Abstract
We explore numerically and experimentally the formation of hybridized modes between a bright mode displayed by a gold nanodisc and either dark or bright modes of a nanorod – both elements being either separated by a nanometer-size gap (disconnected system) or relied on a metal junction (connected system). In terms of modeling, we compare the scattering or absorption spectra and field distributions obtained under oblique-incidence plane wave illumination with quasi-normal mode computation and an analytical model based on a coupled oscillator model. Both connected and disconnected systems have very different plasmon properties in longitudinal polarization. The disconnected system can be consistently understood in terms of the nature of hybridized modes and coupling strength using either QNMs or coupled oscillator model; however the connected configuration presents intriguing peculiarities based on the strong redistribution of charges implied by the presence of the metal connection. In practice, the fabrication of disconnected or connected configurations depends on the mitigation of lithographic proximity effects inherent to top-down lithography methods, which can lead to the formation of small metal junctions, while careful lithographic dosing allows one to fabricate disconnected systems with a gap as low as 20 nm. We obtained a very good agreement between experimentally measured scattering spectra and numerical predictions. The methods and analyses presented in this work can be applied to a wide range of systems, for potential applications in light–matter interactions, biosensing or strain monitoring.
Highlights
Fano profiles in coupled metal nanoparticles displaying localized surface plasmon (LSP) modes have been investigated in recent years with a view to potential applications, like biosensing.[1,2] Following theoretical predictions,[3] Fano resonance has been observed in various types of structures such as inter alia: nanoclusters,[4] plasmonic particles involving nanorods,? metal-insulator-metal systems,[5,6] and between interconnected metal structures.[7]
We explore numerically and experimentally the formation of hybridized modes between a bright mode displayed by a gold nanodisc and either dark or bright modes of a nanorod - both elements being either separated by a nanometer-size gap or relied by a metal junction
If this behaviour suggests a Fano profile resulting from interferences between the NR dark modes and the ND bright mode, quasi-normal modes (QNMs) simulations clearly indicate a different type of interaction between shorter and longer NR: a mode anti-crossing is obtained on Fig. 2(a) for the second order mode, while a mere overlapping occurs for the fourth order mode
Summary
Fano profiles in coupled metal nanoparticles displaying localized surface plasmon (LSP) modes have been investigated in recent years with a view to potential applications, like biosensing.[1,2] Following theoretical predictions,[3] Fano resonance has been observed in various types of structures such as inter alia: nanoclusters,[4] plasmonic particles involving nanorods,? metal-insulator-metal systems,[5,6] and between interconnected metal structures.[7]. If this behaviour suggests a Fano profile resulting from interferences between the NR dark modes and the ND bright mode, QNM simulations clearly indicate a different type of interaction between shorter and longer NR: a mode anti-crossing is obtained on Fig. 2(a) for the second order mode, while a mere overlapping occurs for the fourth order mode. The similarity with the disconnected system for s polarization directly results from the fact that, because surface charges cancel on the incidence plane (which is an anti-symmetry plane for the system), the associated plasmon modes barely ”see” the gap and the resulting spectra are mostly unchanged by the metal connection This can be verified on the movie S2 in Supplementary Materials which compare the QNMs field distributions for disconnected and connected nano-objects in s polarization. The full investigation of this mechanism is beyond the purpose of that paper
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