  Quadrupole topological insulator is a symmetry-protected higher-order
topological phase with intriguing topology of Wannier bands, which, however,
has not yet been realized in plasmonic metamaterials. Here, we propose a
lattice of plasmon-polaritonic nanocavities which can realize quadrupole
topological insulators by exploiting the geometry-dependent sign-reversal of
the couplings between the daisy-like nanocavities. The designed system exhibits
various topological and trivial phases as characterized by the nested Wannier
bands and the topological quadrupole moment which can be controlled by the
distances between the nanocavities. Our study opens a pathway toward plasmonic
topological metamaterials with quadrupole topology.
