  Monolayer transition metal dichalcogenides (TMDs) are the first truly
two-dimensional (2D) semiconductor, providing an excellent platform to
investigate light-matter interaction in the 2D limit. Apart from fundamental
scientific exploration, this material system has attracted active research
interest in the nanophotonic devices community for its unique optoelectronic
properties. The inherently strong excitonic response in monolayer TMDs can be
further enhanced by exploiting the temporal confinement of light in
nanophotonic structures. Dielectric metasurfaces are one such two-dimensional
nanophotonic structures, which have recently demonstrated strong potential to
not only miniaturize existing optical components, but also to create completely
new class of designer optics. Going beyond passive optical elements,
researchers are now exploring active metasurfaces using emerging materials and
the utility of metasurfaces to enhance the light-matter interaction. Here, we
demonstrate a 2D exciton-polariton system by strongly coupling atomically thin
tungsten diselenide (WSe2) monolayer to a silicon nitride (SiN) metasurface.
Via energy-momentum spectroscopy of the WSe2-metasurface system, we observed
the characteristic anti-crossing of the polariton dispersion both in the
reflection and photoluminescence spectrum. A Rabi splitting of 18 meV was
observed which matched well with our numerical simulation. The diffraction
effects of the nano-patterned metasurface also resulted in a highly directional
polariton emission. Finally, we showed that the Rabi splitting, the polariton
dispersion and the far-field emission pattern could be tailored with
subwavelength-scale engineering of the optical meta-atoms. Our platform thus
opens the door for the future development of novel, exotic exciton-polariton
devices by advanced meta-optical engineering.
