  Valley polarization is amongst the most critical attributes of atomically
thin materials. However, achieving a high contrast from monolayer transition
metal dichalcogenides (TMDs) has so far been challenging. In this work, a giant
valley polarization contrast up to 45% from a monolayer WS2 has been achieved
at room temperature by using a single chiral plasmonic nanoparticle. The
increased contrast is attributed to the selective enhancement of both the
excitation and the emission rate having one particular handedness of the
circular polarization. The experimental results were corroborated by the
optical simulation using finite-difference time-domain (FDTD) method.
Additionally, the single chiral nanoparticle enabled the observation of
valley-polarized luminescence with a linear excitation. Our results provide a
promising pathway to enhance valley contrast from monolayer TMDs and utilize
them for nanophotonic devices.

Applied Physics