  Techniques to mold the flow of light on subwavelength scales enable
fundamentally new optical systems and device applications. The realization of
programmable, active optical systems with fast, tunable components is among the
outstanding challenges in the field. Here, we experimentally demonstrate a
few-pixel beam steering device based on electrostatic gate control of excitons
in an atomically thin semiconductor with strong light-matter interactions. By
combining the high reflectivity of a MoSe2 monolayer with a graphene split-gate
geometry, we shape the wavefront phase profile to achieve continuously tunable
beam deflection with a range of 10$^\circ$, two-dimensional beam steering, and
switching times down to 1.6 nanoseconds. Our approach opens the door for a new
class of atomically thin optical systems, such as rapidly switchable beam
arrays and quantum metasurfaces operating at their fundamental thickness limit.
