1,721,135 research outputs found
Polarisation optique, cohérence spatiale et cavités photoniques à haut facteur de qualité en microscopie électronique
No full textLes méthodes d'observation optiques basées sur le champ lointain sont contraintes par la limite de diffraction. C'est une des raisons du succès des spectroscopies électroniques dans les microscopes électroniques à transmission à balayage (STEM), comme la perte d'énergie des électrons (EELS) et la cathodoluminescence (CL). Ces spectroscopies permettent l'études des champs électromagnétiques à l'échelle atomique. Cette propriété ouvre la voie à une meilleure compréhension des phénomènes nano-optiques. Même si les propriétés de ces champs en microscopie électronique (ME) sont mieux comprises depuis les dernières décennies, la ME reste incapable de mesurer certaines quantités en nano-optique. Comment mesurer la polarisation optique en ME ? Malgré vingt ans de recherche, la mesure de la polarisation des champs reste difficile. La polarisation et la chiralité sont deux observables globales. Mais la résolution spatiale en ME permettrait de les définir localement. Des études récentes ont démontré l'existence d'un lien entre l'optique singulière et la polarisation en EELS, tandis que d'autres ont fait des mesures de polarisation en CL sur des MEB et des MET. Pour mesurer ces observables en STEM, j'ai combiné la CL, l'EELS et des simulations sur des objets plasmoniques chiraux faits pour la spectromicroscopie électronique. Ces structures ont été réalisées par D. Gérard et J. Béal de l'Université de Troyes. J'ai aussi développé un système de CL polarisée pour l'acquisition de spectre-images polarisés. Ce dispositif expérimental a démontré que le signal dichroïque en CL est fondamentalement différent de celui attendu en EELS. Cela nous donne des indications sur la physique de ces nanostructures plasmoniques. Une phase-plate dynamique a aussi été installée sur le microscope et utilisée avec des alignements dédiés. Comment mesurer la cohérence spatiale d'excitations plasmoniques ? La cohérence temporelle des excitations optiques peut facilement être mesurée en spectroscopie. Cependant, la cohérence spatiale reste inaccessible. Des travaux théoriques récents, suggèrent qu'une expérience avec un faisceau d'électrons scindé serait capable de mesurer cette quantité entre deux points spécifiques de l'espace. Mais, l'implémentation d'une telle expérience présente de nombreux défis, dont une importante perte de signal, ce qui la rend difficile à réaliser. Pendant cette thèse, j'ai travaillé à la réalisation de cette expérience avec F. Houdellier et H. Lourenço-Martins, du CEMES. Puisque les premiers pas furent non-concluant, je me suis concentré sur la préparation de cette mesure. J'ai donc simulé plusieurs géométries pertinentes pour notre problème et compris quelles sont les observables les plus appropriées. En utilisant des travaux précédents, j'ai trouvé un degré de liberté supplémentaire pour mesurer cette cohérence spatiale sur des nanosctructures plasmoniques. Comment utiliser des électrons pour faire de l'optique quantique ? La communauté en ME cherche à explorer les phénomènes d'optique quantique en utilisant des électrons rapides. Une voie possible nécessite des modes optiques dont les facteurs de qualité (Q) sont très élevés, et un très fort couplage électron-photon. Plusieurs structures ont été proposées, mais la plupart ne sont pas adaptées à une étude EELS. Dans le cadre de cette thèse, on discutera du confinement spatial de la lumière dans des cristaux photoniques spécialement conçus et fabriqués par X. Chécoury, I. Si-Hadj-Mohand, A. Le Roux et P. Baroux du C2N. L'objectif principal de cette étude était de développer une cavité photonique à très fort Q (≈ 10⁶), avec un volume modal très faible (V ≈ 0.04 λ³), pour favoriser le couplage électron-photon. On a montré que cette probabilité de couplage au mode de cavité était d'environ 1% par électron. Ce couplage est comparable à ceux déjà réalisés, mais ici le volume modal est trois ordres de grandeurs plus petit, et devrait donc permettre des expériences d'optique quantique.Far-field optical methods are hindered by the optical diffraction limit. This is one of the reasons for the success of electron-based optical spectroscopies offered by Scanning Transmission Electron Microscope (STEM), such as Electron Energy Loss Spectroscopy (EELS) and Cathodoluminescence (CL). These spectroscopies enable the study of electromagnetic fields at the atomic level. This ability paves the way for a deeper understanding of nano-optical phenomena. Although our understanding of the properties of the fields via Electron Microscopy (EM) has been growing over the last decades, EM still lacks access to nano-optical quantities. How can we use electrons to measure optical polarization ? Despite twenty years, measuring field's polarization in EM remains challenging. Polarization and chirality are global observables, but EM's spatial resolution suggests they can be defined locally, in different ways depending on the spectroscopy method. Recent studies link singular optics with polarization via EELS, while others demonstrate polarization measurements with CL in SEM and TEM. To study these observables in STEM, I combined CL, EELS, and simulations on chiral plasmonic objects designed for electron spectro-microscopy. These structures were made by D. Gérard and J. Béal from the University of Troyes. A new polarized CL setup to acquire dichroic spectral images for circularly and linearly polarized signals was designed. This setup experimentally demonstrated that the CL's dichroic signal is fundamentally different from the one expected in EELS, and gave insights on the physics of chiral plasmonic nanostructures. A dynamical phase plate was also installed on the microscope in combination to the dedicated aligments. How can we assess the spatial coherence of plasmonic excitations with electrons ? The polarization is not the only observable still largely elusive in EM. Temporal coherence of optical modes can be easily measured with spectroscopies. However, the spatial coherence is still elusive. Recent theoretical works have shown that a split-beam experiment would be able to probe the spatial coherence between two specific points in space. But, the implementation of such an experiment presents significant challenges, including signal loss, making them difficult to carry out. During this thesis I worked on the realization of such an experiment helped by F. Houdellier and H. Lourenço-Martins from the CEMES. As the first steps were a bit hazardous, I finally devoted to the preparation of this measurement. I simulated structures to try different geometries making the spatial coherence property varying. This allowed me to find structures suitable for the problem and understand which observables are the most appropriate. Based on previous calculations, I found an additional degree of freedom in the experimental setup. I therefore propose an experimental setup and optimized samples for the exploration of spatial coherence in plasmonic structures at the nanometer scale. How can we use electrons to do quantum optics ? The EM community is looking towards exploring quantum optics phenomena with fast electrons. A possible path requires optical modes with very high-Q factor and strong electron-photon coupling. Several structures have been proposed. But most of them are not suited to EELS investigation. In this thesis, we will discuss the light spatial confinement using photonic cavities designed and manufactured by X. Checoury, I. Si-Hadj-Modand, A. Le Roux and P. Baroux from the C2N. The main goal of this study was to develop a photonic cavity exhibiting very high-Q factor (Q ≈ 10⁶) and very low modal volume (V ≈ 0.04 λ³), to maximize the electron-photon coupling. We proved that the coupling probability to the studied mode is about 1% per electron. This coupling is comparable to the one previously achieved. But here the modal volume is three orders of magnitude smaller, and should then allow quantum optics investigations
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Expérience de Hanbury Brown et Twiss dans un microscope électronique à transmission à balayage : sa physique et ses applications
No full textL'optique quantique réalisée à l'échelle du nanometer est un défit crucial, surtout pour la caractérisation d'émetteur de photon unique. Ces émetteurs sont des défauts ponctuels dans des matériaux (quelques angströms) ou des structures confinées de quelques nanomètres. Une façon d'atteindre cette échelle est d'utiliser la cathodoluminescence (CL) dans un microscope électronique à transmission à balayage (CL-STEM) [1]. Cependant, lorsque l'on cherche à étudier les propriétés statistique d'émission de la lumière sortant d'une expérience de CL, ce qui est nécessaire pour étudier par exemple la nature quantique d'émetteur de photon unique (SPE), une expérience dédiée s'ajoutant à l'expérience de CL-STEM doit être réalisée. Quelques mois avant mon arrivé dans le groupe STEM du LPS, une expérience d'interférence des intensités (HBT) qui mesure la fonction d'autocorrélation g(2)(τ) du signal de CL a été construit [2]. Il est bien connu que la signature univoque d'un SPE en photoluminescence (PL) est l'antibunching, c'est à dire que le g(2)(τ) est toujours inférieur à un. Il a été récemment démontré que lorsque seulement un SPE est excité la CL-STEM est similaire à la PL sur un célèbre SPE, le centre NV dans le diamant. Dans cette thèse nous montrerons comment la CL-STEM a permis de caractériser un nouveau défaut ponctuel dans le h-BN, montrant la pertinence de l'expérience HBT dans un CL-STEM pour découvrir et caractériser de nouveaux SPE. Cependant, en étudiant l'excitation de multiple SPE en CL, on a découvert un nouveau phénomène d'émission, caractérisé par un grand effet de regroupement (bunching) dans la fonction g(2) (g(2)(0) > 35), en complète contradiction avec les mesures de PL et ce que l'on pourrait attendre (g(2)(0) 35), in complete contradiction to PL measurements and expectations (g(2)(0)<1). In my thesis manuscript, this surprising effect will be experimentally investigated, theoretically explained and applied to lifetime measurement at the nanometer scale. Because quantum optics is often linked to quantum plasmonics, I will present, to conclude, a theoretical proposal, in collaboration with J. Garcia de Abajo, about quantum plasmonics measurement in a STEM
Towards Plasmon-Band Engineering in Ordered Plasmonic Nanostructures
Full text linkIn this thesis, the hybridization of localized surface plasmons to generate continuous plasmonic excitation bands was investigated. Localized surface plasmons are quasi-particles corresponding to collective oscillations of charge carriers (for example, conduction electrons in metals). They arise at interfaces between nanoparticles and their surroundings if the signs of the dielectric functions of the facing materials are opposite. If the spatial extension of the plasmon is confined to the nanoparticle, standing plasmon waves (so-called plasmon modes) with localized and amplified electromagnetic fields emerge. The field enhancement is confined to the boundaries of the nanoparticle, with material-dependent evanescent damping (decrease to 1/e in the range of several 10 nm).
The penetration of the fields into the environment allows interaction of plasmon modes of different individual nanoparticles arranged in assemblies with interparticle distances of several nanometers. This interaction can lead to hybridization, i.e., spectral splitting of the coupled plasmons into bonding and anti-bonding modes, which is the basis for the emergence of continuous plasmonic excitation bands. In analogy to electronic band structures, which arise by multiple hybridization of atomic orbitals in periodic lattices, plasmonic band structures can be created in large periodic arrays of plasmonic nanoparticles. Also, tuning of the bands to generate desired band properties is possible in principle by periodically varying either the individual building blocks of the arrangement (shape, size, material) or the coupling strength (distance, dielectric spacer).
In the present dissertation, the spectral and spatial characteristics (excitation energy, localization, etc.) of different arrangements of plasmonic nanostructures and their plasmonic response were investigated. Using a focused electron beam (probe) in a conventional transmission electron microscope, the plasmons were excited by the evanescent electromagnetic fields of the fast beam electrons (around half of the speed of light). By analyzing the energy loss of the beam electrons (which is caused by plasmon excitation) at different probe positions on the sample, the plasmons were characterized in terms of excitation energies and spatial localization. In addition, the measured data were supported by numerical simulations to verify the experiments. To gain a theoretical understanding, appropriate models were adapted to the present experiments. For example, the classical Mie theory (which describes the plasmonic response of a sphere to transverse electromagnetic waves) was generalized to the inhomogeneous case corresponding to the plasmon excitation by the evanescent field of the beam electrons. Furthermore, surface effects (the so-called axion mixing of magnetic and electric field components), for example, present in topological insulators, were taken into account in the generalization of the Mie theory. As a first step towards plasmon band engineering, the plasmonic response of gold nanoparticles of different shapes was studied to get a comprehensive understanding of the plasmonic behavior of isolated single nanoparticles, which can later be arranged into coupled plasmonic nanostructures.
In the next step, gold nanospheres were arranged into chains of different lengths to observe the formation of plasmonic band structures. By examining the hybridization as a function of chain length, the formation of a quasi-continuous plasmonic band with strong dispersion was observed.
To create more complex band structures with band gaps or crossings, gold and silver nanospheres were assembled to heterogeneous chains. Focusing on plasmon hybridization in coupled nanoparticles of different kinds, all possible permutations of four coupled gold and silver nanospheres were analyzed. Considering first pure gold and silver tetra chains, similar hybridized plasmon modes, differing only in a spectral red shift in the case of gold were observed. The mixed chains also show similar hybridized modes with intermediate spectral positions depending on the number of gold and silver spheres in the chains, which proves hybridization in heterogeneous arrangements. In addition, it was found that in particular silver nanoparticles degrade in air, resulting in a bad and undefined plasmonic response. The latter hampers the use of silver for plasmonic band engineering, although it has relatively low dielectric loss.
To address the degradation and to deliberately tune the distance between the coupled nanoparticles, the use of a silicon dioxide shell as a dielectric spacer and protection layer was elicited. Silver nanocubes were encapsulated in silica shells of various controllable thicknesses and investigated in terms of the plasmonic properties. It was found that the coating significantly reduces both degradation and influence of the substrate, resulting in a highly predictable and reproducible plasmonic response. The dielectric silica shell can additionally sustain Mie type resonances, which may couple to plasmons and thus mediate effective plasmonic coupling over relatively large distances (about a factor of two compared to the coupling of uncoated nanoparticles).
In contrast to the delocalized quasi-continuous plasmon bands in periodic nanostructures, localized (spectral and spatial) plasmonic modes can occur in disordered geometries. This effect can hamper the formation of plasmonic bands if the plasmons localize at imperfections (shape, size, or distance deviation) of the coupled nanoparticles. Related to this, the effect of plasmon localization in randomly disordered 2-dimensional gold webs was studied. Stronger localization with increasing plasmon excitation energy was found here. Finally, a geometry-dependent spectral threshold of vanishing localized plasmon modes was observed.
In summary, several fundamental aspects of plasmonic band engineering were investigated, providing a basis for the specific design of plasmonic nanostructures with desired properties.:Abstract
Acronyms
List of Symbols
List of Figures
List of Tables
Contents
1 Introduction
1.1 Synthesis of Plasmonic Systems
1.2 State of the Art
1.3 Outline
2 Theory
2.1 Surface Plasmons at Planar Interfaces
2.2 Modeling Dielectric Functions - the Drude Model
2.3 Axion Electrodynamics of Topological Insulators
2.4 Surface Plasmons at Spherical Geometries - Mie Theory and Generalization to Topological Insulators
2.4.1 Vector Spherical Harmonic Expansion
2.4.2 Axion Boundary Conditions
2.4.3 Homogeneous Case
2.4.4 Inhomogeneous Case
2.5 Complex Geometries and Coupled Nanoparticles
2.5.1 Plasmon Mode Hybridization
2.5.2 Numerical Solvers
2.5.3 Discrete Dipole Model
2.6 Plasmon Mode Classification
2.7 Plasmonics in the Transmission Electron Microscope
2.7.1 Electron Energy-Loss Probability
3 Methods
3.1 Experimental Setup
3.1.1 Energy Filter
3.1.2 Spectroscopy Mode - Direct Imaging of the Energy-Dispersive Plane
3.1.3 Imaging Mode - Energy-filtered Imaging of the Filter Entrance Plane
3.1.4 Alternative Modes
3.1.5 High-Angle Annular Dark Field-Detector
3.2 Data Post-Processing
3.2.1 Zero-Loss Peak Subtraction and Deconvolution
3.2.2 Correction of the Scattering Absorption
3.2.3 Enhancement of the Signal-to-Noise Ratio
3.3 Uncertainties of the Measurement
3.4 Plasma Cleaning of the Sample
4 Results
4.1 Interplay of the Nanoparticle’s Shape and Plasmonic Response
4.2 Self-Assembly of Spherical Nanoparticles to Homogeneous Chains
4.3 Self-Assembly of Spherical Nanoparticles to Heterogeneous Chains
4.4 Silica Encapsulation of Air Sensitive Nanoparticles
4.5 Localization of Surface Plasmon Modes in Disordered 2-Dimensional Webs
5 Summary and Outlook
5.1 Summary
5.2 Outlook
5.2.1 Measurement of the Plasmon Band Dispersion
5.2.2 Generalization of Anderson Localization to Plasmons
5.2.3 Measurement of the Axion Contribution in TIs
5.2.4 Non-Local Measurements
Bibliography
List of Publications
Danksagung
Erklärun
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Bridging nano-optics and condensed matter formalisms in a unified description of inelastic scattering of relativistic electron beams
Full text linkSubmission to SciPost. Updated version: corrected typos, add acknowledgementsInternational audienceIn the last decades, the blossoming of experimental breakthroughs in the domain of electron energy loss spectroscopy (EELS) has triggered a variety of theoretical developments. Those have to deal with completely different situations, from atomically resolved phonon mapping to electron circular dichroism passing by surface plasmon mapping. All of them rely on very different physical approximations and have not yet been reconciled, despite early attempts to do so. As an effort in that direction, we report on the development of a scalar relativistic quantum electrodynamic (QED) approach of the inelastic scattering of fast electrons. This theory can be adapted to describe all modern EELS experiments, and under the relevant approximations, can be reduced to any of the last EELS theories. In that aim, we present in this paper the state of the art and the basics of scalar relativistic QED relevant to the electron inelastic scattering. We then give a clear relation between the two once antagonist descriptions of the EELS, the retarded green Dyadic, usually applied to describe photonic excitations and the quasi-static mixed dynamic form factor (MDFF), more adapted to describe core electronic excitations of material. We then use this theory to establish two important EELS-related equations. The first one relates the spatially resolved EELS to the imaginary part of the photon propagator and the incoming and outgoing electron beam wavefunction, synthesizing the most common theories developed for analyzing spatially resolved EELS experiments. The second one shows that the evolution of the electron beam density matrix is proportional to the mutual coherence tensor, proving that quite universally, the electromagnetic correlations in the target are imprinted in the coherence properties of the probing electron beam
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