics can be optically tuned within a few picoseconds by altering the photoexcited carrier density in graphene. The effective diffusion constant in initial picoseconds in the WSe₂/graphene heterostructure is (40.3 ± 4.5) cm² s^-1, representing a substantial improvement over (2.1 ± 0.8) cm² s^-1, typical for an isolated WSe₂ monolayer. This enhancement can be understood in terms of a transient screening of impurities, charge traps, and defect states in WSe₂ by photoexcited charge carriers in graphene. Furthermore, diffusion within WSe₂ is affected by interlayer interactions, such as charge transfer, varying with the incident excitation fluence. These findings underscore the dynamical nature of screening and diffusion processes in heterostructures of 2D semiconductors and graphene and provide insights for future applications of these systems in ultrafast optoelectronic devices.