Toroidal Dipole Resonances in All‐Dielectric Oligomer Metasurfaces

Abstract

Dynamic toroidal multipoles have been attracting an increasing amount of attention, thanks to their unique electromagnetic properties. The concept bears great promises in the field of optical sensing as well as electromagnetic energy localization. Metal-based structures with specific designs have been widely adopted to obtain toroidal responses. However, applications for such structures are often hindered by the significant intrinsic loss when it comes to optical frequencies. Here, several configurations of all-dielectric metasurfaces, in which the meta-molecules are composed of disk oligomers (trimer, quadrumer, pentamer, and hexamer), are proposed to support toroidal dipole resonances in the near-infrared waveband when the polarization of the normally incident excitation plane wave is directed to one of the symmetry axes of the oligomers. Multipole expansion including the toroidal dipole term along with near-field electromagnetic patterns confirms the existence of strong toroidal dipolar responses within certain wavelength ranges. This work would enrich the diversity of the all-dielectric optical systems with toroidal dipole resonances.

Subwavelength optical localization with toroidal excitations in plasmonic and Mie metamaterials

“…By specially engineering all‐dielectric metamolecules of toroidal topology, electric and magnetic excitations can be suppressed. Spectrally isolated strong toroidal dipole responses can be excited and enhanced to a detectable value 103,157‐166 . A toroidal response can be excited either in a single dielectric particle with larger size or in a cluster (referred as metamolecule) formed by several dielectric particles with a proper arrangement, and the latter excitation often has a stronger toroidal moment.…”

Toroidal Metaphotonics and Metadevices

“…[99,100] In that respect, researchers have extensively focused on inducing toroidal dipoles in an artificial media consisting of meticulously engineered planar metallic and dielectric unit cells (Figure 3). [56,61,[70][71][72][73][74][75][76][77][78][79][80][81][82][83][84][85][86][87]101,102] Easy and cost-effective fabrication steps, simple modeling and quick numerical computations, compatibility with other planar devices in photonic/plasmonic circuits, as well as reduced radiative losses have inspired researchers to develop artificial media based on flatland optics toward this purpose. In principle, in an aligned planar unit cell composed of multiple resonators, the formation of a spinning toroidal feature stems from the discrepancy between the direction of induced magnetic dipole moments in proximal resonators, known as magnetic dipole-induced toroidal dipole moment.…”

“…(c) Reproduced with permission. [79] Copyright 2019, John Wiley & Sons Inc. d) Reproduced with permission. [70] Copyright 2017, American Physical Society.…”

“…[32,33,[65][66][67][68][69][70][71][72][73] Owing to the ability of tightly squeezing electromagnetic fields and supporting ultrasharp resonance lineshapes, very recently, the practical use of toroidal metastructures has obtained considerable research attention. [57,69,74] Considering the unique advantages of these spectral features, well-engineered toroidal unit cells have emerged for designing strategic subwavelength devices, such as large modulation-depth modulators, [75][76][77][78][79][80] augmented nonlinear harmonic signal sources, [81] high-photon yield photodetectors, [82,83] and extremely sensitive and precise immunobiosensors. [84][85][86][87] This contribution aims to highlight and focus on the most recent and important devices enabled by toroidal metasurface technology.…”

Coupled Toroidal Dipole Modes in Silicon Nanodisk Metasurface: Polarization Independent Narrow Band Absorption and Directional Emission

“…TD moments exist not only in an isolated dielectric nanostructure, but also in coupled dielectric nanostructures like oligomers [ 23,24 ] and two dimensional arrays. [ 25–28 ] Basharin et al.…”