  Two-dimensional transition metal dichalcogenides (2D TMDCs) are promising
candidates for ultra-thin active nanophotonic elements due to the strong
tunable excitonic resonances that dominate their optical response. Here we
demonstrate dynamic beam steering by an active van der Waals metasurface that
leverages large complex refractive index tunability near excitonic resonances
in monolayer molybdenum diselenide (MoSe2). Through varying the radiative and
nonradiative rates of the excitons, we can dynamically control both the
reflection amplitude and phase profiles, resulting in an excitonic phased array
metasurface. Our experiments show reflected light steering to angles between
-30{\deg} to 30{\deg} at three different resonant wavelengths corresponding to
the A exciton, B exciton, and trion. This active van der Waals metasurface
relies solely on the excitonic resonances of the monolayer MoSe2 material
rather than geometric resonances of patterned nanostructures, suggesting the
potential to harness the tunability of excitonic resonances for wavefront
shaping in emerging photonic applications.
