From single-particle-like to interaction-mediated plasmonic resonances in graphene nanoantennas.

Marvin M Müller,Andrés Ayuela,Marta Pelc,Garnett W Bryant,Miriam Kosik,Karolina Słowik,Carsten Rockstuhl

doi:10.1063/5.0038883

Abstract

Plasmonic nanostructures attract tremendous attention as they confine electromagnetic fields well below the diffraction limit while simultaneously sustaining extreme local field enhancements. To fully exploit these properties, the identification and classification of resonances in such nanostructures is crucial. Recently, a novel figure of merit for resonance classification has been proposed1 and its applicability was demonstrated mostly to toy model systems. This novel measure, the energy-based plasmonicity index (EPI), characterizes the nature of resonances in molecular nanostructures. The EPI distinguishes between either a single-particle-like or a plasmonic nature of resonances based on the energy space coherence dynamics of the excitation. To advance the further development of this newly established measure, we present here its exemplary application to characterize the resonances of graphene nanoantennas. In particular, we focus on resonances in a doped nanoantenna. The structure is of interest, as a consideration of the electron dynamics in real space might suggest a plasmonic nature of selected resonances in the low doping limit but our analysis reveals the opposite. We find that in the undoped and moderately doped nanoantenna, the EPI classifies all emerging resonances as predominantly single-particle-like and only after doping the structure heavily, the EPI observes plasmonic response.

Highlights

The field of plasmonics experienced huge interest in the past two decades2–6 with a recent shift of focus toward related quantum processes and applications at the nanoscale.7–9 The possibility of confining and enhancing electromagnetic fields5,10–12 attracts attention for fundamental research reasons and due to many potential applications in plasmonic sensing,13–15 photodetection,16–19 medicine,20,21 optical metamaterials,22–24 and single photon sources.25,26Graphene supports intrinsic tunable plasmons and, is a well-suited platform for exploring and exploiting plasmonic phenomena.22,27–29 Recent progress in nanostructure fabrication allows us to produce graphene flakes consisting of only a few hundred atoms30,31 that support a plasmonic response at near infrared frequencies
The energy-based plasmonicity index (EPI) distinguishes between either a single-particle-like or a plasmonic nature of resonances based on the energy space coherence dynamics of the excitation
The structure is of interest, as a consideration of the electron dynamics in real space might suggest a plasmonic nature of selected resonances in the low doping limit but our analysis reveals the opposite

Summary

INTRODUCTION

The field of plasmonics experienced huge interest in the past two decades with a recent shift of focus toward related quantum processes and applications at the nanoscale. The possibility of confining and enhancing electromagnetic fields attracts attention for fundamental research reasons and due to many potential applications in plasmonic sensing, photodetection, medicine, optical metamaterials, and single photon sources.. One can find density functional theory (DFT), the tight binding (TB) model, or quantum fluid dynamics.44,45 Graphene nanoantennas support both single-particle-like resonances and plasmonic ones, which leads to the important question how to identify the nature of resonances in such nanostructures.. Zhang et al defined a dimensionless, but unnormalized metric called generalized plasmonicity index (GPI) to distinguish plasmons from single-particle-like excitations based on a similar aspect.47 Both of these measures can be determined using the real space charge distribution of the structure’s resonances as the only input. The energy-based plasmonicity index (EPI) is a normalized and dimensionless measure for characterizing the nature of resonances in nanostructures It does not rely on charge carrier oscillation patterns on the nanostructure and, cannot be determined by the analysis of atomic site population dynamics. In the 20-fold heavily doped nanoantenna, the EPI observes truly plasmonic response

METHOD

RESONANCE ANALYSIS

Energy space dynamics

CONCLUSIONS AND SUMMARY