Abstract
Abstract Hyperbolic materials offer much wider freedom in designing optical properties of nanostructures than ones with isotropic and elliptical dispersion, both metallic or dielectric. Here, we present a detailed theoretical and numerical study on the unique optical properties of spherical nanoantennas composed of such materials. Hyperbolic nanospheres exhibit a rich modal structure that, depending on the polarization and direction of incident light, can exhibit either a full plasmonic-like response with multiple electric resonances, a single, dominant electric dipole or one with mixed magnetic and electric modes with an atypical reversed modal order. We derive conditions for observing these resonances in the dipolar approximation and offer insight into how the modal response evolves with the size, material composition, and illumination. Specifically, the origin of the magnetic dipole mode lies in the hyperbolic dispersion and its existence is determined by two diagonal permittivity components of different sign. Our analysis shows that the origin of this unusual behavior stems from complex coupling between electric and magnetic multipoles, which leads to very strong scattering or absorbing modes. These observations assert that hyperbolic nanoantennas offer a promising route towards novel light–matter interaction regimes.
Highlights
The optical response of a particular system to external illumination is governed by the oscillator strength of the systems’ electrons [1], and by their spatial distribution
The hyperbolic material which comprises the studied nanoparticles is based on an artificial metal–dielectric multilayer to, on one hand, closely relate to the recently studied hyperbolic Au-SiO2/Al2O3/TiO2 nanostructures [35, 41] and, on the other hand, retain the ability to freely tune the birefringence by changing the composition
For a metal fill factor f m = 0.5 the exemplary dispersion is plotted in Figure 1b, which illustrates the presence of two types of isofrequency surfaces [21] with either two or one (∼3.4–3.8 eV) permittivity tensor elements being negative
Summary
The optical response of a particular system to external illumination is governed by the oscillator strength of the systems’ electrons [1], and by their spatial distribution. They simultaneously support both electric and magnetic resonances and can be chosen to have small or negligible dissipation [6] These were subsequently observed in spherical nanoparticles [7, 8], whose response is characterized by a high degree of symmetry and allows to capture the fundamental properties of scatterers made of a particular type of material. The novel functionalities of large-scale hyperbolic materials [21] can be extended or adapted to nanoparticlebased optical cavities One of these properties is confinement of optical modes, which was initially realized in stacked Ag/Ge multilayers [32], due to large losses the quality factor of the resonances was quite low. Using a combination of finite-difference time-domain (FDTD) modelling, Mie theory, quasistatic (QS), and T-matrix analysis, we explore their modal properties and elucidate their unique spectral response
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