  Transition metal dichalcogenide monolayers are promising candidates for
exploring new electronic and optical phenomena and for realizing atomically
thin optoelectronic devices. They host tightly bound electron-hole pairs
(excitons) that can be efficiently excited by resonant light fields. Here, we
demonstrate that a single monolayer of molybdenum diselenide (MoSe2) can
dramatically modify light transmission near the excitonic resonance, acting as
an electrically switchable mirror that reflects up to 85% of incident light at
cryogenic temperatures. This high reflectance is a direct consequence of the
excellent coherence properties of excitons in this atomically thin
semiconductor, encapsulated by hexagonal boron nitride. Furthermore, we show
that the MoSe2 monolayer exhibits power- and wavelength-dependent
nonlinearities that stem from exciton-based lattice heating in the case of
continuous-wave excitation and exciton-exciton interactions when fast, pulsed
laser excitation is used. These observations open up new possibilities for
studying quantum nonlinear optical phenomena and topological photonics, and for
miniaturizing optical devices.
