Topological Spaser in a Non-uniform Field*

We propose a nanospaser made of an achiral plasmonic-metal nanodisc and a two-dimensional
chiral gain medium, e.g., a monolayer transition-metal dichalcogenide (TMDC). Such a gain medium can pumped by circularly-polarized radiation into a valley-polarized (topologically-charged) state where the population-inversion of the K and K^′valleys may be different. Each valley's population inversion independently feeds a chiral spasing mode of the nanodisk. There are two such modes that carry opposite topological charges; they do not compete with or cross-talk to each other, and are highly independent. The local fields of these modes rotate in time in opposite directions and are highly nonuniform in space.