Electromagnetic Properties of Metamaterials and Anapole Excitations

Metamaterials are artificially structured composites whose subwavelength architecture enables electromagnetic responses beyond those of natural materials, including negative refraction, cloaking and super-resolution imaging. Central to their exotic behaviour is the ability to engineer effective permittivity and permeability, as well as to harness higher-order multipoles. Among these, toroidal dipoles—donut-shaped current distributions distinct from classical electric and magnetic dipoles—play a crucial role. When electric and toroidal dipole moments are tuned to interfere destructively, a nonradiating anapole mode emerges, characterised by suppressed far-field scattering and intense near-field confinement. Such radiationless states, analogous to bound states in the continuum, can exhibit exceptionally high quality factors, offering a platform for enhanced light–matter interaction, low-threshold lasing, nonlinear frequency conversion and nanoscale sensing. Advances in materials processing and nanofabrication have enabled the experimental realisation and mapping of anapole excitations in dielectric and hybrid structures, paving the way to novel photonic devices and fundamental studies of reciprocity and topological effects at optical frequencies.

Research from Nature Portfolio

Recent studies have probed optical anapoles using fast electron beams to achieve subnanometre mapping of nonradiating excitations in layered dielectric nanodisks, demonstrating that beam positioning can selectively control the excitation of pure anapole and hybrid anapole–exciton states. Foundational experimental work has established visible-wavelength anapole modes in high-index dielectric nanoparticles by geometry tuning to overlap electric and toroidal dipolar resonances, producing pronounced dips in far-field scattering alongside characteristic near-field patterns, and enabling investigations into nontrivial electromagnetic phenomena such as reciprocity violation and Aharonov–Bohm-like effects.

Electromagnetic Properties of Metamaterials and Anapole Excitations publication trend

The graph below shows the total number of articles in electromagnetic properties of metamaterials and anapole excitations across all publications each year (not limited to Nature Index journals).

Technical terms

Metamaterial: An artificial structure with subwavelength building blocks designed to exhibit electromagnetic responses not found in natural materials.

Multipole expansion: Decomposition of electromagnetic fields into contributions from basic source configurations such as electric, magnetic and toroidal dipoles, quadrupoles, etc.

Toroidal dipole: A current configuration forming a closed loop on a torus, producing an electromagnetic moment beyond classical electric and magnetic dipoles.

Anapole mode: A nonradiating excitation arising from destructive interference between electric and toroidal dipole moments, resulting in minimal far-field scattering and strong near-field localisation.

Bound state in the continuum (BIC): A discrete, nonradiating state embedded within a continuum of radiating modes, characterised by theoretically infinite quality factor.

Quality factor (Q-factor): A measure of the energy confinement of an electromagnetic resonance, defined as the ratio of stored energy to energy dissipated per cycle.

References

1. Radiationless optical modes in metasurfaces: recent progress and applications. Light: Science & Applications (2024).
2. Probing optical anapoles with fast electron beams. Nature Communications (2023).
3. Nonradiating anapole modes in dielectric nanoparticles. Nature Communications (2015).