Physicists have found a radical new way confine electromagnetic energy without it leaking away, akin to throwing a pebble into a pond with no splash
The theory could have broad ranging applications from explaining dark matter to combating energy losses in future technologies.
However, it appears to contradict a fundamental tenet of electrodynamics, that accelerated charges create electromagnetic radiation, said lead researcher Dr Andrey Miroshnichenko from The Australian National University (ANU).
“This problem has puzzled many people. It took us a year to get this concept clear in our heads,” said Dr Miroshnichenko, from the ANU Research School of Physics and Engineering.
The fundamental new theory could be used in quantum computers, lead to new laser technology and may even hold the key to understanding how matter itself hangs together.
“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,” Dr Miroshnichenko said.
The toroidal dipole moment is associated with the circulating magnetic field M accompanied by electric poloidal current distribution. Since the symmetry of the radiation patterns of the electric P and toroidal T dipole modes are similar, they can destructively interfere leading to total scattering cancelation in the far-field with non-zero near-field excitation.
Nature Communications – Nonradiating anapole modes in dielectric nanoparticles
Arxiv – Nonradiating anapole modes in dielectric nanoparticles (20 pages)
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, successfully tested his new theory with a single silicon nanodiscs between 160 and 310 nanometers in diameter and 50 nanometers 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 (from the Greek, ‘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. Eventually we realized the Cartesian description was missing the toroidal components,” Dr Miroshnichenko said.
“We realized 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 also in the creation of efficient X-ray lasers by high-order harmonic generation.
Nonradiating current configurations attract attention of physicists for many years as possible models of stable atoms. One intriguing example of such a nonradiating source is known as ‘anapole’. An anapole mode can be viewed as a composition of electric and toroidal dipole moments, resulting in destructive interference of the radiation fields due to similarity of their far-field scattering patterns. Here we demonstrate experimentally that dielectric nanoparticles can exhibit a radiationless anapole mode in visible. We achieve the spectral overlap of the toroidal and electric dipole modes through a geometry tuning, and observe a highly pronounced dip in the far-field scattering accompanied by the specific near-field distribution associated with the anapole mode. The anapole physics provides a unique playground for the study of electromagnetic properties of nontrivial excitations of complex fields, reciprocity violation and Aharonov–Bohm like phenomena at optical frequencies.
SOURCES -Eurekalert, Nature Communications, Arxiv
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