Collective plasmon modes in a compositionally asymmetric nanoparticle dimer

Fuyi Chen,Roy L Johnston,Negash Alemu

doi:10.1063/1.3628346

Fuyi Chen, Roy L Johnston + Show 1 more

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https://doi.org/10.1063/1.3628346

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Abstract

The plasmon coupling phenomenon of heterodimers composed of silver, gold and copper nanoparticles of 60 nm in size and spherical in shape were studied theoretically within the scattered field formulation framework. In-phase dipole coupled σ-modes were observed for the Ag-Au and Ag-Cu heterodimers, and an antiphase dipole coupled π-mode was observed for the Ag-Au heterodimer. These observations agree well with the plasmon hybridization theory. However, quadrupole coupled modes dominate the high energy wavelength range from 357-443 nm in the scattering cross section of the D=60 nm Ag-Au and Ag-Cu heterodimer. We demonstrate for the first time that collective plasmon modes in a compositionally asymmetric nanoparticle dimer have to be predicted from the dipole-dipole approximation of plasmon hybridization theory together with the interband transition effect of the constitutive metals and the retardation effect of the nanoparticle size.

Highlights

The optical properties of individual metal nanoparticles are dominated by their localized surface plasmon resonances (LSPR), which are associated with the collective coherent oscillation of conduction electrons in metallic nanostructures.[1]
These results clearly illustrate that only the longitudinal resonance mode (L) is present in the scattering spectrum when the polarizer is oriented along the interparticle axis and the L-mode results from the longitudinal LSPR coupling of the particle plasmons and displays two distinct peaks
In-phase bonding σ -modes were observed for the Ag-Au and Ag-Cu heterodimer and an antiphase bonding π -mode was observed for the Ag-Au heterodimer, and these are in good agreement with that expected from plasmon hybridization theory

Summary

Introduction

The optical properties of individual metal nanoparticles are dominated by their localized surface plasmon resonances (LSPR), which are associated with the collective coherent oscillation of conduction electrons in metallic nanostructures.[1]. Similar to the roles of atoms in molecules and solids, metal nanoparticles can assemble to create a diverse range of new plasmonic nanoclusters,[2] including dimers,[3] trimers,[4] quadrumers[5] and tetramers,[6] etc.[7] Each type of plasmonic nanocluster exhibits its own unique set of collective plasmon modes. The simplest description of collective plasmon modes is the dipole–dipole interaction during the coupling of two nearby oscillators. For the higher energy resonance the coupled dipoles cancel each other, resulting in a resonance with essentially a zero net dipole moment that does not interact with incident light and does not appear in the optical absorption spectrum of the particle pair. Depending on the nanoparticle size and interparticle distance, higher-order multipoles can play an important role in collective plasmon modes.[8]

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