Dark spots along slowly scaling chains of plasmonic nanoparticles

TL;DR

This study numerically explores how mismatched silver nanoparticle chains exhibit unique plasmonic resonances, leading to localized bright and dark spots with asymmetric field enhancements, influenced by symmetry breaking and polarization effects.

Contribution

It reveals the formation of dark spots associated with dark plasmons and demonstrates asymmetric field amplification in slowly scaling nanoparticle chains, highlighting new plasmonic localization phenomena.

Findings

Dark spots linked to dark plasmons with antiparallel dipoles.

Asymmetric field amplification with gain contrast over 200.

Hot-spots form on resonance even for orthogonal polarization.

Abstract

We numerically investigate the optical response of slowly scaling linear chains of mismatched silver nanoparticles. Hybridized plasmon chain resonances manifest unusual local field distributions around the nanoparticles that result from symmetry breaking of the geometry. Importantly, we find localization patterns characterized by bright hot-spots alternated by what we term \textit{dark} spots. A dark spot is associated to dark plasmons that have collinear and antiparallel dipole moments along the chain. As a result, the field amplification in the dark interjunction gap is extinguished for incident polarization parallel to the chain axis. Despite the strong plasmonic coupling, the nanoparticles on the sides of this dark gap experience a dramatic asymmetric field amplification with amplitude gain contrast $> 2 \times 10^2$. Remarkably, also for polarization orthogonal to the axis, gap hot-spots form on resonance.