Dirac plasmons in bipartite lattices of metallic nanoparticles

TL;DR

This paper theoretically investigates Dirac-like plasmons in bipartite metallic nanoparticle lattices, revealing tunable gapless and gapped phases with topological transitions influenced by lattice symmetry and dipole orientation.

Contribution

It introduces a comprehensive analysis of collective plasmon band structures in bipartite lattices, highlighting Dirac plasmons and topological phase transitions due to symmetry breaking.

Findings

Gapless phases exhibit massless chiral Dirac plasmons.

Inversion symmetry breaking induces gapped Dirac modes with Berry phase.

Topological phase transitions occur with sublattice shifts.

Abstract

We study theoretically "graphene-like" plasmonic metamaterials constituted by two-dimensional arrays of metallic nanoparticles, including perfect honeycomb structures with and without inversion symmetry, as well as generic bipartite lattices. The dipolar interactions between localised surface plasmons in different nanoparticles gives rise to collective plasmons that extend over the whole lattice. We study the band structure of collective plasmons and unveil its tunability with the orientation of the dipole moments associated with the localised surface plasmons. Depending on the dipole orientation, we identify a phase diagram of gapless or gapped phases in the collective plasmon dispersion. We show that the gapless phases in the phase diagram are characterised by collective plasmons behaving as massless chiral Dirac particles, in analogy with electrons in graphene. When the inversion symmetry of the honeycomb structure is broken, collective plasmons are described as gapped chiral Dirac modes with an energy-dependent Berry phase. We further relax the geometric symmetry of the honeycomb structure by analysing generic bipartite hexagonal lattices. In this case we study the evolution of the phase diagram and unveil the emergence of a sequence of topological phase transitions when one hexagonal sublattice is progressively shifted with respect to the other.