Fano-Resonances in High Index Dielectric Nanowires for Directional Scattering

TL;DR

This paper investigates Fano resonances in high-index dielectric nanowires, revealing their role in enabling directional scattering and highlighting their potential as low-loss alternatives to plasmonic nanoantennas.

Contribution

The study provides an analytical and numerical analysis of Fano resonances in dielectric nanowires, demonstrating their occurrence and effects in simple-shaped nanostructures.

Findings

Fano resonances occur due to interference between dipolar and higher-order modes.

Shape and coupling of nanowires influence the Fano resonances.

All-dielectric nanostructures offer low-loss, directional scattering capabilities.

Abstract

High refractive index dielectric nanostructures provide original optical properties thanks to the occurrence of size- and shape-dependent optical resonance modes. These modes commonly present a spectral overlap of broad, low-order modes (\textit{e.g}. dipolar modes) and much narrower, higher-order modes. The latter are usually characterized by a rapidly varying frequency-dependent phase, which - in superposition with the lower order mode of approximately constant phase - leads to typical spectral features known as Fano resonances. Interestingly, such Fano resonances occur in dielectric nanostructures of the simplest shapes. In spheroidal nanoparticles, interference between broad magnetic dipole and narrower electric dipole modes can be observed. In high aspect-ratio structures like nanowires, either the electric or the magnetic dipolar mode (depending on the illumination conditions) interferes with higher order multipole contributions of the same nature (electric or magnetic). Using the analytical Mie theory, we analyze the occurrence of Fano resonances in high-index dielectric nanowires and discuss their consequences like unidirectional scattering. By means of numerical simulations, we furthermore study the impact on those Fano resonances of the shape of the nanowire cross-sections as well as the coupling of two parallel nanowires. The presented results show that all-dielectric nanostructures, even of simple shapes, provide a reliable low-loss alternative to plasmonic nanoantennas.