Confining electromagnetic energy

A research collaboration led by The Australian National University (ANU) has found a new way to confine electromagnetic energy without it leaking away, with potential applications ranging from explaining dark matter to combating energy losses in future technologies.

The strange thing is that the theory appears to contradict a fundamental tenet of electrodynamics: that accelerated charges create electromagnetic radiation.

“It took us a year to get this concept clear in our heads,” said lead researcher Dr Andrey Miroshnichenko, whose study has been published in the journal Nature Communications.

“Ever since the beginning of quantum mechanics, people have been looking for a configuration which could explain the stability of atoms and why orbiting electrons do not radiate.”

The absence of radiation is the result of the current being divided between two different components — a conventional electric dipole and a toroidal dipole (associated with poloidal current configuration) — which produce identical fields at a distance.

If these two configurations are out of phase then the radiation will be cancelled out, even though the electromagnetic fields are non-zero in the area close to the currents.

Dr Miroshnichenko, in collaboration with colleagues from Germany and Singapore, tested his theory with a single silicon nanodisc between 160 and 310 nm in diameter and 50 nm high, which he was able to make effectively invisible by cancelling the disc’s scattering of visible light. This type of excitation is known as an anapole (‘without poles’).

Dr Miroshnichenko’s insight came while trying to reconcile differences between two different mathematical descriptions of radiation; one based on Cartesian multipoles and the other on vector spherical harmonics used in a Mie basis set.

“The two gave different answers, and they shouldn’t,” he said. “Eventually we realised the Cartesian description was missing the toroidal components.

“We realised that these toroidal components were not just a correction — they could be a very significant factor.”

Dr Miroshnichenko said the confined energy of anapoles could be important in the development of tiny lasers on the surface of materials, called spasers, and in the creation of efficient X-ray lasers by high-order harmonic generation. The new theory could also be used in quantum computers and may even hold the key to understanding how matter itself hangs together.

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