Past research has shown that the processing of emotional visual stimuli and visual attention are tightly linked together. In particular, emotional stimuli processing can modulate attention, and, reciprocally, the processing of emotional stimuli can be facilitated or inhibited by attentional processes. However, our understanding of these interactions is still limited, with much work remaining to be done to understand the characteristics of this reciprocal interaction and the different mechanisms that are at play. This thesis presents a series of experiments which use eye-tracking, behavioural and event-related potential (ERP) methods in order to better understand these interactions from a cognitive and neuroscientific point of view. First, the influence of emotional stimuli on eye movements, reflecting overt attention, was investigated. While it is known that the emotional gist of images attracts the eye (Calvo and Lang, 2004), little is known about the influence of emotional content on eye movements in more complex visual environments. Using eye-tracking methods, and by adapting a paradigm originally used to study the influence of semantic inconsistencies in scenes (Loftus and Mackworth, 1978), we found that participants spend more time fixating emotional than neutral targets embedded in visual scenes, but do not fixate them earlier. Emotional targets in scenes were therefore found to hold, but not to attract, the eye. This suggests that due to the complexity of the scenes and the limited processing resources available, the emotional information projected extra-foveally is not processed in such a way that it drives eye movements. Next, in order to better characterise the exogenous deployment of covert attention toward emotional stimuli, a sample of sub-clinically anxious individuals was studied. Anxiety is characterised by a reflexive attentional bias toward threatening stimuli. A dot-probe task (MacLeod et al., 1986) was designed to replicate and extend past findings of this attentional bias. In particular, the experiment was designed to test whether the bias was caused by faster reaction times to fear-congruent probes or slower reaction times to neutral-congruent probes. No attentional bias could be measured. A further analysis of the literature suggests that subliminal cue stimulus presentation, as used in our case, may not generate reliable attentional biases, unlike longer cue presentations. This would suggest that while emotional stimuli can be processed without awareness, further processing may be necessary to trigger reflexive attentional shifts in anxiety. Then the time-course of emotional stimulus processes and its modulation by attention was investigated. Modulations of the very early visual ERP C1 component by emotional stimuli (e.g. Pourtois et al., 2004; Stolarova et al., 2006), but also by visual attention (Kelly et al., 2008), were reported in the literature. A series of three experiments were performed, investigating the interactions between endogenous covert spatial attention and object-based attention with emotional stimuli processing in the C1 time window (50–100 ms). It was found that emotional stimuli modulated the C1 only when they were spatially attended and task-irrelevant. This suggests that whilst spatial attention gates emotional facial processing from the earliest stages, only incidental processing triggers a specific response before 100 ms. Additionally, the results suggest a very early modulation by feature-based attention which is independent from spatial attention. Finally, simulated and actual electroencephalographic data were used to show that modulations of early ERP and event-related field (ERF) components are highly dependent on the high-pass filter used in the pre-processing stage. A survey of the literature found that a large part of ERP/ERF reports (about 40%) use high-pass filters that may bias the results. More particularly, a large proportion of papers reporting very early modulations also use such filters. Consequently, a large part of the literature may need to be re-assessed. The work described in this thesis contributes to a better understanding of the links between emotional stimulus processing and attention at different levels. Using various experimental paradigms, this work confirms that emotional stimuli processing is not ‘automated’, but highly dependent on the focus of attention, even at the earlier stages of visual processing. Furthermore, the uncovered potential bias generated by filtering will help to improve the reliability and precision of research in the ERP/ERF field, and more particularly in studies looking at early effects.

Since its inception, Moore's Law has been a reliable predictor of computational power. This steady increase in computational power has been due to the ability to fit increasing numbers of transistors in a single chip. A consequence of increasing the number of transistors is also increasing the power consumption. The physical properties of CMOS technologies will make this powerwall unavoidable and will result in severe restrictions to future progress and applications. A potential solution to the problem of rising power demands is to investigate alternative low power nanotechnologies for implementing logic circuits. The intrinsic properties of these emerging nanotechnologies result in them being low power in nature when compared to current CMOS technologies. This thesis specifically highlights quantum dot celluar automata (QCA) and nanomagnetic logic (NML) as just two possible technologies. Designs in NML and QCA are explored for simple arithmetic units such as full adders and subtractors. A new multilayer 5-input majority gate design is proposed for use in NML. Designs of reversible adders are proposed which are easily testable for unidirectional stuck at faults.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal.

Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal.

This thesis studies the application of silver nanowire transparent conductors in PbS quantum dot photovoltaics. Silver nanowires were synthesized using a colloidal method and characterized using scanning electron microscopy. Nanowires were deposited on glass substrates by a stamp transfer process to generate a low density continuous network of conductive nanowires. This resulted in a highly conductive and transparent film appropriate for optoelectronic applications. Nanowire synthesis, deposition, and processing were optimised to produce transparent conductors suitable for thin film photovoltaics. These nanowire films were used to fabricate lead sulphide (PbS) colloidal quantum dot solar cells. In this structure, p-type PbS quantum dots form a junction with a n-type ZnO nanoparticle layer. A variety of fabrication and processing treatments were developed in order to reduce short-circuiting of devices and to enhance cell performance. Moderate nanowire density, improved ZnO adherence, slight device aging, and increased PbS film thickness proved to result in the highest quality devices. The champion device developed in this thesis achieved a power conversion efficiency of 2.2%.

Estudamos transporte quântico dependente de spin em sistemas de ponto e de poço quântico acoplados a contatos magnéticos. O primeiro passo do nosso estudo foi a dedução de fórmulas originais para a corrente e para o ruído em sistemas com tunelamento dependente de spin, através do formalismo de funções de Green de mão equilíbrio. As equações deduzidas reproduzem casos limites da literatura - em particular as fórmulas de Landauer-Buttiker. Posteriormente aplicamos essas fórmulas para estudar três sistemas distintos: (1) ponto quântico acoplado a contatos ferromagnéticos, (2) um ponto quântico acoplado a múltiplos terminais ferromagnéticos, e (3) um poço quântico acoplado a terminais de semicondutor magnético diluído (DMS). No sistema (1) consideramos os alinhamentos paralelo (P) e anti-paralelo (AP) entre as magnetizações dos terminais. Nesse sistema levamos em conta interação de Coulomb e espalhamento de spin no ponto quântico. Com as fórmulas para corrente e ruído deduzidas aqui, encontramos, por exemplo, que a interação de Coulomb, combinada com o magnetismo dos eletrodos, leva a um bloqueio de Coulomb dependente de spin. Esse efeito por sua vez leva a uma polarização da corrente que pode ser modulada (intensidade e sinal) através de uma tens~ao externa. Também encontramos que o espalhamento de spin leva a comportamentos contrastantes entre corrente e ruído. Enquanto a corrente na configuração AP aumenta com a taxa de espalhamento de spin R, o ruído nessa mesma configuração é suprimido para uma certa faixa de valores de R. Um outro efeito interessante que observamos foi a possibilidade de se suprimir o ruído térmico através de uma tensão de porta. Para o sistema (2) mostramos que é possível injetar corrente &#8593-polarizada no ponto quântico e coletar simultaneamente correntes &#8593 e &#8595 polarizadas em terminais diferentes. Além disso, a corrente ao passar do reservatório emissor para um dos reservatórios coletores tem a sua polarização intensificada. Portanto esse sistema pode operar como inversor e amplificador de polarização de corrente. Por último, analisamos os efeitos de terminais DMS e quantização de Landau (na presença de um campo magnético externo) sobre a corrente e o ruído no sistema (3). Encontramos que o efeito Zeeman gigante nos terminais DMS, gerado pela interação de troca s-d, leva a uma polarização da corrente. Em particular, para uma certa faixa de tensão o efeito Zeeman gigante resulta na completa supressão de uma dada componente de spin no transporte. Com isso é possível controlar a polarização da corrente através de uma tensão externa. Também observamos oscilações na corrente, no ruído e no fator de Fano como função do campo magnético. / We study spin dependent quantum transport in quantum dots and quantum well devices attached to magnetic leads. We first derive general formulas, including electron-electron interaction and spin flip, for both current and noise, using the no equilibrium Green function technique (Keldysh). From our equations we regain limiting cases in the literature - in particular the Landauer-Buttiker formula when we neglect electron-electron interaction. We apply these formulas to study three distinct systems: (1) a quantum dot attached to two ferromagnetic leads, (2) a quantum dot linked to many ferromagnetic leads, and (3) a quantum well coupled to dilute magnetic semiconductor (DMS) terminals. In the first system we consider both parallel (P) and anti-parallel (AP) ferromagnetic alignments of the leads. Coulomb interaction and spin flip scattering are also taken into account. With the formulas for the current and the noise derived here, we find, for instance, that the Coulomb interaction, combined with the magnetism of the electrodes, gives rise to a spin-dependent Coulomb blockade. This effect allows the control (intensity and sign) of the current polarization via the bias voltage. We also observe that spin flip scattering yields contrasting behavior between current and shot noise. While the current in the AP configuration increases with the spin flip, the shot noise becomes suppressed for a range of spin flip rates. Another interesting finding is the possibility to suppress the thermal noise via a gate voltage. For the dot coupled to three magnetic leads, we show that it is possible to inject current &#8593-polarized into the dot from the FM emitter, detect simultaneously &#8593 and &#8595 - polarized currents at distinct collectors. In addition, we find that the current has its polarization amplified when going from the emitter to one of the collectors. Therefore we have a device that operates as both as current polarization inverter and amplifier. Finally, we analyze the effects of DMS leads and Landau quantization on the current and noise of system (3). We and that the giant Zeeman effect in the DMS leads, due to the it s-d exchange interaction, gives rise to a spin polarized current, and for a particular bias voltage range, full suppression of one spin component. This gives rise to the possibility of tuning the current polarization via the bias voltage. We also observe oscillations in the current, the noise and the Fano factor as a function of the magnetic field.

A Terapia Fotodinâmica (TFD) é um método de tratamento do câncer e de outras doenças baseado no efeito conjunto de um composto fotoativo, luz da região visível ou infravermelho próximo e oxigênio molecular. O diagnóstico por fluorescência (DF) é uma técnica para o diagnóstico precoce de diversas doenças que consiste em utilizar algum composto fluorescente para distinguir tecido tumoral de tecido saudável. As porfirinas são amplamente utilizadas como fotossensibilizadores (FS) na TFD e como fluoróforos (FF) no DF. Os pontos quânticos (PQ) são novas nanoestruturas que possuem intenso e largo espectro de absorção na região espectral UV e visível, espectro de luminescência muito intenso e estreito e são fotoestáveis. Isso os torna promissores para o uso em TFD ou em DF, competindo com FS e FF orgânicos. A interação entre PQ e FS orgânicos pode aumentar a eficiência de ambos, devido aos processos de transferência de energia e/ou de carga. Durante o fototratamento os FS podem sofrer fototransformações, perdendo sua fotoatividade e formando produtos estáveis tóxicos. Isso torna importante o estudo da fototransformação dos FS. Neste trabalho, estudou-se, através da espectroscopia de absorção óptica, da fluorescência com resolução temporal e de flash-fotólise, a interação das porfirinas meso-tetrametil piridil (TMPyP) e meso-tetrasulfonatofenil (TPPS4) com o PQ de Telureto de Cádmio (CdTe) encapsulado com ácido 3-mercaptopropiônico (MPA) e sua fotólise individual e em conjunto. Os experimentos entre a TMPyP e o PQ mostraram que existe a formação de um complexo de transferência de carga entre a porfirina e o PQ. Verificou-se que a formação do complexo TMPyP-PQ aumenta a eficiência do processo de fotólise. A interação entre a p TPPS4 e o PQ em pH 4,0 (TPPS4 biprotonada) induz a transferência dos prótons para o PQ. A porfirina TPPS4 em pH 7,0 com adição de PQ não apresentou mudanças espectrais. Entretanto, em ambos os pHs, o PQ causava o aumento da velocidade de fotólise da TPPS4, sendo que o efeito do PQ foi maior em pH 4,0. Os resultados obtidos mostraram a importância da carga dos componentes, tanto na sua interação, como na eficiência de fotólise, devido à sua interação eletrostática. Também foi observado um aumento da intensidade da fluorescência de sistemas porfirina-PQ durante a fotólise. Esse efeito pode ser explicado pela redução da supressão da fluorescência devido à diminuição das concentrações dos componentes durante a fotólise. / The Photodynamic Therapy (PDT) is a method of treatment of cancer and other diseases based on combined effects of a photoactive compound, visible or near infrared light and molecular oxygen. The diagnosis by fluorescence (DF) is a technique for diagnostics of disease that consists in utilizing fluorescence compound to discern tumor tissue from a healthy one. Porphyrins are widely used as photosensitizer (PS) in PDT and as fluorophore (FP) in DF. Quantum dots (QD) are new nanostructures, which have intense and broad absorption spectrum in the UV and visible light region, intense and narrow luminescence spectrum and possess high photostability. They are considered promising for clinical application in PDT and/or DF, competing with organic PS and FP. Interaction between QD and organic PS can increase the efficiencies of both, due to the energy and/or charge transfer processes. The PS phototransformation during phototreatment may lead to loss of the PS photoactivity and formation of toxic products, that is important to study the processes of the PS phototransformation. In this work, with the support of absorption spectroscopy, steady state and time resolved fluorescence and flash-photolysis, we study interaction of meso-tetramethyl pyridyl (TMPyP) and meso-tetrakis sulfonatofenyl (TPPS4) porphyrins with CdTe QD functionalized with 3-mercaptopropionic acid (MPA) and their photolysis individual and in mutual. Experiments with TMPyP and QD demonstrate formation of a charge transfer complex between the porphyrin and the QD. We have verified that TMPyP-QD complex increases the efficiency of the porphyrin photolysis. The photolysis of the proper complex has been observed, as well. The interaction between TPPS4 and QD at pH 4.0 (biprotonated TPPS4) provokes deprotonation of the porphyrin via the proton transfer to QD. At pH 7.0 (deprotonated TPPS4) interaction of the porphyrin with the QD does not change TPPS4 spectral characteristics. Nevertheless, at both pHs QD cause increase in the TPPS4 photolysis rate, the effect being higher at pH 4.0. The observed results demonstrate the importance of the component charges both at their interaction and in photolysis efficiency, associated with their electrostatic interaction. The fluorescence intensity of the porphyrin-QD systems is enhanced during photolysis. This effect may be explained as the fluorescence quenching reduction due to decrease of the compound concentrations in the photolysis process.

O uso das quasi-particulas de Majorana que emergem nas bordas de um supercondutor topológico é uma plataforma promisora para computação quântica. Novas propostas usam quantum dots (QDs) para detectar sinais de Majorana. Este método tem duas vantagens: 1) Os QDs são os melhores dispositivos para estudar a co-existência de Kondo e Majorana, a qual têm sido reportada recentemente em experimentos. 2) O controle experimental preciso sobre os quantum dots que temos hoje em dia oferece a oportunidade única para manipular quasi-partículas de Majoranas dentro de sistemas com vários dots. Esta ideia abriu novos caminhos para o desenho de arquiteturas quânticas, nos aproximando do objetivo de implementar um computador quântico topológico. O caso mais simples em que se é possível manipular tais quasi-partículas é num quantum dot duplo (DQD). Este modelo oferece várias possibilidades para mover os Majoranas, incluindo múltiplas configurações geométricas dos dots como acoplamentos simétricos, lineares e em junções T. Neste trabalho vamos apresentar uma análise teórica das transiç?s dos sinais de Majorana dentro do DQD em sistemas interagentes e não interagentes. Vamos ver que é possível controlar a localização dos modos zero de Majorana mediante o incremento nas voltagens de gate dos QDs. Também vamos explorar como esses sinais interagem com o efeito Kondo que emerge em superposição com o modo zero de Majorana. Principalmente, vamos a usar dois métodos neste projecto: 1) Usamos as equações de movimento no formalismo de funções de Green para obter expressões exatas para a densidade de estados em sistemas não interagentes. Vamos apresentar o método the eliminação de Gauss-Jordan com grafos, o qual permite resolver rapidamente o sistema linear emergente nas equações de movimento. 2) Em sistemas Coulomb interagentes usamos NRG, no qual poderemos observar a interação entre o Majorana e o efeito Kondo. Vamos testar ambos os métodos nos modelos de um double quantum dot e um QD acoplado com uma cadeia de Majorana, com o qual vamos reproduzir os resultados presentes na literatura. Finalmente, incluímos a maior contribuição deste trabalho, o estudo de um DQD acoplado a uma cadeia de Majorana. / Majorana zero modes (MZMs) emerging at the edges of topological superconducting wires are a promising platform for fault-tolerant quantum computation. Novel proposals use quantum dots (QDs) coupled to the end of these wires to detect Majorana signatures. This detection method provides the following advantages: 1) This device allows to study the prospective coexistence of Kondo-Majorana signatures, which have been recently reported in experiments. 2) Today\'s precise experimental control over QDs offers the unique possibility of manipulating MZMs inside multi-dot systems. This innovative idea has enlightened the design of scalable quantum architectures, bringing us closer to the implementation of a topological quantum computer. The simplest case where Majorana manipulation is possible is in a double quantum dot (DQD). This system offers several possibilities for manipulation of MZMs, including different geometric configurations of the dots, from symmetric and linear couplings to T-dot junctions. In this project, we perform a theoretical study of the transitions of the Majorana signature in these geometries in non-interacting and interacting regimes. By tuning the dot\'s gate voltages, we will show that it is possible to control the localization of the MZM inside both dots. We will also explore the interplay of these signatures with the Kondo effect, which emerges in non-interacting dots in superposition with the MZM. We adopt two methods in this project: 1) The Green equations of motion (EOM) allow us to obtain exact expressions for the density of states in coulomb-non-interacting systems. We present the Graph -Gauss-Jordan elimination process as a simple-graphical method to solve the emergent linear systems in the EOM. 2) We use Wilson\'s numerical renormalization group (NRG) in interacting systems, to study the combined Kondo-Majorana physics. We will test these methods, first in a double quantum dot (DQD) (chapter 3) and then in a QD-Majorana model (chapter 4), where we confirm the results of previous papers [1-3]. Finally, we include the main contribution of this thesis, the study of a DQD coupled to a Majorana chain (chapter 5).

Contrôler la lumière avec de la lumière au niveau du photon unique est un objectif fondamental dans le domaine de l'information quantique, ou de l'ordinateur optique à très basse puissance. Un émetteur quantique constitué d'un unique système à deux niveaux est un milieu très non-linéaire, pour lequel l'interaction avec un photon peut modifier la transmission d'un photon suivant. Dans ce scenario, le défi pour obtenir une telle non-linéarité géante est d'optimiser l'interaction lumière matière. Une solution à ce défi est d’insérer l'émetteur quantique dans une structure photonique. Ce système est appelé « atome uni-dimensionnel » : la collection de la lumière, tout comme la probabilité d'absorber un photon se propageant dans la structure est maximum.Dans ce travail, nous avons utilisé ce type de système pour réaliser une non-linéarité géante à deux modes, dans laquelle la réflexion d'un des modes est contrôlée par un autre mode au niveau du photon unique. Le système est constitué d'une boite quantique semi-conductrice InAs/GaAs, qui peut être considéré comme un atome artificiel, insérée dans un fil photonique en GaAs opérant comme un guide d'onde. Le fil photonique définit un mode spatial unique autour de l'émetteur et offre une interaction lumière-matière avec une efficacité quasi-idéale. De plus, ce fil photonique présente cette propriété sur une large bande spectrale. Grâce à ces deux propriétés, nous avons démontré expérimentalement une non-linéarité géante à un mode et à deux modes à un niveau de quelques dizaines de photons par durée de vie de l'émetteur. Cela permet de réaliser un interrupteur tout optique intégré, à très faible seuil. / Controlling light by light at the single photon level is a fundamental quest in the field of quantum computing, quantum information science and classical ultra-low power optical computing. A quantum light emitter made of a single two-level system is a highly non-linear medium, where the interaction of one photon with the medium can modify the transmission of another incoming photon. In this scenario, the most challenging issue to obtain a giant optical non-linearity is to optimize photon-emitter interaction. This issue can be overcome by inserting the quantum emitter inside a photonic structure. This system is known as “one-dimensional atom”: the light collection efficiency as well as the probability for an emitter to absorb a photon fed into the structure is maximum. In this study, we aim at using such kind of system to experimentally realize a two-mode giant non-linearity, in which the reflection of one light mode is controlled by another light mode at the single-photon level. The system consists of a semiconductor InAs/GaAs quantum dot, which can be considered as an artificial atom, embedded inside a GaAs photonic wire, which is an optical waveguide. The photonic wire defines a single spatial mode around the emitter and offers a close to unity light-emitter interaction efficiency. In addition, the photonic wire also possesses a spectrally broadband operation range. Thanks to these two excellent features of the system, we experimentally demonstrate in this thesis a single-mode and a two-mode giant non-linearity obtained at the level of just a few tens of photons per emitter lifetime. This realizes an integrated ultra-low power all-optical switch.

Attentional bias (AB) refers to the tendency to selectively attend to (orientation towards) and/or hold attention on (slowed disengagment from) disorder-relevant stimuli. Females with eating-related concerns are thought to preferentially process threatening stimuli, which in turn is thought to maintain and exacerbate eating concerns. The aim of the present thesis was to explore AB for threatening stimuli in females characterised by restrained, external or emotional eating, and those with high levels of (non-clinical) eating psychopathology. This was carried out with the intention of identifying cognitive processes that contribute to eating behaviours in females, in order to assess the relevance of an attention training (AT) programme for reducing such biases. A pilot study assessed orientation/slowed disengagement, for mood and food words amongst females with high/low levels of restraint. Forty females completed a modified Stroop task with three conditions. Food and mood conditions included sequences of five words ( target food/mood followed by four neutral). The neutral condition consisted of all neutral words. Performance did not significantly differ according to high/low restraint groups. All participants took longest to colour-name word position 2 (demonstrating slowed disengagement lasting one consecutive trial). However, this pattern was also found in the neutral condition. Methodological limitations were then addressed in study one. High/low restrained eaters (n=48) completed a modified Stroop where targets (food, interpersonal threat, animal) were presented prior to four neutral words. Participants were slow to disengage from targets (slowest for word position 2) in all conditions. Patterns of responding indicated that restrained eaters might take longer to disengage (i.e. the carry-over effect from the food word seemed to last longer than one trial). However, more neutral words in the sequence were needed to assess this. As slowed disengagement from animals also arose, a categorical effect may have occurred. Study two explored attention processing of food using modified Stroop and dot probe tasks. In the Stroop task targets (food, interpersonal threat, household objects) were presented prior to six matched neutral words. This task revealed no evidence of AB. No significant pattern of differences between restrained (n=29)/unrestrained eaters (n=31) emerged; however, binge eating scores were significantly negatively correlated with response times. A dot probe task with food/neutral picture pairs also revealed no evidence of AB. Both restrained/unrestrained eaters had negative mean interference scores indicating avoidance of food. None of the following eating behaviours significantly correlated with AB: restraint, disinhibition, external eating, emotional eating and non-clinical eating psychopathology. Study three employed a further modified dot probe task based on image ratings. There was no evidence of AB, and no significant relation between task performance and restrained, emotional or external eating. 2000ms bias scores (assessing disengagement) were significantly negatively correlated with eating psychopathology and age, suggesting that those with high levels of non-clinical eating psychopathology attentionally avoid food stimuli and that younger females are slower to disengage attention from food (although found within a limited age range). Study four employed further modified Stroop and dot probe tasks, and assessed whether AB mediates the negative mood-eating relationship. Participants were allocated to negative or neutral mood conditions. No evidence of AB was found with the dot probe, but greater levels of emotional eating were associated with slower responding. In the Stroop task, all participants displayed an orientation bias towards food. Emotional eating and drive for thinness (DFT) scores were significantly positively correlated with food word colour-naming times but only amongst participants in a negative mood. However, those with high levels of external eating showed greater AB towards food when in a neutral mood. Highly emotional eaters in a negative mood showed a greater desire to eat than those in a neutral mood but did not increase in food intake. Furthermore, those with a high DFT (in a negative mood) showed no evidence of increased desire to eat or food intake. AB was not significantly related to subjective appetite or food intake. Therefore, AB does not seem to mediate the negative-mood eating relationship. The present thesis provides important suggestions for modifications of Stroop and dot probe tasks targeting orientation and disengagement. A modified Stroop has been more sensitive at detecting food AB than the dot probe. Implications of biased attention processing are discussed in relation to the development of harmful eating behaviours, and the present findings have important implications for developing programmes to prevent eating disorders amongst at-risk females (e.g. through AT or training at-risk females how to effectively cope with negative mood).