Topological collective plasmons in bipartite chains of metallic nanoparticles

TL;DR

This paper explores topological properties of collective plasmons in bipartite chains of metallic nanoparticles, revealing Dirac-like excitations, edge states, and topological phase effects analogous to electronic topological insulators.

Contribution

It analytically derives the spectrum and eigenstates of collective plasmons, demonstrating topological phases and edge states in nanoparticle chains with potential for robust plasmonic transport.

Findings

Presence of Dirac-like collective plasmon excitations

Existence of topologically protected edge states

Observation of a nontrivial Zak phase indicating topological phases

Abstract

We study a bipartite linear chain constituted by spherical metallic nanoparticles, where each nanoparticle supports a localized surface plasmon. The near-field dipolar interaction between the localized surface plasmons gives rise to collective plasmons, which are extended over the whole nanoparticle array. We derive analytically the spectrum and the eigenstates of the collective plasmonic excitations. At the edge of the Brillouin zone, the spectrum is of a pseudorelativistic nature similar to that present in the electronic band structure of polyacetylene. We find the effective Dirac Hamiltonian for the collective plasmons and show that the corresponding spinor eigenstates represent one-dimensional Dirac-like massive bosonic excitations. Therefore, the plasmonic lattice exhibits similar effects to those found for electrons in one-dimensional Dirac materials, such as the ability for transmission with highly suppressed backscattering due to Klein tunneling. We also show that the system is governed by a nontrivial Zak phase, which predicts the manifestation of edge states in the chain. When two dimerized chains with different topological phases are connected, we find the appearance of the bosonic version of a Jackiw-Rebbi midgap state. We further investigate the radiative and nonradiative lifetimes of the collective plasmonic excitations and comment on the challenges for experimental realization of the topological effects found theoretically.