Negative Group Velocity from Quadrupole Resonance of Plasmonic Spheres

TL;DR

This paper demonstrates that quadrupole resonances in plasmonic spheres within photonic crystals can produce negative group velocities, revealing an intrinsic negative dispersion mechanism without needing magnetic coupling or effective medium approximation.

Contribution

It analytically shows negative group velocities from quadrupole bands in plasmonic photonic crystals, highlighting an intrinsic negative dispersion mechanism distinct from traditional double negative media.

Findings

Quadrupole bands exhibit negative group velocities in simple cubic photonic crystals.

Negative dispersion is an intrinsic property of quadrupole resonances, not requiring magnetic coupling.

Coupled quadrupole resonances can be tuned to lower optical frequencies in nanoparticle clusters.

Abstract

We study the dispersions of plasmonic bands that arise from the coupling of electric quadrupole resonances in three dimensional photonic crystals (PCs) consisting of plasmonic spheres. Through analytical derivation, we show that two branches of quadrupole bands in simple cubic PCs with a small lattice constant possess negative group velocities. Distinct from double negative media in which the negative responses originates from the coupling of electric and magnetic responses (P and M), the negative dispersion induced by quadrupole resonance is an intrinsic property of quadrupole that does not require coupling to another degree of freedom. In addition, there is no simple effective medium description. In plasmonic systems composed of metallic nanoparticle clusters, the coupled quadrupole resonance may be tuned to lower optical frequencies, and the coupling strength between this quadrupole resonance and external electromagnetic (EM) waves are in the same order of the magnetic dipole M.