Abstract
Monolayers of transition-metal dicalcogenides have emerged as two-dimensional semiconductors with direct bandgaps at degenerate but inequivalent electronic "valleys", supporting distinct excitons that can be selectively excited by polarized light. These valley-addressable excitons, when strongly coupled with optical resonances, lead to the formation of half-light half-matter quasiparticles, known as polaritons. Here we report self-assembled plasmonic crystals that support tungsten disulfide monolayers, in which the strong coupling of semiconductor excitons and plasmon lattice modes results in a Rabi splitting of ∼160 meV in transmission spectra as well as valley-polarized photoluminescence at room temperature. More importantly we find that one can flexibly tune the degree of valley polarization by changing either the emission angle or the excitation angle of the pump beam. Our results provide a platform that allows the detection, control, and processing of optical spin and valley information at the nanoscale under ambient conditions.