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Surface Plasmon modes revealed by fast electron based spectroscopies: from simple model to complex systems

Publication Type:

Thesis

Authors:

Losquin, Arthur

Source:

(2013)

Keywords:

disordered media, fast electron based spectroscopies, scanning transmission electron microscopy, Surface plasmons

Abstract:

Surface Plasmons (SP) are elementary excitations mixing electrons and photons at metal surfaces, which can be seen in a classical electrodynamics framework as electromagnetic surface eigenmodes of a metal-dielectric system. The present work bases on the ability of mapping SP eigenmodes with nanometric spatial resolution over a broad spectral range using spatially resolved fast electron based spectroscopies in a Scanning Transmission Electron Microscope (STEM). Such an ability has been separately demonstrated during the last few years by many spatially resolved experiments of Electron Energy Loss Spectroscopy (EELS), which measures the energy lost by fast electrons interacting with the sample, and CathodoLuminescence (CL), which measures the energy released by subsequently emitted photons. In the case of EELS, the experimental results are today well accounted for by strong theory elements which tend to show that the quantity measured in an experiment can be safely interpreted in terms of the surface eigenmodes of the sample. In order to broaden this interpretation to fast electron based spectroscopies in general, I have performed combined spatially resolved EELS and CL experiments on a simple single nanoparticle (a gold triangular nanoprism). I have shown that EELS and CL results bear strong similarities but also slight differences, which is confirmed by numerical simulations. I have extended the theoretical analysis of EELS to CL to show that CL maps equally well than EELS the radiative surface eigenmodes, yet with slightly different spectral features. This work is a proof of principle clarifiying the quantities measured in EELS and CL, which are shown to be respectively some nanometric equivalent of extinction and scattering spectroscopies when applied to metal-dielectric systems. Based on this interpretation, I have applied EELS to reveal the SP eigenmodes of random metallic media (in our case, semicontinuous metal films before the percolation threshold). These SP eigenmodes constitute a long standing issue in nanooptics. I have directly identified the eigenmodes from measurements and data processing. I have fully characterized these eigenmodes experimentally through an electric field intensity pattern, a resonance energy and a spectral width inherent to the eigenmodes. Doing so, I have shown that the fractal geometry of the medium, which grows towards the percolation, induces random-like eigenmodes in the system at low energies.