Toroidal dipole excitations in metamolecules formed by interacting plasmonic nanorods

TL;DR

This paper demonstrates the existence of toroidal dipole modes in plasmonic nanorod metamolecules, showing how symmetry breaking enables their excitation and proposing methods to optimize their excitation for advanced metamaterial control.

Contribution

It introduces a theoretical and experimental framework for exciting and controlling toroidal dipole modes in plasmonic nanorod metamolecules, highlighting symmetry breaking as a key factor.

Findings

Toroidal dipole modes are subradiant and hard to excite with incident light.

Symmetry breaking enables excitation of toroidal modes by linearly polarized light.

Optimized protocols can maximize toroidal dipole excitation.

Abstract

We show how the elusive toroidal dipole moment appears as a radiative excitation eigenmode in a metamolecule resonator that is formed by pairs of plasmonic nanorods. We analyze one such nanorod configuration - a toroidal metamolecule. We find that the radiative interactions in the toroidal metamolecule can be qualitatively represented by a theoretical model based on an electric point dipole arrangement. Both a finite-size rod model and the point dipole approximation demonstrate how the toroidal dipole moment is subradiant and difficult to excite by incident light. By means of breaking the geometric symmetry of the metamolecule, the toroidal mode can be excited by linearly polarized light and appears as a Fano resonance dip in the forward scattered light. We provide simple optimization protocols for maximizing the toroidal dipole mode excitation. This opens up possibilities for simplified control and driving of metamaterial arrays consisting of toroidal dipole unit-cell resonators.