Dielectric THz Waveguides

Dans cette thèse nous avons exploré une grande variété de guides d’onde diélectriques pour guider la lumière THz (infrarouge lointain). Nous avons exploré théoriquement et expérimentalement un grand nombre de designs, basés sur plusieurs mécanismes de guidage différents, dans le but de réduire les pertes de propagation et les pertes par rayon de courbure élevées qui limitent les guides d’onde actuels. Les différents guides d’onde peuvent être classés selon deux stratégies fondamentalement différentes pour réduire les pertes de propagation : soit une fibre optique monomode dont le petit coeur guide un champ évanescent ou soit un tube creux multimode guidant la lumière à l’intérieur d’un trou d’air. Au delà du design et de la fabrication de nouveaux guides d’onde THz, les résultats présentés dans cette thèse sont basés sur une mesure adéquate des spectres de transmission THz des guides. Un montage innovateur de spectoscopie THz dans le domaine temporel a dû être conçu et réalisé afin de mesurer la propagation de la lumière THz à travers les guides d’onde. Un montage ajustable de spectroscopie THz a été implémenté en fixant des miroirs sur des rails de translation, afin de pouvoir déplacer les miroirs pour accomoder des guides d’onde d’une longueur pouvant atteindre 50 cm. Avec ce montage, nous avons mesuré avec succès des échantillons plans (" 3.25 mm)ainsi qu’une variété de guides d’onde THz, allant jusqu’à 40 cm de longueur. Nos efforts ont d’abord été concentrés sur la stratégie de réduction de pertes de propagation qui repose sur le guidage d’un champ évanescent par une fibre optique dont la taille du coeur est plus petite que la taille de longueur d’onde (lambda) de la lumière guidée. Cette fibre optique est dite de diamètre infra-lambda. Suite à des travaux théoriques par notre groupe sur le potentiel de ces fibres, nous avons entrepris la fabrication et la mesure d’une variété particulaire de fibres de diamètre infralambda dont le coeur est poreux afin de réduire davantage les pertes de propagation. La fabrication des fibres poreuses est un défi qui a nécessité le dévelopement de plusieurs nouvelles méthodes d’étirage de fibre afin d’éviter l’effondrement des trous à l’intérieur des fibres poreuses. La première méthode consistait à emprisonner de l’air dans un assemblage de tubes de polymère en les scellant avec de la colle. La réduction de la taille des trous lors de l’étirage donne lieu à des fibres ayant une faible porosité ----------Abstract In this thesis we have explored a wide variety of dielectric waveguides that rely on many different waveguiding mechanisms to guide THz (far-infrared) radiation. We have explored both theoretically and experimentally a large number of waveguide designs with the aim of reducing propagation and bending losses. The different waveguides can be classified into two fundamentally different strategies for reducing the propagation loss: small-core single-mode evanescent-field fibers or large hollow-core multi-mode tubes. Beyond the design and fabrication of new THz waveguides, the results of this thesis hinged on the proper measurement of waveguide transmission spectra. This required the development of a novel THz time-domain spectroscopy (THz-TDS) setup specifically designed for measuring THz propagation through waveguides. By placing mirror assemblies on a translation rail we implemented a versatile THz-TDS setup with an easily adjustable path length capable of accommodating waveguides up to 50 cm in length. With this setup we successfully measured both thin planar samples (" 3.25 mm) as well as a variety of THz waveguides, some 40 cm in length. Our focus was first set on exploring the small-core evanescent-field fiber strategy for reducing propagation losses. Following initial theoretical work in our group, much effort was spent on the fabrication and measurement of evanescent porous subwavelength diameter plastic fibers, in an attempt to further reduce the propagation losses. The fabrication of such fibers is a challenge and many novel techniques were devised to enable fiber drawing without hole collapse. The first method sealed the holes of an assembly of polymer tubes and lead to fibers of relatively low porosity (#25% air within the core) due to reduction in hole size during fiber drawing. The second method was a novel sacrificial polymer technique whereby drawing a completely solid fiber prevented any hole collapse and the subsequent dissolution of the sacrificial polymer revealed the holes in the fiber. The third method was a combination of preform casting using glass molds and drawing with pressurized air within the holes. This led to fibers of record porosity (86% air). The measurement of these porous fibers began with a collaboration with a group from the university of Sherbrooke. At the time, the only available detector was

G-CSF protects motoneurons against axotomy-induced apoptotic death in neonatal mice

<p>Abstract</p> <p>Background</p> <p>Granulocyte colony stimulating factor (G-CSF) is a growth factor essential for generation of neutrophilic granulocytes. Apart from this hematopoietic function, we have recently uncovered potent neuroprotective and regenerative properties of G-CSF in the central nervous system (CNS). The G-CSF receptor and G-CSF itself are expressed in α motoneurons, G-CSF protects motoneurons, and improves outcome in the SOD1(G93A) transgenic mouse model for amyotrophic lateral sclerosis (ALS). In vitro, G-CSF acts anti-apoptotically on motoneuronal cells. Due to the pleiotrophic effects of G-CSF and the complexity of the SOD1 transgenic ALS models it was however not possible to clearly distinguish between directly mediated anti-apoptotic and indirectly protective effects on motoneurons. Here we studied whether G-CSF is able to protect motoneurons from purely apoptotic cell death induced by a monocausal paradigm, neonatal sciatic nerve axotomy.</p> <p>Results</p> <p>We performed sciatic nerve axotomy in neonatal mice overexpressing G-CSF in the CNS and found that G-CSF transgenic mice displayed significantly higher numbers of surviving lumbar motoneurons 4 days following axotomy than their littermate controls. Also, surviving motoneurons in G-CSF overexpressing animals were larger, suggesting additional trophic effects of this growth factor.</p> <p>Conclusions</p> <p>In this model of pure apoptotic cell death the protective effects of G-CSF indicate direct actions of G-CSF on motoneurons in vivo. This shows that G-CSF exerts potent anti-apoptotic activities towards motoneurons in vivo and suggests that the protection offered by G-CSF in ALS mouse models is due to its direct neuroprotective activity.</p

Multi-band supervised classification for polarimetric SAR

International audienceThis work addresses the potential of multi-band polarimetric SAR imaging for terrains and vegetation classification. A classic supervised Wishart classifier is adapted to polarimetric multi-band datasets, and is applied on the X-, Land UHF-band acquisitions done during the NAOMI campaign (ONERA-Total) in Gabon (Africa) in 2015. The contributions of the different frequencies are shown and discussed. It is shown that the use of the multi-band dataset improves significantly the classification result

Science et fiction dans trois récits de voyage sur la Lune

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal.[À l'origine dans / Was originally part of : Thèses et mémoires - FAS - Département de littérature comparée

Improving predictability of time series using maximum entropy methods

We discuss how maximum entropy methods may be applied to the reconstruction of Markov processes underlying empirical time series and compare this approach to usual frequency sampling. It is shown that, at least in low dimension, there exists a subset of the space of stochastic matrices for which the MaxEnt method is more efficient than sampling, in the sense that shorter historical samples have to be considered to reach the same accuracy. Considering short samples is of particular interest when modelling smoothly non-stationary processes, for then it provides, under some conditions, a powerful forecasting tool. The method is illustrated for a discretized empirical series of exchange rates.Comment: 4 pages, 4 figure

A plural role for lipids in motor neuron diseases: energy, signaling and structure

Motor neuron diseases (MNDs) are characterized by selective death of motor neurons and include mainly adult-onset amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA). Neurodegeneration is not the single pathogenic event occurring during disease progression. There are multiple lines of evidence for the existence of defects in lipid metabolism at peripheral level. For instance, hypermetabolism is well characterized in ALS, and dyslipidemia correlates with better prognosis in patients. Lipid metabolism plays also a role in other MNDs. In SMA, misuse of lipids as energetic nutrients is described in patients and in related animal models. The composition of structural lipids in the central nervous system is modified, with repercussion on membrane fluidity and on cell signaling mediated by bioactive lipids. Here, we review the main epidemiologic and mechanistic findings that link alterations of lipid metabolism and motor neuron degeneration, and we discuss the rationale of targeting these modifications for therapeutic management of MNDs

Mapping complex traits using Random Forests

Random Forest is a prediction technique based on growing trees on bootstrap samples of data, in conjunction with a random selection of explanatory variables to define the best split at each node. In the case of a quantitative outcome, the tree predictor takes on a numerical value. We applied Random Forest to the first replicate of the Genetic Analysis Workshop 13 simulated data set, with the sibling pairs as our units of analysis and identity by descent (IBD) at selected loci as our explanatory variables. With the knowledge of the true model, we performed two sets of analyses on three phenotypes: HDL, triglycerides, and glucose. The goal was to approach the mapping of complex traits from a multivariate perspective. The first set of analyses mimics a candidate gene approach with a high proportion of true genes among the predictors while the second set represents a genome scan analysis using microsatellite markers. Random Forest was able to identify a few of the major genes influencing the phenotypes, such as baseline HDL and triglycerides, but failed to identify the major genes regulating baseline glucose levels

High refractive index composite materials for THz waveguides: trade-off between index contrast and absorption loss

Polymer compounds from titania-doped polyethylene are fabricated and their linear optical properties characterized by THz-TDS. We show that high concentration of dopants not only enhances the refractive index of the composite material, but also can dramatically raise its absorption coefficient. We demonstrate that the design of Bragg reflectors based on lossy composite polymers depends on finding a compromise between index contrast and corresponding losses. A small absorption value is also shown to be favorable, compared to an ideal lossless reflector, as it enables to smooth the transmission passbands. Transmission measurements of a fabricated hollowcore Bragg fiber confirm simulation results