Dispersion and transitions of dipolar plasmon modes in graded plasmonic waveguides

TL;DR

This paper investigates how coupled plasmon modes behave in graded plasmonic waveguides, revealing new mode transitions and localization phenomena based on interparticle spacing and refractive index grading.

Contribution

It introduces a classification of localized plasmonic modes and demonstrates mode transitions driven by interparticle separation in graded waveguides.

Findings

Identification of three localized mode types: light, heavy, and light-heavy gradons.

Discovery of mode transitions when interparticle separation crosses a critical value.

Phase diagrams explain the mode behavior in lossless systems.

Abstract

Coupled plasmon modes are studied in graded plasmonic waveguides, which are periodic chains of metallic nanoparticles embedded in a host with gradually varying refractive indices. We identify three types of localized modes called "light", "heavy", and "light-heavy" plasmonic gradons outside the passband, according to various degrees of localization. We also demonstrate new transitions among extended and localized modes when the interparticle separation $d$ is smaller than a critical $d_c$, whereas the three types of localized modes occur for $d>d_c$, with no extended modes. The transitions can be explained with phase diagrams constructed for the lossless metallic systems.