Ultranarrow and Tunable Fano Resonance in Ag Nanoshells and a Simple Ag Nanomatryushka.

Ping Gu,Zhong Huang,Jing Chen,Xiaofeng Cai,Fanxin Liu,Zhendong Yan,Chaojun Tang,Chenpeng Xue,Guohua Wu,Zhuo Chen,Zuxing Zhang,Wei Du

doi:10.3390/nano11082039

Abstract

We study theoretically the Fano resonances (FRs) produced by the near-field coupling between the lowest-order (dipolar) sphere plasmon resonance and the dipolar cavity plasmon mode supported by an Ag nanoshell or the hybrid mode in a simple three-layered Ag nanomatryushka constructed by incorporating a solid Ag nanosphere into the center of Ag nanoshell. We find that the linewidth of dipolar cavity plasmon resonance or hybrid mode induced FR is as narrow as 6.8 nm (corresponding to a high Q-factor of ~160 and a long dephasing time of ~200 fs) due to the highly localized feature of the electric-fields. In addition, we attribute the formation mechanisms of typical asymmetrical Fano line profiles in the extinction spectra to the constructive (Fano peak) and the destructive interferences (Fano dip) arising from the symmetric and asymmetric charge distributions between the dipolar sphere and cavity plasmon or hybrid modes. Interestingly, by simply adjusting the structural parameters, the dielectric refractive index required for the strongest FR in the Ag nanomatryushka can be reduced to be as small as 1.4, which largely reduces the restriction on materials, and the positions of FR can also be easily tuned across a broad spectral range. The ultranarrow linewidth, highly tunability together with the huge enhancement of electric fields at the FR may find important applications in sensing, slow light, and plasmon rulers.

Highlights

Engineering Fano resonance (FR) in plasmonic nanostrucures and artificial metamaterials has become an important research focus in recent years due to its wide applications such as surface-enhanced Raman scattering (SERS) [1], refractive-index sensing or biosensing sensors [2,3,4,5,6,7], plasmon rulers [8], molecular identification [9], slow-light devices [10], narrow-band absorbers [11], nonlinear optical processes [12,13], and so on
The tunability of the FR whose resonant wavelength and excitation strenghth can be adjusted by internal or external parameters has attracted extensive research attentions, because the tunable FR can largely extend the functionalities of plasmonic metamaterials in practice applications
We investigate theoretically the formation mechanisms and the tunability of the FRs in the Ag nanoshell and the Ag nanomatryushka formed by incorporating a solid silver nanosphere into the center of the same Ag nanoshell

Summary

Introduction

Engineering Fano resonance (FR) in plasmonic nanostrucures and artificial metamaterials has become an important research focus in recent years due to its wide applications such as surface-enhanced Raman scattering (SERS) [1], refractive-index sensing or biosensing sensors [2,3,4,5,6,7], plasmon rulers [8], molecular identification [9], slow-light devices [10], narrow-band absorbers [11], nonlinear optical processes [12,13], and so on. Unlike the metallic nanoparticle assemblies and asymmetry nanostructures, to generate FR with high complexity and high precisely (inter-particle separation on the order of several tens of nanometers) [24,25,26,27,28,35,36,39,40,41], a nobel metal (Au/Ag) nanoshell composed of a spherical dielectric core and a metallic shell is the simplest plasmonic nanostructure [52] These have been demonstrated both theoretically and experimentally to be capable of supporting multiple sharp FRs induced by the multipolar high-Q cavity plasmon modes when the dielectric core size can be comparable to the wavelength of light (beyond the electrostatic limit) [53,54,55,56]. The resonant positions of FR in the Ag nanomatryushka can be tuned across a broad spectral range

Methods

Conclusions

Results