Thesis--Zürich, 1919. / Lebenslauf.

Thesis--Zürich, 1919. / Lebenslauf.

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We introduce a perturbative method for studying photoinduced electronic transitions through conical intersections. Our approach uses a quadratic vibronic coupling Hamiltonian and second order cumulant approximation for the diabatic coupling to derive an analytical expression for time evolution of electronic populations at given temperatures. The formalism is an extension of a previous method called the non-equilibrium Fermi golden rule approach which used the linear vibronic coupling Hamiltonian with the same cumulant treatment for diabatic coupling. The advantage of the quadratic Hamiltonian is that it can include electronic states with different frequencies and normal modes. We explore these advantages with 2D models showing the improved accuracy of the new quadratic method over the linear method. We then apply our formalism to some real molecules, 2,6-bis(methylene) adamantyl cation, and its dimethyl derivative, with parameters obtained from electronic structure calculations followed by diabatization. The results show good agreement with quantum dynamics techniques.

We introduce a perturbative method for studying photoinduced electronic transitions through conical intersections. Our approach uses a quadratic vibronic coupling Hamiltonian and second order cumulant approximation for the diabatic coupling to derive an analytical expression for time evolution of electronic populations at given temperatures. The formalism is an extension of a previous method called the non-equilibrium Fermi golden rule approach which used the linear vibronic coupling Hamiltonian with the same cumulant treatment for diabatic coupling. The advantage of the quadratic Hamiltonian is that it can include electronic states with different frequencies and normal modes. We explore these advantages with 2D models showing the improved accuracy of the new quadratic method over the linear method. We then apply our formalism to some real molecules, 2,6-bis(methylene) adamantyl cation, and its dimethyl derivative, with parameters obtained from electronic structure calculations followed by diabatization. The results show good agreement with quantum dynamics techniques.

This thesis is centered on the study of spontaneous droplet migration along conical fibers. One of the key motivations for this project was to better understand the water-harvesting mechanism used by natural organisms like cacti. These desert plants exploit the conical shape of their spines to make fog condense into small droplets at the tip of theses spines. Then, droplets will spontaneously move towards the thickest end of the spines, bringing water to the main body of the plant. The key force behind drop migration on a conical fiber has been identified as the fluid surface tension by Lorenceau an Quéré in 2004. These authors also suggested and tested a model predicting the speed of drops on conical fibers. Here, we explore a larger range of relative sizes of drops compared to the fiber radius. The present document describes how an experimental set-up was designed in order to study droplet migration with conical glass fibers and silicone oil droplets. Using optical microscopy and data analysis, the droplet speed was measured as well as the other experimental parameters. Finally, a simple theoretical model has been developed to predict the droplet speed as a function of geometrical parameters and fluid characteristics. / Thesis / Master of Science (MSc)

In this thesis the use of thin foil conical profile X-ray mirrors for high energy, large aperture, and lightweight telescopes is investigated. The telescope performance is studied using analytical solutions and ray tracing simulations. Expressions for the number of mirrors in a module, the collecting area, the efficiency of aperture utilisation, and the resolution are calculated for on-axis sources. For off-axis sources ray tracing is used and expressions to describe the performance are derived empirically. The sensitivity and effect of singly reflected rays on the image are also investigated, and the comparison is made with Wolter (I) profile optics. A technique for the production of super-smooth surfaces using a layer of acrylic lacquer is described. Several materials have been coated using this method to assess their suitability as potential mirror substrates. To quantitatively assess the quality of both substrate and lacquer finishes a program of X-ray scattering measurements has been carried out. Several different types of surfaces have been examined both before and after lacquer coating. Particular attention has been payed to the ability of the lacquer to remove surface features of different spatial wavelengths. An epoxy replicated test flat is also examined which provides an interesting comparison with the lacquer coating technique for the production of thin X-ray mirrors. As a consequence of the wide range of materials examined it has been possible to compare most of the different techniques for producing thin X-ray mirrors. There is no one solution for all applications and thin foil mirrors are indeed a competitive technology.

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Cette thèse est consacrée à l’étude du potentiel des cristaux biaxes dans le but de réaliser un laser de puissance. Les cristaux biaxes possèdent deux axes optiques ce qui donnent lieu à un effet appelé "réfraction (ou diffraction) conique". Cet effet, connu depuis 1832, est intensé- ment étudié depuis une décennie. Les progrès en croissance et découpe cristalline permettent d’obtenir des échantillons de longueurs et de qualités optiques suffisantes pour observer ce phénomène. L’utilisation de cet effet en cavité laser ayant déjà été réalisée [79, 78], l’objectif de cette thèse fut dans un premier temps de confirmer ces résultats. Ainsi, un système utilisant des cristaux de KGd(WO4)2 (KGW) dopés au Néodyme a été préalablement testé. Ces premiers résultats furent décevants. La stabilité, la qualité de faisceau ainsi que l’efficacité étaient plus que mé- diocres. L’efficacité maximale fut d’environ 40 % contre 74 % dans la publication [77]. Par ailleurs, le faisceau de sortie présentait un profil elliptique indiquant la présence d’une lentille thermique avec un fort astigmatisme. Ainsi, cette orientation, qui devrait être proche d’une direction athermale (ne présentant pas d’astigmatisme d’origine thermique) selon Biswal [10], ne l’est pas dans une cavité laser pour ce cristal. De plus ce laser est très peu stable et difficile à aligner. Les fluctuations importantes de l’efficacité de celui-ci ont abouti à une interrogation quant à la dépendance des propriétés optiques aux alentours de l’axe optique. Ainsi, dans les premiers chapitres de cette thèse, figurent une étude détaillée des principales propriétés optiques d’absorption et d’émission. Les ré- sultats de cette étude montrent que ces propriétés varient non seulement suivant la polarisation mais également suivant l’orientation aux alentours de l’axe optique. Etant donné que l’état de polarisation autour de l’axe optique varie avec l’orientation, les dépendances en orientation et polarisation sont liées. [...] Enfin, un montage innovant a été testé afin d’utiliser la réfraction conique. La puissance de sortie ainsi que l’efficacité de ce montage étaient très faibles, environ 800 mW en sortie pour 4 W de puissance de pompe. / This thesis is devoted to the study on the potential of biaxial crystals in order to increase laser output power. Biaxial crystals have two optic axes and an effect called conical refraction (or diffraction) can occur. This effect is known since 1832, and intensively studied since the last decade. Thanks to the progress of crystal growth and crystal cutting, it is possible to have long samples of good optical quality in order to ob serve the conical refraction. This effect has already been used in a laser cavity [79, 78]. The aim of this thesis was to confirm the results previously obtained. Thus, Neodymium-doped KGd(WO4)2 (KGW) crystals, cut along the optic axis, have been tested. These first results were disappointing. Stability, beam quality and efficiency were very low. The maximum efficiency achieved was arount 40% compared to the 74% claimed in publication [77]. Thus, this orientation which should be an athermal direction using the calculation of Biswal [10] shows astigmatism when there is a temperature gradient. Furthermore, this laser being difficult to align with strong intensity fluctuations, rise interrogations about the optical property variations around the optic axis. Thus, in the first Chapters of this thesis a study of those properties around the optic axis is given. It shows a strong variation of the optical properties depending on the polarization and orientation around the optic axis. The study of the refraction along the optic axis helps to understand this complex dependency with the orientation. In a second time, holmium-doped KY(WO4)2 (KYW) crystals have been tested for laser emission at 2 µm. The choice of this ion has been done on several criteria. The first one is that the ISL laboratory is used to work with this wavelength and has a lot of equipment. The second one is that Holmium ions have a small "quantum defect" (pump wavelength 1960 nm and lasing wavelength 2074 nm). This low "quantum defect" limits the dispersion between the pump and lasing wavelengths. This decreases the separation between the optic axes of both wavelengths. Thus, when the pump is aligned for conical refraction the lasing wavelength is also aligned along the optic axis. Thanks to a better setup (with new mechanical parts) the polarization and orientation dependencies of the laser efficiency have been investigated. Up to 3 W of output power with a slope efficiency of 70% has been reached near the optic axis. The doping concentration of the Holmium-doped KY(WO4)2 (KYW) being more appropriated for high power laser, no thermal lens effect has been observed. Furthermore, the first conical refraction laser with a quasi-three-level system has been realized. An efficiency of 50% has been achieved with such a conical refraction laser. Finally, a new innovative setup is proposed using the External Conical Refraction. An output power of 800 mW for 4 W of incident pump power has been reached.

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