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Person# Gabriel David Bernasconi

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Second-harmonic generation

Second-harmonic generation (SHG, also called frequency doubling) is a nonlinear optical process in which two photons with the same frequency interact with a nonlinear material, are "combined", and g

Electron energy loss spectroscopy

Electron energy loss spectroscopy (EELS) is a form of electron microscopy in which a material is exposed to a beam of electrons with a known, narrow range of kinetic energies. Some of the electrons

Nanoparticle

A nanoparticle or ultrafine particle is usually defined as a particle of matter that is between 1 and 100 nanometres (nm) in diameter. The term is sometime

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Eigenmodes are central to the study of resonant phenomena in all areas of physics.
However, their use in nano-optics seems to have been hindered and delayed for various
reasons. First, due to their small size, the response of nanostructures to a far-field
optical excitation is mainly dipolar. Thus, preliminary studies of nanosystems through
optical methods meant that only very few eigenmodes of the system were probed, and a
complete eigenmode theory was not required. Second, rigorously defining eigenmodes
of an open and lossy cavity is far from trivial. Finally, only few geometries allow for
an analytical solution of Maxwellâs equations that can be expressed in terms of modes,
rendering the use of numerical methods mandatory to study non-trivial shapes. On the
other hand, modern spectroscopy techniques based on fast electron excitation, instead of
optical excitation, allow going beyond the above-mentioned dipolar regime and enable
the observation of high order modes. In addition, the generation of second harmonic
light (SHG) by nanoparticles permits revealing higher order modes that weakly couple
to planewave far-field probing. Thus, to be able to analyze the data collected with such
experimental methods and comprehend them in order to make appropriate nanostructure
designs, one needs to develop suitable numerical tools for the computation of eigenmodes.
This is the focus of this thesis, where eigenmodes are used throughout to analyze and
understand experimental and numerical results.
First, different approaches used to define and compute eigenmodes are presented in
details together with the surface integral equation method used in this manuscript.
The second chapter presents the use of eigenmodes to study the SHG in plasmonic
nanostructures. A single mode is used as an SHG source to disentangle the modal
contributions from different SHG channels. For three different nanostructures, the
dipolar mode gives a pure quadrupolar second harmonic (SH) response. Then, the
interplay of dipolar and quadrupolar SH radiations in nanorods of different sizes is
revealed through a multipolar analysis, explaining the experimental observation of the
flip between forward and backward maximum SH emissions. Finally, the dynamics of
the SHG from a silver nanorod generated by short pulses is investigated. By tuning the
spectral position and width of the pulses, the dynamics of a single mode is observed,
both in the linear and SH responses, and fits extremely well with a harmonic oscillator
model.
The last chapter presents the utilization of the eigenmodes to interpret electron energy
loss spectroscopy (EELS) measurements. An alternative approach to compute EELS
signal is presented, revealing the different paths through which the energy of the electron
is dissipated. Instead of computing the work done by the electron against the scattered
electric field, the Ohmic and the radiation losses are evaluated. Then, heterodimers with
several shapes and compositions are studied. A rich variety of modes is found, due to
the additional degree of freedom associated with the different metals. Dolmen shaped
nanostructures are also investigated in great details. A rigorous analysis of the eigenmode
evolution when the central horizontal nanorod is moved is performed. Finally, we study
the EELS for three iterations of a Koch snowflake nanoantenna. The evolution of the
modes with the iteration of the fractal is analysed and the modes are linked to the
experimental EELS map

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The relationship between composition and plasmonic properties in noble metal nanoalloys is still largely unexplored. Yet, nanoalloys of noble metals, such as gold, with transition elements, such as iron, have unique properties and a number of potential applications, ranging from nanomedicine to magneto-plasmonics and plasmon-enhanced catalysis. Here, we investigate the localized surface plasmon resonance at the level of the single Au-Fe nanoparticle by applying a strategy that combines experimental measurements using near field electron energy loss spectroscopy with theoretical studies via a full wave numerical analysis and density functional theory calculations of electronic structure. We show that, as the iron fraction increases, the plasmon resonance is blue-shifted and significantly damped, as a consequence of the changes in the electronic band structure of the alloy. This allows the identification of three relevant phenomena to be considered in the design and realization of any plasmonic nanoalloy, specifically: the appearance of new states around the Fermi level; the change in the free electron density of the metal; and the blue shift of interband transitions. Overall, this study provides new opportunities for the control of the optical response in Au-Fe and other plasmonic nanoalloys, which are useful for the realization of magneto-plasmonic devices for molecular sensing, thermo-plasmonics, bioimaging, photocatalysis, and the amplification of spectroscopic signals by local field enhancement.

2019Gabriel David Bernasconi, Andrei Kiselev, Olivier Martin

A large amount of experimental and theoretical works deals with the second harmonic generation from different plasmonic geometries. Since they often consider relatively long optical pulses, many of these studies are focused on the investigation of a quasi-monochromatic response of the system and can be understood through the excitation of one, possibly two, optical modes. On the other hand, when the excitation pulse duration is short (say, below several tens of fs), the excitation spectrum becomes broader and a very interesting dynamics emerges from the interplay between several optical modes. In this work, the dynamics of modes at the second harmonic frequency for two silver spheres of different diameters and a nanorod is investigated numerically and shown to be quite different. For the pulsed illumination with length close to the modes lifetime, apart from different relative contributions of dipolar and quadrupolar multipoles in the far-field, we have been able to observe and explain non constant phase difference between multipoles, which is not accessible in continuous wave regime. Short pulse durations also allow us to observe only one mode, while another one has already decayed. For the case of the nanorod we also perform an eigenmode analysis, which allows to understand the modes interplay that explains the observed spectra. In the paper, we also show a method allowing a significant reduction of required computational steps to find the response of a plasmonic nanostructure to a pulsed illumination with arbitrary frequency-domain method.