  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.
