Submitted by Fernando Augusto Dessotti null (fernandodessoti@hotmail.com) on 2018-01-30T17:37:28Z No. of bitstreams: 1 Tese oficial.pdf: 12552592 bytes, checksum: e44289b205ab39ad49d51a2f976df63c (MD5) / Approved for entry into archive by Cristina Alexandra de Godoy null (cristina@adm.feis.unesp.br) on 2018-01-30T18:39:52Z (GMT) No. of bitstreams: 1 dessotti_fa_dr_ilha.pdf: 12552592 bytes, checksum: e44289b205ab39ad49d51a2f976df63c (MD5) / Made available in DSpace on 2018-01-30T18:39:52Z (GMT). No. of bitstreams: 1 dessotti_fa_dr_ilha.pdf: 12552592 bytes, checksum: e44289b205ab39ad49d51a2f976df63c (MD5) Previous issue date: 2017-12-12 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / O físico italiano Ettore Majorana propôs, no campo da Física de altas energias, a existência de férmions peculiares que têm como característica serem suas próprias antipartículas. No contexto de Física da matéria condensada, tais férmions emergem como quasipartículas de Majorana (MQPs). Da perspectiva da compu- tação quântica, duas MQPs podem compor um férmion regular e atuar como um qubit protegido, que está desacoplado do ambiente e livre do efeito de decoerência. Até onde sabemos, a verificação experimental de uma MQP ainda é questionável, apesar de alguns resultados experimentais, e desta forma, o objetivo desta tese é de propor formas experimentais a fim de ajudar na busca das assinaturas de tais excitações. Como o efeito Fano é um efeito de interferência na qual canais de tunelamento competem entre si pelo transporte eletrônico, ele torna-se uma forma de capturar tais assinaturas das MQPs em sistemas de matéria condensada. Baseado nisto, a ideia é investigar teoricamente três diferentes interferômetros a fim de obter uma assinatura definitiva das MQPs. O primeiro é um interferômetro do tipo Aharanov-Bohm composto por dois quantum dots, sendo um deles acoplado a uma MQP, que se localiza na borda de um fio de Kitaev semi-infinito na fase topológica. Ajustando o nível de Fermi dos terminais e o detuning simétrico dos níveis dos dots, mostrou-se que regimes Fano opostos resultam em uma transmitância caracterizada por distintas regiões condutoras e isolantes, que são marcas de uma MQP isolada. O dispositivo proposto aqui constitui uma alternativa experimental para detectar as MQPs. O segundo interferômetro é composto por pontas de STM e AFM próximas a um dímero de Kitaev de átomos adsorvidos supercondutores, na qual o átomo adsorvido localizado abaixo da ponta de AFM, encerra um par de MQPs. Para uma energia de ligação ∆ do par de Cooper delocalizado nos átomos adsorvidos abaixo das pontas coincidente com a amplitude de tunelamento t entre eles, ou seja, ∆ = t, mostrou-se que somente uma MQP abaixo da ponta de AFM hibridiza com o átomo adsorvido abaixo das pontas de STM, e para esta situação, o padrão de Fano permanece como universal. Mas para o caso das duas MQPs conectadas ao átomo adsorvido abaixo das pontas de STM, foi verificado que tal característica universal foi quebrada. O terceiro e último interferômetro é composto por dois quantum dots assimetricamente acoplados a MQPs isoladas que se localizam em duas cadeias de Kitaev na fase topológica. Este dispositivo habilita a medição da MQP em uma forma distinta do pico zero-bias. Mais importante, o sistema se comporta como um seletor de correntes composto por dois caminhos distintos: (i) para o dot superior conectado a ambas as cadeias, o dispositivo percebe ambas as MQPs como um férmion regular e a corrente atravessa somente o dot inferior, pois a corrente no dot superior é impedida devido a presença de um gap supercondutor; e (ii) pela leve supressão da hibridização do dot superior com a cadeia, a corrente é abruptamente trocada para fluir através deste mesmo dot, uma vez que um elétron é armadilhado como um estado ligado ao contínuo (BIC) surge no dot inferior. Tal seletor de corrente entre os dots inferior e superior caracteriza uma transição de fase quântica, que possibilita não somente a revelação de MQPs, mas também produz um seletor de corrente assistido por elas. / The Italian physicist Ettore Majorana proposed in the field of high-energy Physics the existence of peculiar fermions that constitute their own antiparticles. In the context of condensed matter Physics, these fermions are Majorana quasiparticles (MQPs). From the quantum computing perspective, two MQPs can compose a regular fermion acting as a protected qubit, which is indeed decoupled from the host environment and free of the decoherence effect. To the best of our knowledge, the experimental capture of a MQP up to now is still questionable despite some experimental results, then, the goal of this thesis is to propose helpful experiment manners in revealing signatures from such excitations. As the Fano effect is an interference phenomenon where tunneling paths compete for the electronic transport, it becomes a probe to catch fingerprints of MQPs lying on condensed matter systems. Based on this, the idea is to investigate theoretically three different interferometers in order to obtain a MQP smoking- gun signature. The first one was an Aharonov-Bohm-like interferometer composed by two quantum dots, being one of them coupled to a MQP, which is attached to one of the edges of a semi-infinite Kitaev wire within the topological phase. By changing the Fermi energy of the leads and the symmetric detuning of the levels for the dots, we show that opposing Fano regimes result in a transmittance characterized by distinct conducting and insulating regions, which are fingerprints of an isolated MQP. The setup proposed here constitutes an alternative experimental tool to detect MQPs. The second one is composed by STM and AFM tips close to a Kitaev dimer of superconducting adatoms, in which the adatom placed under the AFM tip, encloses a pair of MQPs. For the binding energy ∆ of the Cooper pair delocalized into the adatoms under the tips coincident with the tunneling amplitude t between them, namely ∆ = t, we find that only one MQP beneath the AFM tip hybridizes with the adatom coupled to the STM tips, and for this situation, the Fano pattern is still universal. But for the case of two MQPs connected to the adatom beneath the STM tips, we verify that such a universality is broken. The third and last one is composed by two quantum dots asymmetrically coupled to isolated MQPs, lying on the edges of two topological Kitaev chains. This setup enables us to probe MQPs in a quite distinct way from the zero-bias peak feature. Most importantly, the system behaves as a current switch made by two distinct paths: (i) for the upper dot connected to both chains, the device perceives both MQPs as an ordinary fermion and the current crosses solely the lower dot, since current in the upper dot is prevented due to the presence of the superconducting gap; and (ii) by suppressing slightly the hybridization of the upper dot with one chain, the current is abruptly switched to flow through this dot, once a trapped electron as a bound state in the continuum (BIC) appears in the lower dot. Such a current switch between upper and lower dots characterizes a quantum phase transition, which enables not only the fundamental revealing of MQPs, but also yields a current switch assisted by them.

Neste trabalho estudamos a forma de linha na absorção de dois fotons nas transições 4f7 de defeitos cristalinos superpostas com bandas de absorção multifonons. Escrevemos o perfil de linha sob a forma de Fano. O íon em estudo é Eu++ como defeito em um cristal de CaF2. Apresentamos o desenvolvimento da instrumentação necessária para realização do experimento, e obtemos resultados de ótima qualidade. / In this work we study the absorption of two photons in the electronic transition within the 4f 7configuration wich is superposed to the vibronic structure of 4f 7 - 4f65d transition. We describe the line shape with the Fano\'s formalism. The study was carried out on Eu+2 doped CaF2. The experimental apparatus was build and the results obtained are in good agreement with the theoretical predictions.

Les méta-surfaces permettent de contrôler efficacement es rayonnements électromagnétiques en manipulant la phase, l'amplitude et la polarisation des ondes. Pour de nombreuses applications, telles que des fonctions d’agilité, de commutations et de détection, des surfaces de haute performance sont nécessaires. L’objectif de cette thèse est d'explorer des méthodes innovantes permettant d'améliorer les facteurs de qualité des méta-surfaces dans les domaines micro-ondes et optique. La limitation des méta-surfaces est généralement liée aux pertes par rayonnement et à l'absorption non radiative. L'étude se concentre sur des solutions permettant la suppression des pertes de rayonnement en utilisant des concepts de mode sombre et de résonance de Fano. Un mécanisme d'excitation direct de mode sombre dans un résonateur individuel basé sur l’adaptation de symétrie est proposé. La possibilité d'améliorer l'efficacité de la réponse magnétique en optique est présentée. L'analyse de l'excitation par résonance de Fano dans un système de résonateurs couplés identiques est également réalisée sur la base du formalisme de la théorie des modes couplés. Deux configurations générales correspondant à l'arrangement spatial symétrique et énantiomères des résonateurs sont considérées. Dans le premier cas, le système de cavité formé par les méta-surfaces très proches fournit des caractéristiques spectrales très étroites et une sensibilité efficace élevée par rapport aux cavités Fabry-Perot conventionnelles. Dans le second cas, les caractéristiques de conception permettent de contrôler la suppression des pertes de rayonnement, permettant d'augmenter considérablement le facteur de qualité de la résonance de Fano. / Metasurfaces allows an effective control of electromagnetic radiation by manipulating phase, amplitude and polarization of electromagnetic waves. For numerous applications including tunable, switchable and sensing functionalities, high performance metasurfaces are required. The goal of this thesis is to explore innovative methods enabling to improve the quality factors of metasurfaces in microwave and optical domains. The limitation of metasurfaces is generally related to radiation losses and non-radiative absorption. The study is focused on solutions allowing the suppression of radiative losses by using concepts of dark mode and Fano resonance. A direct dark mode excitation mechanism in individual resonator based on symmetry matching is proposed. The possibility to enhance magnetic response efficiency in optical frequency range is presented. Analysis of Fano resonance excitation in a system of identical coupled resonators is also performed on the basis of coupled mode theory formalism. Two general configurations are considered corresponding to symmetric and enantiomeric space arrangement of resonators. In the first case, cavity system formed by near-field coupled metasurfaces provides sharp spectral characteristics and high efficient sensitivity compared to conventional Fabry-Perot cavities. In the second case the design features enables to control suppression of radiation losses, allowing to considerably increase Fano resonance quality factor.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18048号 / 理博第3926号 / 新制||理||1566(附属図書館) / 30906 / 京都大学大学院理学研究科数学・数理解析専攻 / (主査)教授 森 重文, 教授 玉川 安騎男, 教授 向井 茂 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.

We employ an extreme ultraviolet (XUV) pulse to impulsively excite dipole polarization in atoms or molecules, which corresponds to coherently prepared superposition of excited states. A delayed near infrared (NIR) pulse then perturbs the fast evolving polarization, and the resultant absorbance change is monitored in dilute helium, dense helium, and sulfur hexafluoride (SF6) molecules. We observe and quantify the time-dependence of various transient phenomena in helium atoms, including laser-induced phase (LIP), time-varying (AC) Stark shift, quantum path interference, and laser-induced continuum structure. In the case of dense helium targets, we discuss nonlinear macroscopic propagation effects pertaining to LIP and resonant pulse propagation, which account for the appearance of new spectral features in transient lineshapes. We then use tunable NIR photons to demonstrate the wavelength dependence of the transient laser induced effects. In the case of molecular polarization experiment in SF6, we show suppression of XUV photoabsorption corresponding to inter-valence transitions in the presence of a strong NIR field. In each case, the temporal evolution of transient absorption spectra allows us to observe and understand the transient laser induced modifications of the electronic structure of atoms and molecules.

Cette thèse est une étude des surfaces de type général dont le fibré cotangent est engendré par ses sections globales et dont l'irrégularité q est supérieure ou égale à 4.<br />L'objet et le moyen de cette étude est l'application cotangente qui est un morphisme du projectivisé du fibré cotangent dans l'espace projectif de dimension q-1. Nous étudions le degré de ce morphisme et le degré de son image.<br />Le fibré cotangent est ample si et seulement s'il n'existe pas de fibre de l'application cotangente de dimension strictement positive.<br />Si le fibré cotangent n'est pas ample, alors il existe une courbe C contenue dans la surface et il existe une section de C dans le projectivisé du fibré cotangent qui est contractée en un point par l'application cotangente. Une telle courbe C est qualifiée de courbe non-ample.<br />Nous donnons une classification des courbes non-amples de la surface suivant leur auto-intersection. Nous donnons ensuite une classification des surfaces possédant une infinité de courbes non-amples.<br />Un exemple pour lequel l'application cotangente intervient naturellement est celui des surfaces de Fano. Nous étudions le diviseur de ramification de leur application cotangente ainsi que leurs courbes non-amples.<br />Cette étude mène à la surface de Fano de la cubique de Fermat qui possède 30 courbes non-amples et dont nous détaillons les propriétés.

Graphene, a one-atom-thick sheet of hexagonally packed carbon atoms, is a novel material with high electron mobility due to its unique linear and gapless electronic band structure. Its broadband absorption and unusual doping properties, along with superb mechanical flexibility make graphene of promising application in optoeletronic devices such as solar cell, ultrafast photodetectors, and terahertz modulators. How- ever, the current performance of graphene-based devices is quite unacceptable owning to serious limitations by its inherently small absorption cross section and low quan- tum efficiency. Fortunately, nanoscale optical antennas, consisting of closely spaced, coupled metallic nanoparticles, have fascinating optical response since the collective oscillation of electrons in them, namely surface plasmons, can concentrate light into a subwavelength regime close to the antennas and enhance the corresponding field considerably. Given that optical antenna have been applied in various areas such as subwavelength optics, surface enhanced spectroscopies, and sensing, they are also able to assist graphene to harvest visible and near-infrared light with high efficiency. Moreover, the efficient production of hot electrons due to the decay of the surface plasmons can be further implemented to modulate the properties of graphene. Here we choose plasmonic oligomers to serve as optical antenna since they pos- sess tunable Fano resonances, consisting of a transparency window where scattering is strongly suppressed but absorption is greatly enhanced. By placing them in di- rect contact with graphene sheet, we find the internal quantum efficiency of hybrid antenna-graphene devices achieves up to 20%. Meanwhile, doping effect due to hot electron is also observed in this device, which can be used to optically tune the elec- tronic properties of graphene.

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No / Providing a guaranteed quality-of-service (QoS) is essential to many real-time applications. The existing distributed QoS routing algorithms are based on either shortest path or flooding and both tend to have high message overhead. A new distributed unicast QoS routing algorithm based on Fano¿s decoding method is studied. Fano¿s decoding method is a technique used in error control coding and it attempts to trace an optimal path probabilistically. The similarity of various aspects of Fano¿s decoding method to a QoS routing algorithm and the benefits it can provide encourages us to investigate the possibility of using it in QoS routing. This is the first known attempt to modify an error control technique using Fano¿s decoding method for the purpose of QoS routing in fixed wired networks. Simulation results demonstrate the effectiveness of the proposed algorithm in terms of message overhead and success ratio (% of paths obtained that satisfy the given QoS constraints). It is shown that the message overhead in the proposed algorithm is reduced compared to flooding based algorithms while maintaining a similar success ratio. Message overhead is also reduced compared to distance vector based algorithms for all but very sparsely connected networks, while success ratio is not compromised. Nodal storage is also considerably lower than for most other contemporary QoS routing algorithms.