Collective scattering of subwavelength resonators in metamaterial systems

Abstract

In this thesis, we model the electromagnetic (EM) interactions between plasmonic resonators and an incident EM field. By capturing the fundamental physics of each resonator, such as its resonance frequency and radiative decay rate, the dynamics of the EM interactions can be modeled by a linear set of equations. The system's eigenmodes exhibit characteristic line shifts and linewidths that may be subradiant or superradiant.

For simple resonator systems, we approximate each resonator as a point electric dipole. Nanorods and nanobars, are such resonators where the magnetization can be assumed to be negligible. However, closely spaced parallel electric dipoles can exhibit EM properties similar to higher order multipoles, e.g., electric quadrupoles. We show how, in an original way, simple systems comprising closely spaced parallel pairs of electric dipoles can be approximated as electric quadrupole and magnetic dipole resonators. When the resonators are close, their finite-size and geometry is important to the EM interactions, and we show how we cope with these important factors in our model.

We analyze in detail a nanorod configuration comprising pairs of plasmonic nanorods - a toroidal metamolecule, and show how the elusive toroidal dipole moment appears as a radiative eigenmode. In this original work, we find that the radiative interactions in the toroidal metamolecule can be qualitatively represented by our point electric dipole approximation and finite-size resonator model. The results 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, we show the toroidal mode can be excited by linearly polarized light and that it 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.

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