  Recently, light-matter interaction has been vastly expanded as a control tool
for inducing and enhancing many emergent non-equilibrium phenomena. However,
conventional schemes for exploring such light-induced phenomena rely on uniform
and diffraction-limited free-space optics, which limits the spatial resolution
and the efficiency of light-matter interaction. Here, we overcome these
challenges using metasurface plasmon polaritons (MPPs) to form a sub-wavelength
optical lattice. Specifically, we report a ``nonlocal" pump-probe scheme where
MPPs are excited to induce a spatially modulated AC Stark shift for excitons in
a monolayer of MoSe$_2$, several microns away from the illumination spot.
Remarkably, we identify nearly two orders of magnitude reduction for the
required modulation power compared to the free-space optical illumination
counterpart. Moreover, we demonstrate a broadening of the excitons' linewidth
as a robust signature of MPP-induced periodic sub-diffraction modulation. Our
results open new avenues for exploring power-efficient light-induced lattice
phenomena below the diffraction limit in active chip-compatible MPP
architectures.
