Abstract
Here, we examine the electromagnetic (EM) energy coupling and hybridization of plasmon resonances between closely spaced concentric nanoshells known as “nanomatryoshka” (NM) units in symmetric and antisymmetric compositions using the Finite Difference Time Domain (FDTD) analysis. Utilizing plasmon hybridization model, we calculated the energy level diagrams and verified that, in the symmetric dimer (in-phase mode in a homodimer), plasmonic bonding modes are dominant and tunable within the considered bandwidth. In contrast, in the antisymmetric dimer (out-of-phase mode in a heterodimer), due to the lack of the geometrical symmetry, new antibonding modes appear in the extinction profile, and this condition gives rise to repeal of dipolar field coupling. We also studied the extinction spectra and positions of the antibonding and bonding modes excited due to the energy coupling between silver and gold NM units in a heterodimer structure. Our analysis suggest abnormal shifts in the higher energy modes. We propose a method to analyze the behavior of multilayer concentric nanoshell particles in an antisymmetric orientation employing full dielectric function calculations and the Drude model based on interband transitions in metallic components. This study provides a method to predict the behavior of the higher energy plasmon resonant modes in entirely antisymmetric structures such as compositional heterodimers.
Highlights
The optical characteristics of noble metallic structures in nanoscale dimensions have been described by localized surface plasmon resonances (LSPRs), which correspond to free electron coherent oscillations in the conduction band when illuminated by an incident light at the optical frequencies.[1]
First, we examine the quality of bonding and antibonding plasmon resonance modes for a NM homodimer (symmetric (Ψ1 + Ψ2)) with the same geometrical parameters and metal compositions for both sides, while the separation or gap distance between NM units is approximately Din∼15 nm, which resembles the strong coupling regime and the thickness of the rings is kept fix as 25 nm throughout the study
We observed a minor blue-shift in the bonding mode due to the symmetry breaking which is in complete agreement with the plasmon hybridization model
Summary
The optical characteristics of noble metallic structures in nanoscale dimensions have been described by localized surface plasmon resonances (LSPRs), which correspond to free electron coherent oscillations in the conduction band when illuminated by an incident light at the optical frequencies.[1]. Reducing the offset gap between nanoparticles to a touching regime gives rise to a remarkable blue-shift of the LSPR to the shorter wavelengths, which is not considered for the proposed heterodimer structure in this study.[11] In contrast, in the heterodimer regime (out-of-phase mode), the adjacent heterodimer shows the antibonding modes as well as bonding modes resulting localized electric field enhancement at the outer sides of the dimer configurations and blue-shift of the plasmon resonance frequency, which is denoted by σ∗.16. We examine the quality of the plasmon resonance coupling in the NM structures with various geometrical dimensions to achieve enhanced and ultra-sensitive nanoplasmonic configurations
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