Topological Spaser

We theoretically introduce a topological spaser, which consists of a hexagonal array of plasmonic metal nanoshells containing an achiral gain medium in their cores. Such a spaser can generate two mutually time-reversed chiral surface plasmon modes in the \mathbf{K}and {\mathbf{K}}^{\ensuremath{'}}valleys, which carry the opposite topological charges, \ifmmode\pm\else\textpm\fi{}1 and are described by a two-dimensional...