  Two-dimensional semiconductors have emerged as a new class of materials for
nanophotonics for their strong exciton-photon interaction and flexibility for
engineering and integration. Taking advantage of these properties, we engineer
an efficient lasing medium based on dipolar interlayer excitons, in
rotationally aligned atomically thin heterostructures. Lasing is measured from
a transition metal dichalcogenide hetero-bilayer integrated in a silicon
nitride grating resonator. A sharp increase in the spatial coherence of the
emission was observed across the lasing threshold. The work establishes
interlayer excitons in two-dimensional heterostructures as a silicon-compatible
coherent medium. With electrically tunable light-matter interaction strength
and long-range dipolar interactions, these interlayer excitons promise both
applications to low-power, ultrafast laser and modulators and rich many-body
quantum phenomena.
