1,720,962 research outputs found
Toroidal excitations in free-space and metamaterials
No full textToroidal 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
Electromagnetic Toroidal Excitations in Matter and Free Space
No full textData used in Figure 5 (“Focused Doughnut” (FD) pulses) in Papasimakis, N., Fedotov, V.A., Savinov, V., Raybould, T.A. and Zheludev, N.I. (2016) Electromagnetic toroidal excitations in matter and free space. Nature Materials.
The toroidal dipole is a localized electromagnetic excitation independent from the familiar magnetic and electric dipoles. While the electric dipole can be understood as separated opposite charges and the magnetic dipole as a current loop, the toroidal dipole corresponds to currents flowing on the surface of a torus. Resonant interactions of induced toroidal dipoles with electromagnetic waves have recently been observed at microwave, terahertz and optical frequencies. They provide distinct and physically significant contributions to the basic characteristics of matter including absorption, dispersion, and optical activity, the origin of which cannot be comprehensively interpreted in the context of standard multipoles alone.
Interference of radiating induced toroidal and electric dipoles leads to transparency windows in artificial materials as a manifestation of the dynamic anapole. Toroidal excitations also exist in free-space as spatially and temporally localized electromagnetic pulses propagating at the speed of light and interacting with matter.</span
Dataset for "Pulse Generation Scheme for Flying Electromagnetic Doughnuts"
No full textThis file contains the dataset for "Pulse Generation Scheme for Flying Electromagnetic Doughnuts" by Nikitas Papasimakis, Tim Raybould, Vassili A. Fedotov, D. P. Tsai, Ian Youngs, and Nikolay I. Zheludev in Physical Review B. </span
Exciting dynamic anapoles with electromagnetic doughnut pulses
No full textAs was predicted in 1995 by Afanasiev and Stepanovsky, a superposition of electric and toroidal dipoles can lead to a non-trivial non-radiating charge current-configuration, the dynamic anapole. The dynamic anapoles were recently observed first in microwave metamaterials and then in dielectric nanodisks. However, spectroscopic studies of toroidal dipole and anapole excitations are challenging owing to their diminishing coupling to transverse electromagnetic waves. Here, we show that anapoles can be excited by electromagnetic Flying Doughnut (FD) pulses. First described by Helwarth and Nouchi in 1996, FD pulses (also known as "Flying Toroids") are space-time inseparable exact solutions to Maxwell's equations that have toroidal topology and propagate in free-space at the speed of light. We argue that FD pulses can be used as a diagnostic and spectroscopic tool for the dynamic anapole excitations in matter
Dataset for focused electromagnetic doughnut pulses and their interaction with interfaces and nanostructures
No full textData presented in Figs. 2-6 in "Focused electromagnetic doughnut pulses and their interaction with interfaces and nanostructures" by T. Raybould, V. Fedotov, N. Papasimakis, I. Youngs, N. Zheludev</span
Pulse generation scheme for flying electromagnetic doughnuts
No full textTransverse electromagnetic plane waves are fundamental solutions of Maxwells equations. It is less known that a radically different type of solutions has been described theoretically, but has never been realized experimentally, that exist only in the form of short burst of electromagnetic energy propagating in free-space at the speed of light. They are distinguished from transverse waves by a doughnut-like configuration of electric and magnetic fields with a strong field component along the propagation direction. Here, we demonstrate numerically that such Flying Doughnuts can be generated from conventional pulses using a singular metamaterial converter designed to manipulate both the spatial and spectral structure of the input pulse. The ability to generate Flying Doughnuts is of fundamental interest, as they shall interact with matter in unique ways, including non-trivial field transformations upon reflection from interfaces and the excitation of toroidal response and anapole modes in matter, thus offering new opportunities for telecommunications, sensing, and spectroscopy
Dataset for "Exciting electromagnetic anapoles with Flying Doughnut pulses"
No full textThis file contains the dataset for "Exciting electromagnetic anapoles with Flying Doughnut pulses" by Tim Raybould, Vassili A. Fedotov, Nikitas Papasimakis, Ian Youngs, and Nikolay I. Zheludev. A readme file is contained within the zip.</span
Chiral phenomena in toroidal metamaterials
No full textWe present the first observation of circular dichroism due to a combination of resonant toroidal dipole and electric quadrupole excitations in a toroidal metamaterial
Dataset for "Toroidal Circular Dichroism"
No full textData presented in Figs. 2-4 in "Toroidal Circular Dichroism"" by T. Raybould, V. Fedotov, N. Papasimakis, I. Kuprov, I. Youngs, W. T. Chen, D. P. Tsai, N. I. Zheludev</span
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