  Advent of new materials such as van der Waals heterostructures, propels new
research directions in condensed matter physics and enables development of
novel devices with unique functionalities. Here, we show experimentally that a
monolayer of MoSe2 embedded in a charge controlled heterostructure can be used
to realize an electrically tunable atomically-thin mirror, that effects 90%
extinction of an incident field that is resonant with its exciton transition.
The corresponding maximum reflection coefficient of 45% is only limited by the
ratio of the radiative decay rate to the linewidth of exciton transition and is
independent of incident light intensity up to 400 Watts/cm2. We demonstrate
that the reflectivity of the mirror can be drastically modified by applying a
gate voltage that modifies the monolayer charge density. Our findings could
find applications ranging from fast programmable spatial light modulators to
suspended ultra-light mirrors for optomechanical devices.
