The response of coupled metallic nanoparticles of various shapes to excitations by plane waves and an electron beam is investigated numerically within an efficient eigenvector expansion of the discrete-dipole approximation (DDA). It is shown that this procedure allows to reduce significantly the size of the system of equations to be solved (hence the computationnal effort) when one considers multiparticle systems. The results are then confronted to standard DDA for validation.
Localized surface plasmon resonances (LSPR) govern the optical properties of metallic nanoparticles at the
nanoscale level and depend strongly on their shape, size and environment. When particles are a few nanometers
apart, new plasmon modes, that can be either bright or dark, arise from the electromagnetic coupling of the
plasmon modes of the individual nanoparticles. In the case of 3 gold nanorods assembled in a dolmen-like
structure, a transmission window has been observed experimentally from 0.9 and 1.3 eV. It has been attributed
to Fano interferences between a broad bright mode coming from the monomer and a narrow dark mode coming
from the dimer. Because it is optically inactive, the latter has to be probed locally with an electron beam. This
is why we investigate numerically the energy electron loss (EEL) response of coupled metallic nanorods together
with their optical properties to get insight into the origin of these Fano resonances. To achieve this, calculations
are performed in the frame of the discrete dipole approximation both for optical and EEL excitations.
A short review of electron-energy-loss spectroscopy (EELS) experiments of carbon nanotubes and onions is presented. The dielectric response function of these nanostructures is derived from electrodynamics. Loss spectra computed with the dielectric theory are compared with spatially-resolved experimental spectra. The main features of the loss spectra obtained with non-penetrating electrons can be attributed to surface plasmon excitations (π plasmon at 6 eV and π + σ plasmon at 15 and 17-18 eV).
One of the most versatile formalism for the study of the electrodynamic response of solids, surfaces and interfaces or nanoparticles is the continuum dielectric model. In this contribution, we develop an application of this dielectric approach to nanocylinders and more particularly to the simulation of near-field electron energy loss (EEL)spectra of nanotube bundles. On the experimental side, EELS in a Scanning Transmission Electron Microscope (STEM) combines both spatial and energy resolutions in the plasmonic energy range and then permits the spectroscopic analysis of the surface and volume excitations of nanoparticles.
Amongst the challenges brought about by the discovery of carbon nanotubes, one can cite the understanding of their optical properties. In this contribution, pursuing this goal within a dielectric continuum model, we focus on the dispersion and coupling of surface plasmon excitations of hollow nanocylinders and on the near-field EELS of nanotube nanocrystals (bundles). Experimental EELS in a STEM have also been obtained on bundles of carbon nanotubes. The interpretation in terms of effectif medium theory is successfuly performed both for surface and bulk losses associated with the σ plasmon.
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