Fano resonances in plasmonic heptamer nano-hole arrays.

Akram Hajebifard,Pierre Berini

doi:10.1364/oe.25.018566

Abstract

The optical properties of gold heptamer nanohole arrays have been investigated theoretically and numerically. This structure support pronounced Fano resonances with high transmittance (~50%) and narrow bandwidths (down to 12 nm). The Fano features arise from the interference between light directly transmitted through the holes, and light indirectly scattered through the excitation of localized surface plasmon polaritons (LSPPs), propagating surface plasmon polaritons (SPPs), or/and waves related to Wood's anomaly (WA). The mechanisms behind the generation of these resonances are revealed by observing near-field distributions, altering the structural parameters and applying the Bloch wave model. Furthermore, it is shown that Fano resonances associated with LSPPs exhibit high surface (2 nm/nm) and bulk sensitivities (400 nm/RIU). However, the highest figure of merit (~24 RIU-1) occurs for a Fano resonance involving a WA and SPP mode.

Highlights

Compared to the symmetric Lorentzian lineshape supported in various nanostructures [1], asymmetric Fano resonances in plasmonic structures have recently attracted intense attention [2,3] due to their remarkable characteristics such as a narrow asymmetric line-shape, strong electromagnetic field enhancement, light confinement to the nanometer scale, and high spectral sensitivity to changes in the local dielectric environment [1,4,5,6]
We show that arrangements of nanoholes in metal films produce more complex spectral responses because the structure supports many excitations, including localised surface plasmon polaritons (LSPPs) in the vicinity of holes, and propagating surface plasmon polaritons (SPPs) or Wood’s anomaly (WA) waves excited on the metal film that interact with the holes
Considering the heptamer-arranged nanoholes (HNH) array as a grating of lattice constant P = Py = Pz illuminated by light incident at an angle θ, the light gains additional momentum given by |Gy| = |Gz| = 2π/P, which can excite propagating SPP modes (Bloch waves) on the metal film following the Bragg coupling condition [41]: Kspp = K0 sinθ + S yGy + SzGz

Summary

Introduction

Compared to the symmetric Lorentzian lineshape supported in various nanostructures [1], asymmetric Fano resonances in plasmonic structures have recently attracted intense attention [2,3] due to their remarkable characteristics such as a narrow asymmetric line-shape, strong electromagnetic field enhancement, light confinement to the nanometer scale, and high spectral sensitivity to changes in the local dielectric environment [1,4,5,6]. A Fano peak is a consequence of constructive interference between such modes, whereas a Fano dip arises as the two modes interfere destructively such as when the dark mode is out of phase with respect to the continuum Taking advantage of this principle, different types of plasmonic structures, such as metamaterials [5,8], clusters [9,10], single and multi-surface nanoparticles [11,12] and periodic structures [13,14,15,16,17] have been proposed and studied, seeking Fano resonances in their spectral responses. Far less attention has been paid to complementary structures - arrangement of metallic nanoholes - which are expected to support Fano resonances [5,27]. The increased number of resonances, and the ability to independently tune the position of SPP and LSPP resonances (both providing near-field enhancement) is favorable for double-resonance SERS applications, where simultaneous enhancement at both the excitation and Stokes wavelengths is needed [28,29]

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Conclusion