  Monolayers of semiconducting transition metal dichalcogenides are a strongly
emergent platform for exploring quantum phenomena in condensed matter, building
novel opto-electronic devices with enhanced functionalities. Due to their
atomic thickness, their excitonic optical response is highly sensitive to their
dielectric environment. In this work, we explore the optical properties of
monolayer thick MoSe$_2$ straddling domain wall boundaries in periodically
poled LiNbO$_3$. Spatially-resolved photoluminescence experiments reveal
spatial sorting of charge and photo-generated neutral and charged excitons
across the boundary. Our results reveal evidence for extremely large in-plane
electric fields of 3000\,kV/cm at the domain wall whose effect is manifested in
exciton dissociation and routing of free charges and trions toward oppositely
poled domains and a non-intuitive spatial intensity dependence. By modeling our
result using drift-diffusion and continuity equations, we obtain excellent
qualitative agreement with our observations and have explained the observed
spatial luminescence modulation using realistic material parameters.
