MAT-701: PREDICTING THE COMPRESSIVE STRENGTH OF ULTRA-LIGHTWEIGHT CONCRETE BY AN ARTIFICIAL NEURAL NETWORK

Ultra-lightweight concrete (ULWC) has potential applications for floating structures and architectural elements because of its dry density coming in at under 1000 kg/m3. The objective was to develop an artificial neural network (ANN) to aid the ULWC designer according to his needs. Boundary conditions were set for each material and 13 constraints based on the water binder ratio, density, air content, binder and aggregate content. The ANN predicted the compressive strength with a comfortable margin of error, with the gap encountered being attributed to variability in workability. Precise constraints and boundary conditions are needed to ensure a lower variability in workability. The ANN, coupled with a genetic algorithm, can generate millions of mixes for a given compressive strength in a short amount of time. The designer is able to choose mixes according to additional needs, such as the carbon footprint, absolute density, polymer content, cost, etc

Activation cérébrale en fonction de l'âge et de la complexité du traitement sémantique des mots par imagerie par résonance magnétique fonctionnelle

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

On the physics of multimode polaritons

This thesis is devoted to the study of microcavity polariton systems, in which the strong coupling occurs between more than one exciton or photon modes i.e. multimode polaritons. The first part of this work states the theoretical background of light-matter interaction starting from quantum well excitons to the non-linear regime of interactions between polaritons. We apply this knowledge to the case of GaAs based microcavities with InGaAs/GaAs quantum wells which are experimentally tested. Sample characterization and optimization are also discussed. We first report on the observation of multiple polariton modes, originating from an electronic coupling between quantum wells inside a planar microcavity. When shallow quantum well stacks are placed at the antinodes of a microcavity, we measure a series of anticrossings when the cavity mode energy crosses that of the different excitonic levels. Comparing our experimental results with a coupled oscillator model that includes the electron and hole wave functions allows us to show that the exciton binding energy is affected by the interwell coupling. We then study the non-linear properties of these InGaAs based microcavities, in the search for polariton condensation. We study the effect of Indium content, number of quantum wells, types of quantum well stacks, number of pairs of Bragg mirrors and magnetic field on the non-linearity of the system. In all cases, we measure a single threshold, and a clear signature of the transition from strong to weak coupling regime. We discuss the limiting factors for condensation in our system, namely the cavity losses induced by optical disorder, the light-matter coupling strength, and the saturation of the quantum wells. In the second part of the thesis, we demonstrate the occurrence of spatial multistability using laterally confined polariton modes. We measure a multihysteresis curve of the transmitted intensity when we cycle the excitation power of a blue detuned laser with respect to the polariton modes. At each threshold of the hysteresis loop, we measure a switching of the spatial profile of the transmitted beam, and an energy jump of all the polariton modes. We reproduce all main characteristics of our experiment using a multimode generalization of the Gross-Pitaevskii equations in the exciton photon basis. The mechanism behind the spatial multistability is identified as a repulsive cross-interaction between polaritons in different modes. Following this experiment, we investigate the effect of decoherence on polariton bistability. We demonstrate how the polariton hysteresis loop collapses when increasing either the temperature or the power of a weak non-resonant laser. We explain this effect by the population of an incoherent reservoir that induces dephasing and repulsive interactions on the driven polaritons. All experimental findings are accurately simulated with the excitonic Bloch equations, and indicate that reservoir induced dephasing can be dominant over the reservoir induced energy blue shift. In the final chapter, we present ongoing work on polariton lattices, namely measurements of the band structure for a square, and an hexagonal lattice. We also present preliminary results on the effect of a magnetic field on the band structure of a polariton square lattice. We conclude by discussing a series of future experiments to continue the investigation of multimode polaritons

Magnétophotoluminescence de dyades d'azote uniques dans le GaAs

Dans le but de réaliser une source de lumière quantique, plusieurs émetteurs ont été envisagées allant des lasers aux boites quantiques. Un des candidats pourrait être une dyade d’azote dans le GaAs. Cette nanostructure est composée de deux atomes d’azote en proche voisin se substituant à deux atomes d’arsenic. De même valence que l’arsenic, ce sont les effets combinés de la différence d’électron égativité et de la petite taille des atomes d’azote qui forment un puits de potentiel attractif pour l’électron. Celui-ci lie un trou par interaction coulombienne, formant ainsi un exciton lié à la dyade dont on peut observer la luminescence. Ce mémoire présente une étude expérimentale portant sur la structure fine de l’émission provenant de dyades d’azote uniques. La photoluminescence de ces dyades est réalisée à l’aide d’un microscope confocal à haute résolution spatiale et sous un champ magnétique pouvant aller jusqu’à 7 T. La résolution spatiale de ce montage combinée à la densité surfacique de dyades de l’échantillon étudié permet de faire l’étude des propriétés d’une seule dyade à la fois. Puisque la symétrie C2v des dyades étudiées lève la dégénérescence des niveaux excitoniques sans champ magnétique, il est possible observer quatre ou cinq transitions excitoniques, tout dépendant de l’orientation de la dyade avec la direction d’observation. À l’aide d’un hamiltonien considérant l’interaction d’échange, le champ cristallin et l’effet Zeeman, on modélise les énergies des niveaux excitoniques ainsi que leurs probabilités de transitions. De plus, on confirme les différents états de polarisation linéaire de ces transitions. Ce modèle nous permet de déterminer des plages de validité pour le facteur g de l’électron ainsi que les facteurs isotropes et anisotropes de l’interaction du trou avec le champ magnétique. Enfin, le coefficient diamagnétique nous permet de calculer un rayon de l’électron de 16.2 °A, confirmant que celui-ci est fortement localisé à la dyade. Parmi les dyades étudiées, un certain nombre d’entre elles possèdent une émission qui contraste avec celle généralement observée. Dans un premier cas, on trouve que l’environnement de celles-ci vient perturber les états excitoniques de sorte que seulement les deux transitions à plus hautes énergies sont observables. Dans un second cas, on observe une transition supplémentaire qui est d´epolarisée et toujours à plus basse énergie. On démontre que cette transition est associée à un exciton chargé, indiquant pour la première fois que ces états sont observables sur une dyade isoélectronique. L’ensemble de ces travaux permet donc une meilleure compréhension des états excitoniques liés à une dyade d’azote et ouvre la voie pour la réalisation de diverses applications.----------Abstract On the goal to achieve an efficient quantum light source, there are many possibilities ranging from lasers to quantum dots. One of those candiate is to use a single nitrogen dyad in GaAs. This nanostructure is composed of two nitrogen atoms in nearest neigbors subsituting for two arsenic atoms. Since both of those atoms have the same valence, the combined effet of the electronegativity and the small size of the nitrogen atoms form a potential well which attracts an electron. A hole is then bound to the electron via coulomb interaction, creating a bound exciton at the dyad from which the luminescence can be studied. In this work, we present an experimental study of the fine structure of the emission from single nitrogen dyads. The photoluminescence measurements are realised using a high resolution confocal microscope and under a magnetic field of up to 7 T. The spatial resolution combined with the sample’s surface density of nitrogen dyads allows studying the properties of individual dyads. Since the C2v symmetry of the dyad lifts the degeneracy of the excitonic levels without magnetic field, four or five transitions are observed, depending on the orientation of the dyad with respect to the observation axis. Using a Hamiltonian taking into account the exchange interaction, the local crystal field and the Zeeman effect, the energie of excitonic states as well as their transition probabilites are modelised. This model reproduce the linear polarization of the emmited photons and is used to determine a range of acceptable value for the g-factor of the bound electron as well as the isotropic and anisotropic factors of the interaction of the weakly-bound hole with the magnetic field. Furthermore, from the diamagnetic shift, the radius of the wavefunction of the electron is evalutated at 16.2 °A, confirming that it is strongly localized to the dyad. Of all the dyads studied, a certain number of them had an emission strickingly different from the ones usually observed. In a first case, the environment perturbed the excitonic states making only the two states at higher energy observable. In a second case, an additional depolarised transition is observed at lower energy. We show that this transition is associated to a charged exciton, indicating for the first time that these nanotructures can bind multiple charges like their larger epitaxial and collo¨ıdal counterpart. This work gives a better comprehension of excitons bound to a nitrogen dyad and opens the way to many applications

Effet de l'aération d'appoint, de la saison et de l'espèce de macrophytes dans le traitement d'un effluent piscicole par marais artificiel

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

A unified moment tensor potential for silicon, oxygen, and silica

Si and its oxides have been extensively explored in theoretical research due to their technological and industrial importance. Simultaneously describing interatomic interactions within both Si and SiO$_2$ without the use of \textit{ab initio} methods is considered challenging, given the charge transfers involved. Herein, this challenge is overcome by developing a unified machine learning interatomic potentials describing the Si/ SiO$_2$/ O system, based on the moment tensor potential (MTP) framework. This MTP is trained using a comprehensive database generated using density functional theory simulations, encompassing a wide range of crystal structures, point defects, extended defects, and disordered structure. Extensive testing of the MTP is performed, indicating it can describe static and dynamic features of very diverse Si, O, and SiO$_2$ atomic structures with a degree of fidelity approaching that of DF

Spontaneous self-ordered states of vortex-antivortex pairs in a Polariton Condensate

Polariton condensates have proved to be model systems to investigate topological defects, as they allow for direct and non-destructive imaging of the condensate complex order parameter. The fundamental topological excitations of such systems are quantized vortices. In specific configurations, further ordering can bring the formation of vortex lattices. In this work we demonstrate the spontaneous formation of ordered vortical states, consisting in geometrically self-arranged vortex-antivortex pairs. A mean-field generalized Gross-Pitaevskii model reproduces and supports the physics of the observed phenomenology

Nanocrystalline Cellulose as Effect Pigment in Clear Coatings for Wood

Nanocrystalline cellulose (CNC) is a renewable material with high potential in many applications. Due to its unique self-assembly and optical properties, CNC tends to behave as an iridescent pigment. The aim of this research was to explore the potential of CNC as an effect pigment in wood coatings. CNC-based coatings were developed from an aqueous CNC solution, a UV-curable water-based clear coating formulation, several colorants, and specialized additives. In this paper, the morphology of the resulting CNC films was investigated through circular dichroism and optical microscopy under polarized light. The effect of the CNC surface charge changes was monitored through zeta potential measurements. Color changes, or travel, and flop index were used to assess the iridescent effect of the coatings containing CNC. The experimental wood coatings contained CNC showed that the enhancement of the iridescent effect depends on the distribution and alignment of the CNC rod-like particles in order to generate the right pitch in the helical structure and their interaction with the polymer matrix as well with the additives. In conclusion, CNC could be successfully used as effect pigment in finishing systems, which can enhance the attractiveness and bring out the special grain of various types of wood.ISSN:2090-874