| Affiliation: | 1. ITMO University, St. Petersburg, 197101 Russia;2. Institute of Telecommunications, Riga Technical University, Riga, 1048 Latvia;3. National Research University Moscow Power Engineering Institute, 14 Krasnokazarmennaya st., Moscow, 111250 Russia National University of Science and Technology (MISIS), 4 Leninsky pr., Moscow, 119049 Russia;4. School of Optical and Electronic Information, Suzhou City University, Suzhou, 215104 China;5. School of Engineering and Technology, University of New South Wales Canberra Campbell, Canberra, ACT, 2600 Australia |
| Abstract: | Multipoles are paramount for describing electromagnetic fields in many areas of nanoscale optics, playing an essential role in the design of devices in plasmonics and all-dielectric nanophotonics. Challenging the traditional division into electric and magnetic moments, toroidal moments are proposed as a physically distinct family of multipoles with significant contributions to the properties of matter. However, the apparent impossibility of separately measuring their response sheds doubt on their true physical significance. Here, the possibility of selectively exciting toroidal moments is confirmed without any other multipole. A set of general conditions is developed that any current distribution must fulfill to be entirely described by toroidal moments and prove the results in an analytically solvable case. The new theory allows to design and verify experimentally an artificial structure supporting a pure broadband toroidal dipole response in the complete absence of the electric dipole and other “ordinary” multipole contributions. In addition, a structure capable of supporting a novel type of non-radiating source is proposed- a “toroidal anapole,” originating from the destructive interference of the toroidal dipole with the unconventional electromagnetic sources known as mean square radii. The results in this work provide conclusive evidence on the independent excitation of toroidal moments in electrodynamics. |
