Metallo-dielectric core–shell nanospheres as building blocks for optical three-dimensional isotropic negative-index metamaterials

Abstract

Materials showing electromagnetic properties that are not attainable in naturally occurring media, the so called metamaterials, have been lately, and still are, among the most active fields in optical and materials physics and engineering. Among those properties, one of the most attractive is the sub-diffraction resolving capability predicted for media having index of refraction of -1. Here we propose a fully 3D, isotropic metamaterial with strong electric and magnetic responses in the optical regime, based on spherical metallo-dielectric core-shell nanospheres. The magnetic response stems from the lowest, magnetic-dipole resonance of the dielectric shell with high refractive index, and can be tuned to coincide with the plasmon resonance of the metal core, responsible for the electric response. Since the response does not originate from coupling between structures, no particular periodic arrangement needs to be imposed. Moreover, due to the geometry of the constituents, the metamaterial is intrinsically isotropic and polarization independent. It could be realized with current fabrication techniques with materials such as Silver (core) and Silicon or Germanium (shell). For these particular realistic designs, the metamaterials present negative index in the range within 1.2 − 1.55 µm.

Semiconductor-Metal-Semiconductor Core-Multishell Nanowires as Negative-Index Metamaterial in Visible Domain

“…Notably, increasing the fill factor of the meta-atoms in the NIM has been shown to substantially improve the FOM [16][17][18] . For instance, high FOM in Ag-Si CS nanowire NIM was obtained for hexagonal close packing arrangement with 10 nm separation between adjacent nanowire surfaces resulting in a fill factor of about 0.856 17 .…”

“…More recently, theoretical designs have been reported for low loss plasmonic NIM utilizing the geometry dependant Mie excitations in Ag-GaP hybrid rods 15 , Ag-Si core-shell (CS) nanospheres 16 and nanowires 17 . Specifically, in the CS structures the core diameter and the shell thickness are chosen so that the magnetic dipole resonance arising from the dielectric shell and the electric dipole resonance (localized surface plasmon resonance -LSPR) in the metal core spectrally overlap.…”

“…Since the natural magnetic resonance occurring in the dielectric shell is coupled to the LSPR within the same CS nanostructure, the losses due to the NIM design in CS structures are significantly suppressed, resulting in higher FOM. While NIM based on CS nanospheres are polarization independent 16,18 , nanowire (NW) structures are polarization dependent due to their high aspect ratio. This anisotropy enables stronger light coupling and leads to stronger resonance, hence the NIM based on nanowires exhibit substantially higher FOM 17 .…”

“…Compared to the previous NIM studies based on core-shell meta-atoms, the uniqueness of the proposed approach is two-fold. First, distinguished from scaling down the meta-atom diameter or introducing low-permittivity intermediate layer as suggested in previous studies [16][17][18] , we utilize plasmon hybridization to tune the electric resonance. Consequently, the CMS structures avoid the reduction in the resonance strength accompanied by scaling down the size and also circumvent the need for a low-permittivity intermediate layer.…”

The directivity of a compact antenna: an unforgettable figure of merit

“…It is interesting to speculate that in the future one might be able to impregnate the essentially dispersionless (at these UHF frequencies), low-loss Teflon substrate in the Duroid sheets with combinations of passive and active core-shell nanoparticles, e.g., [67][68][69][70][71][72], to create dispersion-tailored macroscopic metamaterial substrates or metasurfaces to enhance the impedance bandwidth of these metamaterialinspired antennas as suggested in [73]. The resulting metastructures could conceivably replace the non-Foster circuits and their complexities to enhance the bandwidth performance of electrically small antennas.…”