Harmonic generation in gases using Bessel-gauss beams

The generation and propagation of harmonics in an atomic gas are described for the case of an incident Bessel-Gauss beam. Theoretical expressions are derived for the far-field amplitude of the harmonic field by solving the propagation equation using an elaborate integral formalism. We establish simple rules which determine the optimum Bessel-Gauss beam with respect to phase-matching as a function of the medium properties, such as the dispersion and the gas density. Target depletion due to photoionization and refractive index variations originating from both free electrons and dressed linear atomic susceptibilities are taken into account. The intensity-dependent complex atomic dipole moment is calculated using nonpertur- bative methods. Numerical propagation calculations for hydrogen, xenon and argon are presented. For hydrogen we consider the third harmonic of a 355-nm, 15-ps pump beam up to 3 X 10(^13) W/cm(^2) intensity, similarly for xenon, but at lower intensities. For argon we consider the 17th and 19th harmonic of a 810-nm, 30-fs pump beam around 10(^14) W/cm(^2) intensity. We compare conversion efficiencies and both spatial and temporal far-field profiles for an optimized Bessel-Gauss beam with respect to a Gaussian beam of same power and/or peak focal intensity. For the case of hydrogen, we investigate the effect of an ac-Stark-shift induced atomic resonance. We find all results in good agreement with our theoretical predictions. We conclude from our studies that Bessel-Gauss beams can perform better in terms of conversion efficiency than a comparable Gaussian beam. We find this to originate essentially from the more flexible phase-matching conditions for Bessel-Gauss beams. Bessel-Gauss beams also allow for spatial separation of the harmonic and the incident field in the far-field region, owing to the conical shape of their spatial far-field profile. Both features make Bessel-Gauss beams an attractive alternative to Gaussian beams in a limited but substantial number of experimental conditions

Efficient Grid Resource Selection for a CEM Application

National audienceComputational Electromagnetics (CEM) is a domain which provides numerical solutions to compute an- tenna performance, electromagnetic compatibility, radar cross section and electromagnetic wave propaga- tion. The ever-increasing need for more precision and larger meshes raises the natural question whether it is worth porting CEM algorithms to computer grids. Due to the nature of the computations, CEM al- gorithms are not trivially parallelisable, as data dependency inside the mesh implies communication. The goal of this paper is to answer the question: given a set of resources, what is the subset of resources one should chose among, to minimise the time it takes to solve a CEM problem. After presenting a model of the application execution time, several algorithms for selecting resources are described. The limits of the theoretical model is then compared against experimental results, obtained from the Grid'5000 platform

Three Essays on the Political Determinants of Public Policy in the U.S. States

This dissertation consists of three articles that seek to understand policy developments related to the death penalty, same-day registration, and felon voting rights in the American states, respectively. Leveraging annual estimates of state death penalty opinion, the first paper proposes, and finds, that direct democracy strengthens the connection between public preferences and policy and, in doing so, is partly responsible for the continued widespread existence of capital punishment in the U.S. I proceed to demonstrate, however, that the representation-enhancing effect of direct democracy does not extend to the punishment’s application, as measured by states’ issuances of death sentences, suggesting that the policy effects of these institutions can, in certain instances, be primarily symbolic in nature. In the second paper, I examine whether the Democratic (Republican) Party’s support of (opposition to) same-day registration is driven by strategic electoral considerations. I find that states have an increased probability of enacting the reform when legislative Democrats are in the precarious position that comes with having just experienced minority status in one or both chambers. Relatedly, I show that the presence of a Republican legislature does not make adoption less likely until the size of the Black population reaches a certain threshold. In fact, provided the Black population is small enough, Republican control of the legislature encourages reform. The third paper applies the overarching theory that politicians pursue electoral reform with the goal of manipulating the racial and, relatedly, partisan composition of the electorate to explain the introduction and adoption of felon voting rights legislation. As predicted by the racial threat explanation, lawmakers in states with larger Black populations propose more anti-felon voting rights bills. Further, in a sign that lawmakers view felon voting reform as a tool for solidifying their hold on power, I find that switches to Democratic (Republican) control are associated with higher numbers of pro- (anti-) felon voting rights bill introductions. A pooled event history analysis of state adoptions of three liberalizing reforms similarly confirms that the positive effect of Democratic control is limited to the party’s first year of rule over a state.Doctor of Philosoph

Accélération d'algorithmes permettant le traitement de la vidéo à l'aide d'une puce GPU

Le traitement de la vidéo exige une quantité phénoménale de calculs. La venue de format d'image aux dimensions encore plus grandes, tel que la télévision haute définition par exemple, contribue à accroître le nombre de calculs requis pour traiter une image. Conséquemment, il existe toujours un besoin pour traiter les images de façon plus rapide. Diverses méthodes exploitent le parallélisme afin d'accélérer le traitement de la vidéo. Sur une architecture de Von Neumann, certains calculs peuvent être exécutés en parallèle avec des instructions SIMD. Sur une machine biprocesseur, deux pixels peuvent être traités à la fois, exploitant ainsi le parallélisme spatial. En traitant simultanément des images sur différentes machines, c'est le parallélisme temporel qui est exploité. Ces méthodes d'accélération du traitement de la vidéo ne sont pas confinées aux méthodes de programmation logicielle s'exécutant sur des architectures de Von Neumann. Elles peuvent également être réalisées sur des puces dédiées exploitant la programmation matérielle comme les FPGA."--résumé abrégé par UMI

Public Opinion and Death Penalty Policy Under Direct Democracy Institutions: A Dynamic Analysis of the American States

This paper assesses the conditional effect of direct democracy on death penalty policy in the American states. I argue that these institutions have played an integral role in maintaining capital punishment in the United States by enhancing responsiveness to public preferences, which are generally supportive of the death penalty. The findings provide some support for my expectations, showing that direct democracy amplifies the effect of public opinion on policy, even if it does not necessarily increase the probability that policy will be congruent with majority preferences. This paper also overcomes a methodological limitation of prior studies on this subject, which employ a cross-sectional research design. Because public opinion---especially on the death penalty---and policy evolve over time, such a design can produce misleading results due to its static nature. Using the longitudinal research design promoted by Lewis and Jacobsmeier (2017), in combination with yearly state-level death penalty opinion estimates computed by Gelman and Shirley (2015), I am able to account for the dynamic nature of opinion and policy.Master of Art

New Representations of the Perturbative S-Matrix

We propose a new framework to represent the perturbative S-matrix which is well-defined for all quantum field theories of massless particles, constructed from tree-level amplitudes and integrable term-by-term. This representation is derived from the Feynman expansion through a series of partial fraction identities, discarding terms that vanish upon integration. Loop integrands are expressed in terms of "Q-cuts" that involve both off-shell and on-shell loop-momenta, defined with a precise contour prescription that can be evaluated by ordinary methods. This framework implies recent results found in the scattering equation formalism at one-loop, and it has a natural extension to all orders---even non-planar theories without well-defined forward limits or good ultraviolet behavior.Comment: 4+1 pages, 4 figure

Direct Observation of Quadrupolar Strain Fields forming a Shear Band in Metallic Glasses

For decades, scanning/transmission electron microscopy (S/TEM) techniques have been employed to analyze shear bands in metallic glasses and understand their formation in order to improve the mechanical properties of metallic glasses. However, due to a lack of direct information in reciprocal space, conventional S/TEM cannot characterize the local strain and atomic structure of amorphous materials, which are key to describe the deformation of glasses. For this work, 4-dimensional-STEM (4D-STEM) is applied to map and directly correlate the local strain and the atomic structure at the nanometer scale in deformed metallic glasses. Residual strain fields are observed with quadrupolar symmetry concentrated at dilated Eshelby inclusions. The strain fields percolate in a vortex-like manner building up the shear band. This provides a new understanding of the formation of shear bands in metallic glass

Diet-ethic: Fair Scheduling of Optional Computations in GridRPC Middleware

Most HPC platforms require users to submit a pre-determined number of computation requests (also called jobs). Unfortunately, this is cumbersome when some of the computations are optional, i.e., they are not critical, but their completion would improve results. For example, given a deadline, the number of requests to submit for a Monte Carlo experiment is difficult to choose. The more requests are completed, the better the results are, however, submitting too many might overload the platform. Conversely, submitting too few requests may leave resources unused and misses an opportunity to improve the results. This paper introduces and solves the problem of scheduling optional computations. An architecture which auto-tunes the number of requests is proposed, then implemented in the DIET GridRPC middleware. Real-life experiments show that several metrics are improved, such as user satisfaction, fairness and the number of completed requests. Moreover, the solution is shown to be scalable.La plupart des plate-formes HPC demandent à l'utilisateur de soumettre un nombre pré-déterminé de requêtes de calcul (aussi appelées " job "). Malheureusement, cela n'est pas pertinent quand une partie des calculs est optionnelle, c'est-à-dire, que l'exécution des requêtes n'est pas critique pour l'utilisateur, mais que leur complétion pourrait améliorer les résultats. Par exemple, étant donnée une date limite, le nombre de requêtes à soumettre pour une expérience Monte Carlo est difficile à choisir. Plus il y a des requêtes qui sont exécutées, meilleures sont les résultats. Cependant, en soumettant trop de requêtes, on risque de surcharger la plate-forme. À l'opposé, en ne soumettant pas assez de requêtes, les ressources sont sous-exploitées alors qu'elles auraient pu être utilisées pour améliorer les résultats. Cet article introduit et résout le problème d'ordonnancer des requêtes optionnelles. Une architecture qui choisit automatiquement le nombre de requêtes est proposée puis implémentée dans l'intergiciel GridRPC DIET. Les expériences faites sur de vraies plate-formes - telles que Grid'5000 - montrent que plusieurs métriques peuvent être améliorées, telles que la satisfaction des utilisateurs, l'équité et le nombre des requêtes exécutées. Enfin, la solution proposée passe à l'échelle

Mise en valeur des archives au Québec : expérimentation d'une plateforme participative en ligne

Les plateformes numériques contribuent à l’émergence d’un nouveau mode de participation, l’externalisation ouverte (crowdsourcing) « qui consiste, pour une organisation, à externaliser une activité par l'entremise d'un site Web, en faisant appel à la créativité, à l'intelligence et au savoir-faire de la communauté des internautes pour créer du contenu, développer une idée, résoudre un problème ou réaliser un projet innovant, et ce, à moindre coût. » (Office de la langue française, Le grand dictionnaire terminologique). Grâce à l’externalisation ouverte, la population est vue comme un bassin de contributeurs aux compétences diverses que les organisations peuvent mobiliser et mettre à contribution pour réaliser des activités ou tâches pour lesquelles les ressources internes ne suffisent plus (Brabham, 2013; Erickson, 2013). D’abord adoptée par des entreprises privées à des fins d’innovation, l’externalisation ouverte s’est ensuite déployée dans de nombreux projets en sciences citoyennes (astronomie, biochimie, ornithologie, mathématiques, langues anciennes et plantes). Les institutions culturelles ne sont pas en reste. Au contraire, l’externalisation ouverte y gagne en popularité à mesure que les institutions culturelles numérisent massivement leurs documents patrimoniaux et leurs oeuvres pour diffusion sur le Web, sans pour autant disposer des ressources humaines suffisantes pour décrire et indexer finement ces contenus (Oomen et Aroyo, 2011). Cela occasionne des difficultés de repérage et d’interprétation par les internautes des documents d’archives diffusés en ligne, faute d’information les décrivant et les indexant. De plus, les institutions présentent d’importants arrérages dans le traitement des documents (Néroulidis, 2016). L’externalisation ouverte est vue comme une solution à ce problème, les amateurs et autres citoyens venant prêter main-forte aux professionnels dans le traitement des documents patrimoniaux (Oomen et Aroyo, 2011). Elle est aussi vue comme une façon utile d’engager activement les citoyens dans la valorisation du patrimoine documentaire (Ridge,3 2013; 2016). De plus en plus d’institutions culturelles ont recours à l’externalisation ouverte pour solliciter une collaboration citoyenne dans le traitement des nombreux documents qu’elles numérisent (Holley, 2009; Moirez, 2012; Oomen et Aroyo, 2011; Ridge, 2016; Smith- Yoshimura et Shein, 2011). Alors que l’on dénombre en France plus d’une vingtaine de centres d’archives ayant une plateforme collaborative permettant aux citoyens de corriger et retranscrire des textes ou d’indexer des images (Néroulidis, 2015), aucune plateforme d’externalisation ouverte n’a encore été déployée spécifiquement au Québec pour appuyer le traitement du patrimoine documentaire. Les centres d’archives et l’externalisation de certaines de leurs tâches seront au coeur de ce projet de recherche

Simultaneous mapping of magnetic and atomic structure for direct visualization of nanoscale magnetoelastic coupling

Achieving a correlative measurement of both magnetic and atomic structures at the nanoscale is imperative to understand the fundamental magnetism of matters and for fostering the development of new magnetic nanomaterials. Conventional microscopy methods fall short in providing the two information simultaneously. Here, we develop a new approach to simultaneously map the magnetic field and atomic structure at the nanoscale using Lorentz 4-dimensional scanning transmission electron microscopy (Ltz-4D-STEM). This method enables precise measurement of the characteristic atomic and magnetic structures across an extensive field of view, a critical aspect for investigating real-world ferromagnetic materials. It offers a comprehensive visualization and statistical evaluation of the different structural information at a pixel-by-pixel correlation. The new method allows to directly visualize the magnetoelastic coupling and the resulting complex magnetization arrangement as well as the competition between magnetoelastic and magnetostatic energy. This approach opens new avenues for in-depth studying the structure-property correlation of nanoscale magnetic materials.Comment: 28 pages, 14 figure