Nanoparticle above a Dielectric Interface: Plasmon Hybridization Model, Comparison with the Dimer System, and against Exact Electrodynamics Calculations
Résumé
In the presence of a nearby highly reflecting dielectric substrate the optical response of a metal nanoparticle exhibits a rich multipolar plasmonic structure. For interpreting and quantifying the spectral patterns, the quasistatic hydrodynamic plasmon hybridization (PH) method developed by the Nordlander group has been applied to the particle/interface system in the simple generic case of a jellium sphere (PI–PH model). In the quasistatic limit, the method of images allows the PI–PH model to be tightly related to the two-sphere dimer PH model (D–PH model). However, because the dynamics of the electron fluid in the mirror particle is enslaved to that of the particle above the interface, strong differences between both PH models result from the reduction by a factor two of the number of dynamical variables describing the electron fluids. In particular the PI–PH model involves only bonding bright modes, whereas bonding and antibonding, bright and dark modes are involved in the homodimer case. A very detailed description of both PH models is provided, with particular emphasis on the coupling with an external field and the transfer of the oscillator strength of the dipole excitations of the isolated particle(s) to high-order plasmon modes of the particle/interface or dimer systems. The predictions of both PH models are also compared, through absorption and extinction cross-section spectra, to exact results computed in the frame of the multipole expansion method, allowing the range of applicability and limits of both models to be determined. Some comments about heterodimers are also provided.