Toroidal excitations in free-space and metamaterials

Abstract

Toroidal excitations represent an intriguing new concept in classical electrodynamics that is rapidly becoming increasingly important in the interpretation of light-matter interactions. This thesis reports on new manifestations of electromagnetic toroidal excitations, both localised in matter, and propagating through free-space.

I have developed a set of new computational frameworks for evaluating the creation, propagation, and interaction of toroidal excitations. This framework shall form a key basis for the future study of toroidal excitations in free-space and matter. Key results of this framework are outlined below.

I have developed a methodology for generating "focused doughnut" pulses from metasurface arrays of dipole resonators. Numerical simulations indicate the generated pulse from such an array would be a 93% match for an ideal "focused doughnut". The generation scheme has been tested and found to perform well under anticipated experimental bandwidth limitations. Generation of a continuous train of "focused doughnut" pulses from a frequency-comb-like excitation of the array has been demonstrated.

I have, for the first time, investigated the interactions of "focused doughnut" pulses with continuous and particulate matter. I report on the peculiar field transformations that the "focused doughnut" pulse experiences under reflection at an interface, and the broadband modal excitation when incident on small particles. This includes the excitation of multiple, dominant toroidal dipolar resonances and dynamic anapole modes.

I have demonstrated that toroidal excitations contribute to optical activity. Using a chiral metamaterial structure, I observed a new microscopic mechanism for circular dichroism, underpinned by the resonant excitation of the toroidal dipole and the electric quadrupole. This toroidal circular dichroism supplements the well-established mechanism involving electric dipole and magnetic dipole transitions.

I have demonstrated the superior coupling of radially polarised vector beams to toroidal excitations in matter. The toroidal modes excited possess high quality factors of ∼ 124 and radiate with an almost exclusively dipolar radiation pattern.

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Identifiers

ORCID for Nikolai Zheludev: orcid.org/0000-0002-1013-6636

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