Ultrafast Optical Control of Exciton Diffusion in WSe_2/Graphene Heterostructures Revealed by Heterodyne Transient Grating Spectroscopy

Using heterodyne transient grating spectroscopy, we observe a significant enhancement of exciton diffusion within a monolayer WSe_2 stacked on top of graphene. We further demonstrate that the diffusion dynamics can be optically tuned on the ultrafast time scale (i.e. a few picoseconds) by altering the photoexcited charge carrier density in graphene. The results reveal that, on a time scale of a few picoseconds, the effective diffusion constant in the WSe_2/graphene heterostructure is approximately 40 cm^2/s, representing a substantial improvement over the 2 cm^2/s typical for an isolated monolayer of WSe_2. The enhanced diffusion can be understood in terms of a transient screening of impurities, charge traps, and defect states in WSe_2 by photoexcited charge carriers in graphene. Furthermore, we observe that the diffusion within WSe_2 is affected by interlayer interactions, like charge transfer, exhibiting different dynamic states depending on the incident excitation fluence. These findings underscore the dynamic nature of screening and diffusion processes in heterostructures of 2D semiconductors and graphene and provide valuable insights for future applications of these systems in ultrafast optoelectronic devices.