Tailoring the Valley Polarization in Monolayer MoS₂ by Plasmonic Chiral Structures

Monolayer transition metal dichalcogenides (TMDs) are semiconductors exhibiting direct band gaps at two degenerate but inequivalent valleys of electronic bands, which can endow carriers with valley pseudo-spins for future valleytronic applications. Although the control of valley polarization can be created by the helicity of light, it remains difficult to create valley polarization by lifting the valley degeneracy using magnetic field due to the small valley Zeeman splitting for typical monolayer TMDs. Here, we report on manipulating valley polarization in monolayer MoS₂ by plasmonic chiral structures. The chiral structures are based on Archimedean spiral antennas fabricated on single-crystalline Au flakes using focused ion beam. The resonance wavelength of these structures can be controlled by their geometry. As the chiral structure is on resonance with the excitonic emission of MoS₂, an imbalance between the left- and right-circularly polarized components of the optical density of states is created, which in turn affects the valley decay dynamics, giving rise to a net valley polarization and a high degree of circular polarization even at room temperature. Our results demonstrate a route for tailoring the valley-dependent dynamics of carriers in monolayer TMDs.