Excellent Anapole by Decoupling Electric Multipoles of Ag/Si Core–Shell Nanoparticles

Abstract

The interplay of Mie's multipole resonances provides a huge opportunity for discovering new phenomena in nanophotonics and developing nanophotonic devices of new functionalities. One of the keys to this task is to control the spectral overlap or separation between resonances, for which Ag/Si core−shell nanostructures, that is, nanospheres and nanodisks, are proposed here. The Ag core and the dielectric shell have different roles in dominating spherical electric dipole (ED) and electric quadrupole (EQ) resonances with different variational trends to the change of the structural parameter such as the core radius. Thus, the EQ and ED may be decoupled in a great spectral separation, such that an excellent anapole can be achieved and lies in a broad valley of the scattering spectrum. The anapole mode displays a perfect far-field radiation pattern of the Cartesian ED and toroidal dipole with destructive interference and radiates more than 70 times less than that of the solid nanodisk of the same size. Only one tightly focused radially polarized beam is needed for illumination in order to avoid excitation of magnetic-type resonances. This work of the spectral characteristics of Ag/Si core−shell nanoparticles provides detailed physical insight into experimental demonstration of nonradiating scatterers and suggests a scheme to study the particular electromagnetic modes in a hybrid nanostructure.

Multipolar scattering analysis of hybrid metal-dielectric nanostructures

“…Actually, this observation is important for engineering directional scattering [9,10,63,64], or producing a far-field response constituting of only one particular mode, e.g. the MD for "magnetic light" [31,65]. The ED and MD resonances of dielectric structures can have strong spectral overlap between them; a difference in their shift rate enables their efficient spectral separation.…”

“…by appending a few nm thick metallic disk on top of the Si cylinder. Indeed, quite significant level of control over the position of the resonances can be achieved by introducing a metal in the system [31,69]. We choose a Si cylinder with 𝐻 = 220 nm and 𝑅 = 235 nm since it has prominent additional multipoles, like MD and EQ.…”

“…To the author's knowledge, only a few publications have unveiled some of these dependencies. For example, the scattering characteristics of dielectric core coated with metal shell have been implemented experimentally and studied analytically for spherical and numerically for co-axial cylindrical configurations [8,[31][32][33]. A hybrid geometry with metal nanorods on top of Si cylinders was studied, noting the influence of a spacer between a metal rod and a dielectric cylinder to enhance directional radiation [34,35].…”

Broadband Polarization‐Insensitive Coherent Rasorber in Terahertz Metamaterial with Enhanced Anapole Response and Coupled Toroidal Dipole Modes

“…The former can be due to the strong interference in the far-field originating from the toroidal dipole and antiphase electric dipole moments, and the latter can be contributed to the induced displacement currents existing in the nanostructure, which attain confined fields to effectively boost the intensity of the electromagnetic field. [46][47][48][49] The destructive interference induced by the anapole mode can induce the EIT effect, a quantum interference effect that exists in three-level atomic systems, with sharp transparency when the radiated losses are restrained owing to the interaction of toroidal dipole and out-of-phase electric dipole modes. However, a large amount of literature regarding the transmission response of anapole mode is limited to the narrow-band characteristics, hence greatly hindering the practical application.…”

Optical Characteristics of Metasurfaces at Meta-Atom Anapole

“…It can even be directly proved theoretically that one LR can completely null the transmittance at the resonance wavelength for an array composed of non-absorbent scatterers[5]. In order to further study optical responses of MLRs at the anapole wavelength, we replace the homogenous Si nanosphere with the Au/Si core-shell nanosphere that allows altering spectral separations between multipolar resonances of the scatterer[33]. In the two cases illustrated in Figs.3(a) and 3(d), MQ and MD are adjusted to contribute significant components at anapole wavelengths, respective.…”