Collective plasmonic modes of metal nano-particles in two-dimensional periodic regular arrays

TL;DR

This paper studies the collective plasmonic modes in 2D periodic arrays of metal nanoparticles, revealing how their dispersion and mode qualities are influenced by inter-particle interactions and coupling with free photons.

Contribution

It introduces an efficient method to compute the full-dynamic dispersion relations and effective polarizability for 2D nanoparticle arrays, highlighting intrinsic properties like vanishing group velocities.

Findings

Transverse quasi-modes have high quality inside the light-cone.

In-plane modes experience significant radiation damping.

Strong photon coupling occurs near the light-line.

Abstract

We investigate the collective plasmonic modes of metal nano-particles in periodic two-dimensional (2D) arrays within a point-dipole description. As an open system, the full-dynamic dispersion relations of the 2D arrays are obtained through an efficient method which gives an effective polarizability describing the collective response of a system. Both the dispersion relations and mode qualities are simultaneously related to the imaginary part of the effective polarizability, which has contributions from the single-particle response as well as the inter-particle coupling. The transversal long-range dipolar interaction is dominated by a wave term together with a purely geometrical constant representing the static geometrical contribution to resonant frequencies. As concrete examples, we considered small Ag spheres arranged in a square lattice. We find that inside the light-cone, the transverse quasi-mode has a reasonably high mode quality while the two in-plane modes show significant radiation damping. Near the light-line, we observe strong coupling with free photons for the bands of the transverse mode and the transversal in-plane mode, and the longitudinal in-plane mode exhibits a negative group-velocity inside the light-cone. Vanishing group velocities in the light-cone for all the quasi-modes are found to be intrinsic properties of the 2D metal nano-sphere dense arrays.