Segui questo link per vedere altri tipi di pubblicazioni sul tema: Diffraction.
Tesi sul tema "Diffraction"
Cita una fonte nei formati APA, MLA, Chicago, Harvard e in molti altri stili
Vedi i top-50 saggi (tesi di laurea o di dottorato) per l'attività di ricerca sul tema "Diffraction".
Accanto a ogni fonte nell'elenco di riferimenti c'è un pulsante "Aggiungi alla bibliografia". Premilo e genereremo automaticamente la citazione bibliografica dell'opera scelta nello stile citazionale di cui hai bisogno: APA, MLA, Harvard, Chicago, Vancouver ecc.
Puoi anche scaricare il testo completo della pubblicazione scientifica nel formato .pdf e leggere online l'abstract (il sommario) dell'opera se è presente nei metadati.
Vedi le tesi di molte aree scientifiche e compila una bibliografia corretta.
1
Hamam, Habib. "De la diffraction a la synthese des elements diffractifs". Rennes 1, 1995. http://www.theses.fr/1995REN10109.
Testo completoAbstract (sommario):
Parmi les facteurs les plus remarquables qui ont suscite l'interet pour les elements optiques diffractifs, on enumere deux facteurs. La technologie actuelle a vecu recemment un progres important qui a touche notamment la mise en uvre d'elements optiques ayant des hautes resolutions et des structures complexes. En outre apres l'apparition du laser, l'holographie synthetique a connu un essor remarquable qui s'est traduit notamment par des techniques de synthese des hologrammes generes par ordinateur operant en regime paraxial. Nous avons choisi de nous situer dans la continuite de ces trois dernieres decennies de progres et nous nous sommes limites a la diffraction scalaire dont l'exploitation est la plus abordable. Pour ce faire, il a fallu se pencher sur le phenomene de diffraction et analyser les mecanismes associes. Dans une premiere partie de ce travail, on a avance une nouvelle formulation surmontant la difficulte operatoire, associee a la these de huygens-fresnel, pour de nombreuses positions le long de l'axe de propagation. Deux parties supplementaires, ou l'on a considere le cadre theorique ainsi que la verification experimentale, ont ete essentiellement consacrees aux applications. On a aborde la synthese d'elements diffractifs mettant a profit le couplage latero-longitudinal qui fait la matiere de la transformee de fresnel fractionnaire. Dans le but de la synthese d'elements diffractifs a haute efficacite, on a considere, dans un premier temps, la combinaison de deux elements diffractifs lies par l'operateur de propagation en espace libre. Une extension de cette configuration holographique nous a menes a definir un nouveau type d'elements diffractifs que l'on a appele element diffractif multi-couches. Ce dernier permet d'injecter l'information dans plusieurs plans holographiques repartis parallelement dans l'espace libre et couples par l'operateur de propagation. Dans le cadre de cette technique, on s'est interesse particulierement a une fonction d'usage frequent a savoir la generation de tableaux et on a propose et experimente le principe d'un illuminateur de tableaux multi-couches. En outre, on s'est interesse a la synthese d'elements diffractifs pour laquelle on dispose d'un ensemble d'informations implicites concernant les signaux optiques desires dans le plan de sortie ou dans un plan intermediaire. Ce probleme, depassant le cadre standard de l'holographie synthetique, se ramene a l'optimisation sous contraintes d'elements diffractifs. A titre d'application, on a propose l'implantation d'un regenerateur optique dans un reseau d'interconnexions multi-couches
2
White, Thomas Ashley. "Structure solution using precession electron diffraction and diffraction tomography". Thesis, University of Cambridge, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.611748.
Testo completo
3
DeSandre, Lewis Francis. "Extinction theorem analysis of diffraction anomalies in overcoated-gratings". Diss., The University of Arizona, 1989. http://hdl.handle.net/10150/184853.
Testo completoAbstract (sommario):
A rigorous analysis based upon the extinction theorem is presented to study anomalous resonance effects from single- and multilayer-overcoated, low-efficiency diffraction gratings. Anomalously high diffraction efficiency at resonance results from the coupling of the incident beam into guided waves that can be propagated within the composite structure. Both the traditional characteristic matrix technique and a recursive or R-matrix propagation technique are presented. The R-matrix propagation algorithm was found to be stable numerically, and computational results agree favorably with both experimental and other theoretical work. Numerical results are presented in order to investigate the influence of certain parameters (i.e., groove depth and shape and the number of high- and low-index overlayers) on the diffraction efficiency at resonance. In this analysis, a wavelength of 0.6328 μm and grating period of 0.7 μm were chosen so that only a -1 diffracted order other than the specular is reflected from the gratings. Perfect transfer of the grating relief to the film boundaries does not occur in all instances; it depends on the grating and film characteristics together with the conditions during deposition. Investigated in this work is the effect of nonreplication of the grating profile at film interfaces on anomalous diffraction; a transition from trapezoidal profile at the grating substrate to a rounded relief at the top surface of the multilayer structure is assumed. For the cases studied, it was found that nonreplication has the effect of reducing the strength of the resonance outcoupling. Finally, experimental results on anomalous resonance effects for multilayer-coated gratings are presented. Good agreement with computational results was attained.
4
Persson, Roger. "Breaking the diffraction limit using conical diffraction in super resolution fluorescence microscopy : Breaking the diffraction limit using conical diffraction in super resolution fluorescence microscopy". Thesis, KTH, Tillämpad fysik, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-140725.
Testo completo
5
Grant, Stephen D. "Conical diffraction photonics". Thesis, University of Dundee, 2016. https://discovery.dundee.ac.uk/en/studentTheses/c4c0c9b8-f54a-406b-b73f-a84bc07f456e.
Testo completoAbstract (sommario):
Recent interest in conical diffraction (CD) has led to a large increase in experimental and theoretical investigations over the last two decades, a marked change from the previous 160 quiet years in the field. Once dismissed as an optical curiosity, the phenomenon has emerged as a fascinating area with potential for a large number of practical applications many of which have been realised while others are still being discovered. In this thesis a number of aspects of the theory as recently described are experimentally investigated with a view to strengthening the current theoretical understanding of the phenomenon. Developing from single crystal CD (the simplest case), through cascade CD, nonlinear CD, and with a particular emphasis on the polarisation effects of the phenomenon, a number of areas are investigated. Single crystal CD with circularly (CPL), linearly (LPL), azimuthally and radially polarised light (APL and RPL) is examined. The effect of LPL in removing a section of the ring orthogonally polarised to the incident beam is shown, along with the first investigation into the effects of RPL and APL polarisation effects in CD. The effect of the incident beam spot size on the pattern developed is also investigated and shown to conform to the theory. All findings show good agreement with the current theory. Cascade CD with various numbers of crystals and incident beam polarisations is investigated. Included in these experiments are a variable two crystal cascade and the first demonstration of the different patterns produced for a three crystal cascade when left and right circularly polarised light (LCPL and RCPL, respectively) are used, as recently predicted. As with the single crystal case, results are in agreement with theory. In both the single and cascade cases a cross section of the beam is captured to demonstrated the free space evolution of a CD beam. Simultaneous second harmonic generation (SHG) and CD from a single crystal is described in an update of Bloembergen et al.'s pioneering 1970's investigations with added emphasis on polarisation. SHG in non-phase matched conditions, as well as the influence of incident polarisation on the pattern and type of SHG, are observed. And finally a sensor based on CD demonstrating a simple, practical application of the phenomenon is outlined and a prototype device made and trialled. Using the effect of LPL described earlier to determine the polarisation angle of an incident beam, the device's use as a polarimeter is also tested to determine the specific rotation introduced by optically active liquids with the initial prototype showing results comparable to current methods. This work contains 7 chapters. The first is an introduction to the history of the phenomenon along with a thesis statement. Chapter 2 deals with single crystal CD, chapter 3 contains the expansion from single to cascade CD. The complexities introduced by various types of polarisation are described in chapter 4 for both single and cascade setups. Chapter 5 deals with SHG using a CD crystal and chapter 6 outlines the design and operation of the novel sensor based on the phenomenon. The final chapter is a summary of the work and outlook on the future of the field.
6
Nishantha, Hewamarappulige Indunil. "Powder Diffraction Methods". The Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=osu1222116031.
Testo completo
7
Ihee, Hyotcherl Zewail Ahmed H. Zewail Ahmed H. "Ultrafast electron diffraction /". Diss., Pasadena, Calif. : California Institute of Technology, 2001. http://resolver.caltech.edu/CaltechETD:etd-04072008-112244.
Testo completo
8
Patton, Forest S. "Coherent atom beam diffraction /". view abstract or download file of text, 2005. http://wwwlib.umi.com/cr/uoregon/fullcit?p3190537.
Testo completoAbstract (sommario):
Thesis (Ph. D.)--University of Oregon, 2005.Typescript. Includes vita and abstract. Includes bibliographical references (leaves 81-83). Also available for download via the World Wide Web; free to University of Oregon users.
9
Edwards, Philip John. "Diffraction theory and radiometry". Thesis, Imperial College London, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.408858.
Testo completo
10
Walsh, Sheridan John T. P. "Diffraction by volume gratings". Thesis, University of Oxford, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.303660.
Testo completo
11
Goodwin, A. L. "Dynamics from powder diffraction". Thesis, University of Cambridge, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.599515.
Testo completoAbstract (sommario):
This dissertation explores the extent to which dynamical information is preserved in neutron powder diffraction data. Total scattering data collected on the GEM instrument in ISIS have been used as input for the reverse Monte Carlo (RMC) method to generate large ensembles of independent atomistic configurations consistent with the data. A method of analysing these configurations for evidence of correlated atomic motion has been developed. The method is based on a similar technique originally used to analyse molecular dynamics (MD) configurations. It has been extended further in the dissertation to account for lattice symmetry, allowing unambiguous assignment of the normal modes at each wavevector. The theory has also been used to show that phonons are determined uniquely by first- and second-order displacement correlation functions alone, and should consequently be fixed by diffraction data. The impact of various restraints and constraints in the RMC method on the dynamical information stored within configurations has been systematically studied and used to develop a new constraint (the DW constraint) that helps prevent the incorporation of damaged regions into RMC configurations without affecting the form of the phonon dispersion curves obtained. The phonons-from-diffraction approach, implemented in the program PHONON, has been applied to MgO and SrTiO3. Calculated phonon dispersion curves resemble those obtained independently in spectroscopic experiments in the low-frequency region; however, the higher-frequency modes were generally not well preserved. Temperature-dependent mode softening of the R25 and Γ15 modes in SrTiO3 could be observed, and the mode displacement vectors used to characterise the atomic translations involved in the 105 K displacive phase transition.
12
Lahme, Stefan. "Femtosecond single-electron diffraction". Diss., Ludwig-Maximilians-Universität München, 2014. http://nbn-resolving.de/urn:nbn:de:bvb:19-178346.
Testo completoAbstract (sommario):
Die grundlegenden Funktionsprinzipien der Natur zu verstehen, ist seit jeher Antrieb der Naturwissenschaften. Verhalten und Eigenschaften von Festkörpern werden dabei häufig von dynamischen Prozessen auf atomarer Skala (< 10^-10 m) bestimmt, welche typischerweise auf Zeitskalen im Bereich von zehn Femtosekunden (10^-15 s) bis hin zu vielen Picosekunden (10^-12 s) ablaufen. Zeitaufgelöste Elektronenbeugung an kristallinen Festkörpern ermöglicht die direkte Beobachtung solcher Prozesse in Raum und Zeit. Die bislang mit diesem Verfahren erreichte Zeitauflösung von etwa 100 fs eignet sich jedoch nicht zur Beobachtung der schnellsten Prozesse in Festkörpern. Auch die, zur zuverlässigen Auflösung von großen Elementarzellen molekularer Kristalle erforderliche, transversale Kohärenz ist unzureichend. Eine wesentliche Ursache für diese beiden Probleme liegt in der gegenseitigen Coulomb-Abstoßung der Elektronen innerhalb eines Pulses und den daraus resultierenden Veränderungen der Geschwindigkeitsverteilungen in radialer und longitudinaler Richtung. Während erstere zu verringerter transversaler Kohärenz führt, hat letztere längere Elektronenpulsdauern und damit eine begrenzte Zeitauflösung zur Folge.
In dieser Arbeit wird ein Messaufbau zur zeitaufgelösten Elektronenbeugung vorgestellt, welcher auf der Erzeugung von nur einem Elektron pro Puls basiert. Aufgrund der Vermeidung von Coulomb-Abstoßung innerhalb der Pulse ist dieser Ansatz eine vielversprechende Basis zur konzeptionell nahezu unbegrenzten Verbesserung der Zeitauflösung. Eine hier eigens entwickelte, thermisch stabilisierte Elektronenquelle garantiert einen hohen Grad an Kohärenz bei gleichzeitig hervorragender Langzeitstabilität der Photoelektronenausbeute. Insbesondere letzteres ist für zeitaufgelöste Beugungsexperimente mit Einzeleelektronen aufgrund der längeren Integrationszeit unerlässlich, konnte jedoch durch vorhergehende Quellen nicht erreicht werden. Darüber hinaus werden in dieser Arbeit die besonderen Ansprüche der Einzelelektronenbeugung an die zu untersuchenden Materialien diskutiert und Strategien zur Vermeidung von Schäden an der Probe durch akkumulierte Anregungsenergie entwickelt. Diese erfordern neue Schwerpunkte bei der Probenpräparation, welche entwickelt und diskutiert werden. Die Beobachtung der komplexen Relaxationsdynamik in Graphit-Dünnfilmen mit zeitaufgelöster Einzelelektronenbeugung demonstriert abschließend die generelle Eignung dieses Verfahrens als zuverlässige Methodik zur Untersuchung von reversibler, struktureller Dynamik in Festkörpern mit atomarer Auflösung.
Nicht-relativistische Einzelelektronenpulse können mit Hilfe von zeitabhängigen Feldern bei Mikrowellenfrequenzen bis in den 10 fs-Bereich komprimiert werden, eventuell sogar
bis in den Attosekundenbereich. Die hier demonstrierte langzeitstabile und hochkohärente Elektronenquelle, sowie die Methodiken zur Probenpräparation und zeitaufgelösten Beugung
mit Einzelelektronenpulsen liefern die Basis für zukünftige Experimente dieser Art.The understanding of nature’s fundamental processes has always been the goal of science. Often, the behavior and properties of condensed matter are determined by dynamic pro- cesses on the atomic scale (< 10^-10 m). The relevant time scales for these processes range from tens of femtoseconds (10^−15 s) to several picoseconds (10^−12 s). Time-resolved electron diffraction on crystalline solids allows the direct observation of such processes in space and time. However, the state-of-the-art temporal resolution is insufficient to observe the fastest processes in solids. The transverse coherence is insufficient to resolve large unit cells of molecular crystals. One major origin for both of these problems is that the electron within the pulse repel each other, resulting in a change of the radial and longitudinal velocity distribution. The former leads to a decrease transverse coherence while the former leads to a significant increase in electron pulse duration, limiting temporal resolution. In this work, a setup for time-resolved electron diffraction is introduced that works with electron pulses each containing only a single electron. Circumventing Coulomb repulsion, this approach can lead to in principle nearly unlimited, improvement of temporal resolu- tion. The novel, thermally stabilized single-electron gun developed here provides a high degree of transverse coherence and excellent long-term stability of the photoemission yield at the same time. The latter is crucial for time-resolved diffraction experiments due to the long integration times required when working with single-electron pulses and has not been achieved prior to this work. Furthermore, the special requirements of single-electron diffraction on the materials under study are discussed. Strategies for avoidance of sam- ple damage from accumulated excitation energy are developed, requiring new emphases in sample preparation. The observation of the complex relaxation dynamics of graphite thin films using time-resolved single-electron diffraction finally demonstrates the general feasi- bility of this technique as a reliable methodology for investigation of reversible, structural dynamics in solids with atomic resolution. Using time-dependent fields at microwave frequencies, non-relativistic single-electron pulses can be compressed to 10 fs and possibly even down to the attosecond regime. The long-term stable and high-coherence electron gun demonstrated here as well as the method- ology developed for sample preparation and time-resolved electron diffraction using single- electron pulses provide the basis for such experiments in the future.
13
Zora, J. A. "X-ray diffraction studies". Thesis, University of Sussex, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.374467.
Testo completo
14
Senyshyn, A., M. Monchak, O. Dolotko e H. Ehrenberg. "Lithium Diffusion and Diffraction". Diffusion fundamentals 21 (2014) 4, S.1, 2014. https://ul.qucosa.de/id/qucosa%3A32392.
Testo completoAbstract (sommario):
In the current contribution the application of bond valence method for the prediction (and diffraction-based techniques for the evalution) of ion diffusion pathways in different materials
for electrochemical energy conversion and storage will be presented and discussed.
15
Bonner, Bradley David. "Calculating conical diffraction coefficients". Thesis, University of Bath, 2003. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.425888.
Testo completo
16
Northey, Thomas. "Ab initio molecular diffraction". Thesis, University of Edinburgh, 2017. http://hdl.handle.net/1842/28772.
Testo completoAbstract (sommario):
In 1915, Debye derived his well-known equation for the X-ray scattering from a sample of randomly orientated gas-phase molecules. He approximated the molecular scattering by adding the contributions of isolated atomic constituents. This is known as the Independent Atom Model (IAM). However, it omits the redistribution of valence electrons due to bonding, and is limited to the electronic ground state. The main proposition of this thesis is that it is worthwhile going beyond the IAM when interpreting X-ray scattering data. In part, this is motivated by the arrival of new X-ray sources called X-ray Free-Electron Lasers (XFELs). A new method called Ab Initio X-ray Diffraction (AIXRD) is introduced. It calculates the elastic X-ray molecular scattering factor directly from wave functions calculated by ab initio electronic structure theory, for instance Hartree-Fock or multiconfigurational self-consistent field. In this way, the valence electrons are correctly taken into account, and calculations based on electronically excited wave functions become possible. The wave functions must be constructed from spatial orbitals made up of Gaussian-Type Orbitals (GTOs), giving an analytical solution to the Fourier transform integrals involved, and is key to computationally efficient and accurate results. This is compared to a fast Fourier transform (FFT) method, where the electron density is computed on a 3D grid and an FFT algorithm is used to obtain the elastic X-ray molecular scattering factor. Inspired by post-crystallography experiments such as serial femtosecond crystallography and single-particle imaging at XFELs, the AIXRD method is expanded to allow accurate X-ray diffraction calculations from large molecules such as proteins. To make the underlying ab initio problem tractable, the molecule is split into fragments. In other words, the electron density is constructed by a sum of fragment contributions, as is the corresponding molecular form-factor. In this way, it is analogous to the IAM approach except that instead of isolated atoms, there are isolated fragments. A pairwise summation of fragment contributions is also used to account for fragment-fragment interactions. Various fragment definitions are compared based on their effect on the X-ray diffraction signal, and are compared to the IAM method. Finally, X-ray diffraction from molecules in specific quantum states is calculated, revealing a distinct quantum fingerprint in the X-ray diffraction, and a comparison to experiment is made. In particular, the elastic X-ray diffraction is calculated from gas-phase H2 pumped to various electronic, vibrational, and electronic states. This is expanded upon for polyatomic molecules using the harmonic approximation for the vibrational states.
17
Malyarenko, Eugene V. "Lamb wave diffraction tomography". W&M ScholarWorks, 2000. https://scholarworks.wm.edu/etd/1539623991.
Testo completoAbstract (sommario):
As the worldwide aviation fleet continues to age, methods for accurately predicting the presence of structural flaws, such as hidden corrosion and disbonds, that compromise air worthiness become increasingly necessary. Ultrasonic guided waves, Lamb waves, allow large sections of aircraft structures to be rapidly inspected. However, extracting quantitative information from Lamb wave data has always involved highly trained personnel with a detailed knowledge of mechanical waveguide physics. In addition, human inspection process tends to be highly subjective, slow and prone to errors. The only practical alternative to traditional inspection routine is a software expert system capable of interpreting data with minimum error and maximum speed and reliability. Such a system would use the laws of guided wave propagation and material parameters to help signal processing algorithms automatically extract information from digitized waveforms. This work discusses several practical approaches to building such an expert system.;The next step in the inspection process is data interpretation, and imaging is the most natural way to represent two-dimensional structures. Unlike conventional ultrasonic C-scan imaging that requires access to the whole inspected area, tomographic algorithms work with data collected over the perimeter of the sample. Combined with the ability of Lamb waves to travel over large distances, tomography becomes the method of choice for solving NDE problems. This work explores different tomographic reconstruction techniques to graphically represent the Lamb wave data in quantitative maps that can be easily interpreted by technicians. Because the velocity of Lamb waves depends on the thickness, the traveltimes of the fundamental modes can be converted into a thickness map of the inspected region. Lamb waves cannot penetrate through holes and other strongly scattering defects and the assumption of straight wave paths, essential for many tomographic algorithms, fails. Diffraction tomography is a way to incorporate scattering effects into tomographic algorithms in order to improve image quality and resolution. This work describes the iterative reconstruction procedure developed for the Lamb Wave tomography and allowing for ray bending correction for imaging of moderately scattering objects.
18
Morishita, Shigeyuki, Jun Yamasaki, Keisuke Nakamura, Takeharu Kato e Nobuo Tanaka. "Diffractive imaging of the dumbbell structure in silicon by spherical-aberration-corrected electron diffraction". American Institite of Physics, 2008. http://hdl.handle.net/2237/11975.
Testo completo
19
Shukayr, Riman. "Evaluation of the uniform theory of diffraction for edge diffraction at low frequency". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2001. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp05/MQ64062.pdf.
Testo completo
20
Ye, Hong. "X-ray diffraction, neutron diffraction and circular dichroism studies of nucleic acids and proteins". Thesis, Keele University, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.245601.
Testo completo
21
Kumari, Maini S. M. "Development of a breast tissue diffraction analysis system using energy dispersive X-ray diffraction". Thesis, University College London (University of London), 2012. http://discovery.ucl.ac.uk/1370578/.
Testo completoAbstract (sommario):
Research groups have shown that diffraction techniques could be applied for characterising materials. In particular, Energy Dispersive X-ray Diffraction (EDXRD) technique has been successfully used in characterising materials such as plastics, drugs and biological tissues. The size of breast tissues used for characterisation so far has been small, in the range of mm. In order to exploit the fullness of the EDXRD technique in characterising breast tissues and hence enable early and precise breast tumour detection, the presented research work takes the existing research work a step forward by developing a breast tissue diffraction analysis system wherein breast-sized tissue-equivalent materials have been studied for tumour detection and an optimised EDXRD system for breast tissue analysis has been presented. For the development of this breast tissue analysis diffraction system, a ray-tracing model of the EDXRD system has been developed. The model has been used to predict diffraction spectra. These model predictions have been further used to optimize system parameters for an EDXRD system so it could be used for breastsized samples. Materials like plastics, pharmaceutical drugs and tissues have been characterised on this optimized system. The diffraction spectra collected have been used to build a diffraction spectrum database which has been further used to generate diffraction images for detection of tumour of size as small as 0.5 cm. Following this abstract, in the thesis, Chapter 1 introduces how X-rays interact with matter and what research groups have achieved so far in breast tissue diffraction. Ray-tracing model of EDXRD system forms Chapter 2 wherein the system parameters along with the corrections used in the model and model predictions have been presented. The characterisation of materials using optimized EDXRD system has been detailed in Chapter 3. Chapter 4 elaborates the generation of diffraction images. Chapter 5 presents the conclusions and suggests future work. The thesis ends with a list of references.
22
Petrovic, Novak S. "Modelling diffraction in optical interconnects /". [St. Lucia, Qld.], 2004. http://adt.library.uq.edu.au/public/adt-QU20050129.135451/index.html.
Testo completo
23
Liu, Yuan. "Procédé Tactile à Diffraction Ultrasonore". Phd thesis, Ecole nationale supérieure d'arts et métiers - ENSAM, 2010. http://pastel.archives-ouvertes.fr/pastel-00552186.
Testo completoAbstract (sommario):
Cette thèse est une contribution au développement d'un procédé tactile acoustique basé sur la perturbation des ondes de Lamb, permettant de détecter un toucher simple ou des touchers multiples sur des objets minces de forme quelconque. Après avoir étudié la propagation des modes de Lamb dans les objets minces par exemple les plaques de cuivre d'épaisseur 450 µm, nous avons observé, que l'énergie des ondes de Lamb antisymétriques était essentiellement due à la composante normale de déplacement et qu'il était aisé de perturber un mode de flexion par un toucher surfacique réalisé avec le doigt ou ponctuel avec la pointe d'un stylo, tandis que dans le cas d'un mode symétrique, l'énergie était essentiellement distribuée sur la composante de déplacement tangentielle et en conséquence plus difficilement perturbée par une interaction tactile. Nous avons en conséquence exploité le toucher sur une plaque mince perturbant le rayonnement d'une onde acoustique dans l'objet et réalisé en pratique une mesure de la perturbation d'un signal de diffraction des ondes de Lamb en un ou deux points récepteurs d'une plaque. Ainsi, une méthode de localisation basée sur la diffraction des ondes a été proposée, pour obtenir une interface tactile compacte, performante et faible coût. Cette méthode nommée méthode de la Figure de Diffraction Perturbée en Amplitude (FDPA) peut rendre une surface tactile avec une résolution spatiale de l'ordre du millimètre et un temps de réponse inférieur à 10 ms. Deux méthodes d'optimisation sont proposées pour obtenir un taux de localisation correcte proche de 100%. Le procédé tactile et les méthodes d'optimisation présentés dans cette thèse ont été testés sur plusieurs types d'objets, réalisés dans des matériaux différents et sous différentes formes géométriques. Ces réalisations ont montré des performances satisfaisantes en termes d'encombrement, de consommation et de fiabilité de localisation pour des touchers simples et multiples.
24
Ayzenberg, Milana. "Three-Dimensional Seismic Diffraction Modeling". Doctoral thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for petroleumsteknologi og anvendt geofysikk, 2008. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-5131.
Testo completoAbstract (sommario):
Synthetic modeling of seismic wavefields scattered in the subsurface is of growing interest today due to its applicability in various forward and inverse problems of geophysics. It has been extensively used for general evaluation of the subsurface structure, in survey design and illumination studies, and also as the basis for imaging and inversion algorithms. A particular challenge is presented by models with complex geological structure containing strong-contrast or irregular reflectors and shadow zones, where conventional algorithms fail to simulate realistic wavefields. Therefore, the demand for advanced modeling techniques has dramatically increased. This thesis is based on a new analytic approach to the description and modeling of three-dimensional reflected wavefields. The method combines surface singular integrals describing the wave propagation inside smoothly heterogeneous layers and effective reflection and transmission coefficients (ERC and ETC) at reflectors. The propagators are implemented using a seismic-frequency approximation of the Helmholtz-Kirchhoff integral. The approximation is based on the assumption that a small (compared to the predominant wavelength) part of the reflector acts as a secondary source, which, in accordance with Huygens’ principle, emits a wave beam. The beam comprises not only the main reflection or transmission, but also the edgediffracted and tip-diffracted waves. Because the tip-diffracted waves contribute most to the beam, the method is called the “tip-wave superposition method” (TWSM). ERC and ETC generalize classical plane-wave reflection and transmission coefficients (PWRC and PWTC) for point sources and curved reflectors. Their definition accounts for the local interface curvature, sphericity of the wavefront, and finite frequency content of the incident wavelet. Therefore, they produce correct reflected and transmitted amplitudes at near-critical and post-critical incidence angles. Numerical experiments indicate that ERC and ETC also accurately reproduce amplitudes of the head waves. For a plane interface, ERC or ETC represents the exact reflection or transmission response at the reflector. For a curved interface, they are approximate and based on an “apparent“ source location, which depends on the incidence angle and the mean reflector curvature. Because ERC and ETC account for all amplitude-related effects, they are a useful tool for evaluation of reflected and transmitted wavefields at the reflector. The new approach comprising the TWSM with ERC and ETC gives the possibility of reproducing complex wave phenomena such as caustics, diffractions, and head waves. The modeled full wavefield is represented as a set of separate events, each corresponding to a particular wavecode. This makes the approach event-oriented: an event of interest can be modeled separately. Numerical experiments demonstrate that the method simulates scattered 3D synthetic wavefields in layered media with accurate traveltimes and amplitudes. Chapter 1 presents the basic principles of wave propagation in 3D media and an overview of existing modeling techniques. Chapter 2 contains a paper about 3D diffraction modeling of singly scattered acoustic wavefields. The paper introduces ERC and ETC for acoustic waves and gives a detailed overview of the acoustic version of TWSM. Chapter 3 contains a paper about 3D diffraction modeling of acoustic scattering in layered media. The paper deals with the extension of the acoustic version of TWSM for layered media. It provides a thorough analysis of the modeling results for 3D models with smoothly varying reflectors and for models containing diffracting edges. Chapter 4 contains a paper about ERC for curved interfaces in TI media. The paper analyzes the dependence of ERC on anisotropy parameters and reflector shape and demonstrates their advantages over PWRC in 3D diffraction modeling of PP and PS reflection data.
25
Diehl, Markus. "Diffraction in electron-proton collisions". Thesis, University of Cambridge, 1996. https://www.repository.cam.ac.uk/handle/1810/265482.
Testo completoAbstract (sommario):
Subject of this thesis is diffractive collisions of electrons and protons, i.e. collisions wher~ the proton remains intact or is slightly excited, and loses only a small fraction of its initial energy. Such processes can be described by the exchange of a pomeron, which is well-known from the phenomenology of highenergy hadron scattering but far from being understood in the framework of quantum chromodynamics. We use the simple model of Landshoff and Nachtmann, where the pomeron is approximated by two nonperturbative gluons, to calculate the cross section for diffractive production of a quark-antiquark pair from a real or virtual photon. This allows us to give detailed predictions for diffractive electro- or photoproduction of a jet pair, for diffractive charm production, and for the inclusive cross section in diffractive deep inelastic scattering at not too large masses of the diffractive system. For the latter we find rather good agreement with the data taken at the HERA collider, taking into account the uncertainties of our calculation. We then turn our attention to azimuthal correlations in diffractive scattering. The dependence of the electron-proton cross section on the angle between the lepton plane and some direction in the hadronic final state has a simple relation with cross sections and interference terms of the photon-proton subprocess for definite photon helicity. Its measurement can be used to constrain the cross section for longitudinally polarised photons. We compute the distribution of the azimuthal jet angle in diffractive dijet production in the Landshoff-Nachtmann model and find that useful bounds on the longitudinal cross section for such events might be obtained from its measurement. Going further we extend our calculation of quark-antiquark production to finite squared momentum transfer from the proton. This enables us to investigate the distribution of another important azimuthal angle, that of the scattered proton, which contains information about the helicity of the pomeron. The last part of this thesis is devoted to the phenomenological pomeron model of Donnachie and Landshoff. When applied to diffractive quark-antiquark production it violates electromagnetic gauge invariance. We suggest a prescription to deal with this problem and compare the results we then obtain with those of the Landshoff-Nachtmann model. We find remarkable similarities between the two models, but they also show important differences, especially in details of kinematical distributions.
26
Hnatiuk, Heather Joanne. "Integrated linear diffraction grating spectrometer". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape9/PQDD_0021/NQ45000.pdf.
Testo completo
27
Jafari, Amir. "Distributed etched diffraction grating demultiplexer". Thesis, McGill University, 2012. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=107746.
Testo completoAbstract (sommario):
This doctoral thesis studies the concept of a distributed etched diffraction grating (DEDG) and presents a methodology to engineer the spectral response of the device. The design which incorporates a distributed Bragg reflector (DBR) at the facets of a conventional etched diffraction grating demultiplexer promises for a superior performance in multiple aspects. Where in a conventional etched diffraction grating, smooth vertical deep etched walls are required in order to realize a low insertion loss device; in the DEDG such requirement is significantly mitigated. Deep etched walls are replaced with shallowly etched diffraction grating facets followed by a DBR structure and as a result devices with significantly lower insertion loss are achievable. The feasibility of the application of DEDG as a wavelength demultiplexer was demonstrated through fabrication and characterization of a prototype device. The proof of concept device was fabricated using the state of the art deep UV optical lithography and reactive ion etching in a nano-photonic silicon-on-insulator (SOI) material platform. The fabricated device was then characterized in the lab. Furthermore, incorporation of the DBR structure at the facets of the conventional etched diffraction grating decouples the reflection and diffraction functionalities, rendering the DEDG suitable for spectral response engineering. According to the application, the output spectral response of the device can be tailored through careful design and optimization of the incorporated DBR. In this thesis, through numerical simulations we have shown that functionalities such as polarization independent performance and at top insertion loss envelop are viable. A methodology to engineer the spectral response of the DEDG is discussed in details.Cette thèse de doctorat examine le concept d'un réseau de diffraction gravé et distribué (DEDG) et présente une méthodologie pour concevoir la réponse spectrale de l'appareil. Cette conception qui incorpore un réflecteur de Bragg distribué (DBR)aux facettes d'un démultiplexeur conventionnel basé sur un réseau de diffraction gravé (RDG), promet une performance supérieure sur plusieurs aspects. Alors que dans un RDG conventionel, il faut des murs gravés qui soient verticals et profonds afin d'obtenir une faible perte d'insertion; dans le DEDG, une telle exigence est considérablement atténuée. Les murs profondément gravés sont remplacés par des facettes peu profonde, suivis par un DBR; par conséquent des appareils avec une perte d'insertion nettement inférieure sont réalisables. La faisabilité de l'application de DEDG comme un démultiplexeur de longueur d'onde a été démontré grace à la fabrication et la caractérisation d'un prototype. Un exemple de l'appareil a été fabriquéen utilisant la lithographie optique et la gravure ionique réactive sur une plateformede silicium sur isolant(SOI). L'appareil a été ensuite caractérisé dans le laboratoire. Par ailleurs, l'incorporation de la structure de DBR aux facettes du RDG conventionel découple la réflexion et la diffraction, rendant le DEDG approprié pour l'adaptationde la réponse spectrale. Selon l'application, la réponse spectrale de l'appareil peut etre adapté grace à une conception et une optimisation méticuleuses de la DBR incorporé. Dans cette thése nous avons démontré par des simulations numériques que des fonctionnalités telles que la performance indépendante de la polarisation et la performance uniforme en ce qui concerne la perte d'insertion sont réalisable. Une méthodologie pour adapter la réponse spectrale de la DEDG est discutée en détails.
28
Saward, Victoria Helen. "Some problems in diffraction theory". Thesis, University of Oxford, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.364212.
Testo completo
29
Jackson, Daryl Christopher. "Photoelectron diffraction of surface structures". Thesis, University of Warwick, 2010. http://wrap.warwick.ac.uk/3740/.
Testo completo
30
Huxley, David W. "Diffraction studies of disordered materials". Thesis, University of Kent, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.293796.
Testo completo
31
Geake, Peter John. "Smoke characterisation by laser diffraction". Thesis, London South Bank University, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.329148.
Testo completo
32
Jing, Cao. "Spray diagnostics by laser diffraction". Thesis, University of Sheffield, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.321852.
Testo completo
33
Webb, S. "Unusual effects in particle diffraction". Thesis, University of Manchester, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.234226.
Testo completo
34
Forsyth, G. A. "Molecular structure by diffraction methods". Thesis, University of Reading, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.384586.
Testo completo
35
Wang, Duan Qiang. "Strain measurement using neutron diffraction". Thesis, Open University, 1996. http://oro.open.ac.uk/57651/.
Testo completoAbstract (sommario):
This thesis contains a study of improvement and application of strain measurement technique using neutron diffraction. A dedicated neutron strain scanner - ENGIN, funded by Ee. has been developed at ISIS. It has two radial collimators, allowing for the first time, measurements taken simultaneously at two orthogonal directions; uses Pawley refinement permitting both whole pattern and individual peak profile analysis and its positioner allow's specimens weighing 250 kg to be placed with an accuracy of 100 um. Strain measurement using neutron diffraction has been investigated under two extreme circumstances: very shallow (within 1 mm near surface) and very deep in materials (hundred of mm). Near-surface measurement requires both the accurate determination of the effective measurement position associated with precise location of specimen, calculation of centroid and correction for an anomalous near-surface effect. The strain measurements on a shot-peened surface in titanium alloy were carried out using the methodology mentioned above. The result is comparable to that obtained from X-ray diffraction. The latter was exploited by studying the effect of wavelength-dependent attenuation in materials. Experiments and theoretical analysis on aluminium and iron show that the effect is small for a strain scanner using radial collimator and time-of-flight technique. However, the reduction of the diffraction peak intensity, as a function of the amount of material in the beam path, reveals that great care should be taken when measuring texture as a function of depth in materials. The neutron diffraction technique was applied to several sets of engineering strain measurements. Firstly, 3-D residual stresses surrounding a cold expanded hole in a high strength aluminium alloy plate were measured. The result agrees well with that from modified Sachs' method after taking the effect of gauge volume averaging into account. Second, measurements of strain distributions in a Q-joint under zero and 30 kN load have been carried out, which provides a confirmation of the excellence of the joint design as the fastener system has completely shielded the fastener hole from deleterious tensile strains under operational loading. Finally, calibration strain measurement for an energy dispersive neutron transmission spectrometer was performed for the first time and it is found that the transmission spectrometer is feasible for strain and phase transformation measurement.
36
Lobban, Colin. "Neutron diffraction studies of ices". Thesis, University College London (University of London), 1998. http://discovery.ucl.ac.uk/10065569/.
Testo completoAbstract (sommario):
This thesis reports on a series of neutron diffraction measurements on ices in the medium-pressure range of the phase diagram in order to address several outstanding issues. The structure of ice II under its thermodynamic conditions of stability is unknown. Previous studies have used helium gas as the pressurising medium, which is included within the ice structure. Argon gas has been used in this work as the pressure medium, and the detailed structure of ice II is presented and discussed. Similarly, the structure of ice V under its conditions of stability is established for the first time by this work. For ices III and V, the presence of orientational order is controversial with the results obtained from structural studies on recovered samples differing from conclusions drawn from dielectric and spectroscopic measurements. The results obtained in this work indicate partial ordering of the water molecules which, for ice V, appears to be both temperature and pressure dependant. Ice IV is a metastable phase of ice that is difficult to form. A structure has been proposed on the basis of samples recovered to ambient pressure at 110 K . This work has successfully prepared ice IV at high temperature and pressure and a structure refinement confirms, for the first time, that the proposed structure is correct. During studies within the established stability region of ice V, features were observed which could not be identified as any ice or clathrate phase. Further work in this region has successfully formed and retained one such phase. The structure has been solved and the phase identified as a new ice structure. The topology of this phase is unlike that of any known solid water structure, and contains a mixture of seven- and eight-membered rings. General issues concerning hydrogen-bonding, compressibility and expansivity of these phases are discussed.
37
Watson, Andre James. "Diffraction gratings in ray tracing". Diss., [La Jolla] : University of California, San Diego, 2009. http://wwwlib.umi.com/cr/ucsd/fullcit?p1464665.
Testo completoAbstract (sommario):
Thesis (M.S.)--University of California, San Diego, 2009.Title from first page of PDF file (viewed July 7, 2009). Available via ProQuest Digital Dissertations. Includes bibliographical references (p. 66-67).
38
Mercer, Carolyn Regan. "Liquid crystal point diffraction interferometer". Diss., The University of Arizona, 1995. http://hdl.handle.net/10150/187127.
Testo completoAbstract (sommario):
A new instrument, the liquid crystal point diffraction interferometer (LCPDI), has been developed for the measurement of phase objects. This instrument maintains the compact, robust design of Linnik's point diffiaction interferometer (PDI) and adds to it phase stepping capability for quantitative interferogram analysis. The result is a compact, simple to align, environmentally insensitive interferometer capable of accurately measuring optical wavefronts with very high data density and with automated data reduction. This dissertation describes the theory of both the PDI and liquid crystal phase control. The design considerations for the LCPDI are presented, including manufacturing considerations. The operation and performance of the LCPDI are discussed, including sections regarding alignment, calibration, and amplitude modulation effects. The LCPDI is then demonstrated using two phase objects: a defocus difference wavefront, and a temperature distribution across a heated chamber filled with silicone oil. The measured results are compared to theoretical or independently measured results and show excellent agreement. A computer simulation of the LCPDI was performed to verify the source of observed periodic phase measurement error. The error stems from intensity variations caused by dye molecules rotating within the liquid crystal layer. Methods are discussed for reducing this error. Algorithms are presented which reduce this error; they are also useful for any phase-stepping interferometer that has unwanted intensity fluctuations, such as those caused by unregulated lasers. It is expected that this instrument will have application in the fluid sciences as a diagnostic tool, particularly in space based applications where autonomy, robustness, and compactness are desirable qualities. It should also be useful for the testing of optical elements, provided a master is available for comparison.
39
Verma, Rupesh. "Fractal diffraction: signatures and applications". Thesis, IIT Delhi, 2015. http://localhost:8080/iit/handle/2074/6663.
Testo completo
40
Simonne, David. "Catalytic properties at the nanoscale probed by surface x-ray diffraction and coherent diffraction imaging". Electronic Thesis or Diss., université Paris-Saclay, 2024. http://www.theses.fr/2024UPASP001.
Testo completoAbstract (sommario):
Le principal objectif de ce travail est d'étudier des catalyseurs hétérogènes in situ et operando pendant l'oxydation de l'ammoniac en se rapprochant des valeurs de température et pression industrielles. Actuellement, ce processus catalytique et les changements structurels associés sont mal compris, et nous proposons d'utiliser différents échantillons en platine, nanoparticules et monocristaux afin de réduire l'écart entre les études scientifiques sur échantillons modèles et les catalyseurs utilisés en agro-industrie. L'activité catalytique des différents échantillons est mesurée pour lier structure et sélectivité durant la réaction, qui peut être focalisée vers la production d'azote (N₂) ou d'oxyde nitrique (NO). La production de protoxyde d'azote (N₂O) doit être évitée de par son importante contribution à l'effet de serre. Le développement d'une catalyse hétérogène avec une sélectivité ciblant les 100 % est un défi constant, ainsi que la compréhension de la durabilité, du vieillissement et de la désactivation du catalyseur lui-même. Mesurer la structure de nanoparticules à l'échelle nanométrique permet de révéler les effets de volume, de tension et de compression de surface et d'interface, ainsi que l'existence de différents types de défauts. En complément des études d'imagerie par diffraction cohérente de rayons X en condition de Bragg sur des nanoparticules individuelles, l'étude d'un ensemble de nanoparticules sera effectuée via la diffraction des rayons X à incidence rasante. La diffraction cohérente de rayons X en condition de Bragg étant une nouvelle technique, une organisation typique de la réduction et analyse des données est proposée. De plus, chaque type de surface présente sur les nanoparticules (e.g. (111), (100)) est ensuite étudiée à l'aide de monocristaux par diffraction des rayons X en surface et spectroscopie photoélectronique par rayons X. De ce fait, la structure de surface ainsi que la présence d'espèces adsorbées peut être reliée à l'activité catalytique mesurée, permettant une meilleure compréhension du mécanisme de réaction. Finalement, l'évolution de la structure d'ensemble et de monocristaux est comparée à celle des nanoparticules uniques pour confirmer/infirmer le lien entre structure moyenne et structure individuelleThe main objective of this work is to study heterogeneous catalysts in situ and operando during the oxidation of ammonia by approaching industrial temperature and pressure values. Currently, this catalytic process and the associated structural changes are poorly understood. We propose to use different samples in platinum, nanoparticles and single crystals in order to reduce the gap between scientific studies on model samples and catalysts used in the fertiliser industry. The catalytic activity of the different samples is measured to link structure and selectivity during the reaction, which can be focused towards the production of nitrogen (N₂) or nitric oxide (NO).The production of nitrous oxide (N₂O) must be avoided due to its significant contribution to the greenhouse effect. Developing heterogeneous catalysis with selectivity targeting 100 % is an ongoing challenge, as is understanding the durability, ageing, and deactivation of the catalyst itself. Measuring the structure of nanoparticles at the nanoscale makes it possible to reveal the effects of volume, surface and interface tension and compression, as well as the existence of different types of defects. In addition to imaging studies by Bragg coherent X-ray diffraction imaging on individual nanoparticles, the study of a set of nanoparticles will be carried out via grazing incidence X-ray diffraction. Bragg coherent X-ray diffraction imaging being a new technique, a typical workflow for data reduction and analysis is proposed. In addition, each type of surface present on the nanoparticles (e.g. (111), (100)) is then studied using single crystals by surface X-ray diffraction and X-ray photoelectron spectroscopy. Therefore, the surface structure as well as the presence of adsorbed species can be linked to the measured catalytic activity, allowing a better understanding of the reaction mechanism. Finally, the evolution of the nanoparticle ensemble structure and of single crystals is compared to that of single nanoparticles to confirm/refute the link between average structure and individual structure
41
Schwiegerling, Jim. "Diffraction efficiency and aberrations of diffractive elements obtained from orthogonal expansion of the point spread function". SPIE-INT SOC OPTICAL ENGINEERING, 2016. http://hdl.handle.net/10150/622533.
Testo completoAbstract (sommario):
The Point Spread Function (PSF) indirectly encodes the wavefront aberrations of an optical system and therefore is a metric of the system performance. Analysis of the PSF properties is useful in the case of diffractive optics where the wavefront emerging from the exit pupil is not necessarily continuous and consequently not well represented by traditional wavefront error descriptors such as Zernike polynomials. The discontinuities in the wavefront from diffractive optics occur in cases where step heights in the element are not multiples of the illumination wavelength. Examples include binary or N-step structures, multifocal elements where two or more foci are intentionally created or cases where other wavelengths besides the design wavelength are used. Here, a technique for expanding the electric field amplitude of the PSF into a series of orthogonal functions is explored. The expansion coefficients provide insight into the diffraction efficiency and aberration content of diffractive optical elements. Furthermore, this technique is more broadly applicable to elements with a finite number of diffractive zones, as well as decentered patterns.
42
Blums, Valdis Roberts. "Ion Fluorescence Collection And Diffraction Limited Imaging From A Microfabricated Ion Trap With Integrated Diffractive Mirrors". Thesis, Griffith University, 2016. http://hdl.handle.net/10072/365461.
Testo completoAbstract (sommario):
In this thesis I discuss the collaborative monolithic design and testing of microfabricated diffractive mirrors integrated into a microfabricated ion trap. I discuss the necessary optical and electrical systems needed to trap 174Yb+ ions. From the diffractive mirrors we measured a collection efficiency of more than 4% per ion fluorescence photon, while obtaining diffraction limited ion images.Thesis (Masters)
Master of Philosophy (MPhil)
School of Natural Sciences
Science, Environment, Engineering and Technology
Full Text
43
Recaldini, Valentino. "Optical diffraction tomography: a resolution study". Master's thesis, Alma Mater Studiorum - Università di Bologna, 2020. http://amslaurea.unibo.it/21258/.
Testo completoAbstract (sommario):
In the past years, optical diffraction tomography (ODT) has been used both in cell imaging and to investigate the three-dimensional refractive index (RI) of large-scale (millimetre-sized) samples. In this technique, the projections at different illumination angles are acquired through digital holography (DH) and used to estimate the complex wave fields, which can be refocused with the aid of numerical diffraction algorithms. However, real extended specimens may not completely lie on a single plane. In this case, the (refocused) projections retain a certain amount of defocus which will affect the tomographic reconstruction.
For this reason, this thesis aims to study the spatial resolution of an ODT system when a point-like object is allowed to go in and out of focus and is reconstructed without numerical refocusing. Two-dimensional rotation and computational Fourier optics will be used to track and model defocus and lenses during the simulation of the projections. Spatial resolution will be assessed both qualitatively and quantitatively by numerically computing the full width at half maximum (FWHM) in relation to the maximum defocus to which a simulated point was subjected during acquisition.
Lastly, deconvolution is used to remove unwanted blur.
44
Burvall, Anna. "Axicon imaging by scalar diffraction theory". Doctoral thesis, KTH, Microelectronics and Information Technology, IMIT, 2004. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3736.
Testo completoAbstract (sommario):
Axicons are optical elements that produce Bessel beams,i.e., long and narrow focal lines along the optical axis. Thenarrow focus makes them useful ine.g. alignment, harmonicgeneration, and atom trapping, and they are also used toincrease the longitudinal range of applications such astriangulation, light sectioning, and optical coherencetomography. In this thesis, axicons are designed andcharacterized for different kinds of illumination, using thestationary-phase and the communication-modes methods.
The inverse problem of axicon design for partially coherentlight is addressed. A design relation, applicable toSchell-model sources, is derived from the Fresnel diffractionintegral, simplified by the method of stationary phase. Thisapproach both clarifies the old design method for coherentlight, which was derived using energy conservation in raybundles, and extends it to the domain of partial coherence. Thedesign rule applies to light from such multimode emitters aslight-emitting diodes, excimer lasers and some laser diodes,which can be represented as Gaussian Schell-model sources.
Characterization of axicons in coherent, obliqueillumination is performed using the method of stationary phase.It is shown that in inclined illumination the focal shapechanges from the narrow Bessel distribution to a broadasteroid-shaped focus. It is proven that an axicon ofelliptical shape will compensate for this deformation. Theseresults, which are all confirmed both numerically andexperimentally, open possibilities for using axicons inscanning optical systems to increase resolution and depthrange.
Axicons are normally manufactured as refractive cones or ascircular diffractive gratings. They can also be constructedfrom ordinary spherical surfaces, using the sphericalaberration to create the long focal line. In this dissertation,a simple lens axicon consisting of a cemented doublet isdesigned, manufactured, and tested. The advantage of the lensaxicon is that it is easily manufactured.
The longitudinal resolution of the axicon varies. The methodof communication modes, earlier used for analysis ofinformation content for e.g. line or square apertures, isapplied to the axicon geometry and yields an expression for thelongitudinal resolution. The method, which is based on abi-orthogonal expansion of the Green function in the Fresneldiffraction integral, also gives the number of degrees offreedom, or the number of information channels available, forthe axicon geometry.
Keywords:axicons, diffractive optics, coherence,asymptotic methods, communication modes, information content,inverse problems
45
Herbison, Sarah. "Ultrasonic diffraction effects on periodic surfaces". Diss., Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/41180.
Testo completoAbstract (sommario):
Although the study of the interaction of acoustic and elastic waves with periodic surfaces and structures has a rich history dating back to Lord Rayleigh, it has recently been attracting new research efforts due to its value in the study of phononic crystals and in methods for ultrasonic non-destructive evaluation (NDE). The objective of the research described in this thesis is to provide new numerical and experimental tools capable of capturing important features that occur due to the diffraction of ultrasound on periodic solid surfaces. This thesis is divided into four main parts. First, the Rayleigh-Fourier (R-F) method will be used to simulate diffracted fields generated by structures containing multiple periodic surfaces and/or multiple solid layers. The second part of this thesis examines diffraction effects and compares ultrasonic NDE techniques for surfaces with imperfect periodicities. The third portion of this thesis focuses on one unusual phenomenon that has been observed on periodic surfaces, namely the lateral backward displacement of a bounded ultrasonic beam along the surface. This effect is currently understood to occur due to backward propagating surface waves that result from diffraction and mode conversion on the surface. The fourth and final part of this thesis describes the diffraction of bulk ultrasonic waves that can occur on the surfaces of phononic crystals.
46
Chen, Yixin. "Electron diffraction analysis of amorphous Ge2Sb2Te5". Thesis, University of Oxford, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.669987.
Testo completo
47
Kreikemeyer, Lorenzo D. "Photoelectron diffraction determination of adsorbate structures". Thesis, University of Warwick, 2011. http://wrap.warwick.ac.uk/49672/.
Testo completoAbstract (sommario):
Scanned-energy mode photoelectron diffraction (PhD) is a well-known method to determine quantitatively the local structure of adsorbates at surfaces. In this thesis, it has been employed to determine the adsorption site of a selection of molecules on surfaces. The adsorption on Cu(110), of methoxy (CH3O), an intermediate in the catalytic decomposition of methanol (CH3OH), has been studied. O 1s PhD spectra show the strongest modulation at 30° and 40° polar emission angles, both in the [1 1 0] azimuth, which is consistent with a bridge position adsorption site in the [1 1 0] azimuth. The subsequent analysis, as well as parallel DFT studies, confirms two bridge adsorption sites, with different bond lengths to the underneath copper atoms. A tilt of the molecules of 37° in the [1 1 0] azimuth is also observed, with the carbon atoms pointing in opposite directions for every adsorption site. This tilt creates a zig-zag model, which fits with an old STM [1] study. Formate (HCOO), a surface intermediate of the catalytic decomposition of formic acid (HCOOH), has been studied on two different faces of copper, Cu(110) and Cu(111). Although the adsorption sites obtained for both surfaces is similar, namely a short-bridge site slightly off atop, a significant difference of = 0.1 Å in the copper-oxygen bond lengths is found, being 1.99 Å for Cu(111) and 1.90 Å for Cu(110). In this thesis, it is demonstrated that it is possible, though very challenging, to perform PhD successfully under higher pressures. Methanol oxidation on Cu(110) has been studied under reaction conditions. At temperatures below = 450 K, the adsorption sites of methoxy and formate, the most important surface intermediates of this reaction, have been proved to be similar as in the previous studies performed in ultra high vacuum. A recent investigation of two different organic molecules, azobenzene (C12H10N2) and aniline (C6H7N), on rutile TiO2(110) and anatase TiO2(101) surfaces with scanning tunneling microscopy (STM) [2] indicates that both molecules lead the formation of the same superstructure, believed to be of a common species, phenyl imide (C6H5N). PhD has been exploited to determine the local adsorption site of adsorbed species formed by both molecules on rutile TiO2(110). N 1s photoelectron diffraction data are almost identical for both molecules, providing further support for a common surface species with the same, or a closely similar. Additional NEXAFS results support these results, implying that the local adsorption site of azobenzene and aniline is indeed the same. PhD results, which show the largest modulation amplitude at normal emission, suggests that the phenyl imide bonds via the N atoms atop a five-fold coordinated surface Ti atom, with the molecular plane tilted with respect to the surface normal, with a N-Ti bond length of 1.77 Å. [1] F. Leibsle, S. Francis, S.Haq, and M. Bowker, Aspects of formaldehyde synthesis on Cu(110) as studied by STM, Surf. Sci. 318, 46 (1994). [2] S.-C. Li, and U. Diebold, Reactivity of TiO2 rutile and anatase surfaces toward nitroaromatics, J. Am. Chem. Soc. 134, 64 (2010).
48
Duncan-Jones, George. "Noise removal for powder diffraction profiles". Thesis, University of Oxford, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.543553.
Testo completo
49
Hyslop, Michael. "Electron diffraction studies of unsupported clusters". Thesis, University of Canterbury. Physics and Astronomy, 2002. http://hdl.handle.net/10092/5563.
Testo completoAbstract (sommario):
The motivation to study the structure of clusters is the possible observation of unusual structures for small cluster sizes. The presence of such structural size effects is generally associated with the optimization of surface energy in clusters. For metals that have face centred cubic (FCC) structure in the bulk, optimization of the surface energy typically results in icosahedral or decahedral structures being preferred for nanometre sized clusters.
The inert gas aggregation (IGA) technique has been used to produce a beam of clusters (diameter ≤10 nm) for structural studies using electron diffraction. Studying the clusters while in a molecular beam, as opposed to on a substrate, means that the clusters are unsupported and thus free of any perturbing effects due to a substrate. The use of a beam also means each cluster is subjected to only a brief exposure to the electron beam, minimizing effects due to the electron beam. Attempts to obtain diffraction patterns from Zn clusters were unsuccessful, however using Pb it was possible to obtain diffraction patterns from clusters using a wide range of parameters in the IGA source.
The experimental diffraction patterns result from the range of different sized and structured clusters produced by the source. The analysis reflects the distribution of cluster sizes and structures by combining diffraction patterns from model clusters with a range of sizes and structures to produce a best fit to the experimental pattern. In general, two sets of model clusters are used: the first set contains models with up to ~6500 atoms, created using bulk and symmetry properties for clusters with FCC, decahedral and icosahedral structure. The second set contains the same sizes and structures as the first, however each model has been relaxed using molecular dynamics (MD) simulations. In the analysis of several experimental diffraction patterns, models with twinned FCC, liquid, anti-Mackay icosahedral and shaved icosahedral structures are also considered. Domain size estimates are obtained using the fit results; cluster size estimates are made from samples collected from the beam and observed in a TEM. Size estimates are also made using the Scherrer formula and the Fourier inversion method .
Analysis of diffraction patterns from Pb clusters shows that changing the type of inert gas produces the greatest variation in the size and structure of the clusters. The small clusters produced using He are found to be based on icosahedral structures. The clusters produced using Ar are larger than those produced using He and the diffraction patterns are difficult to interpret. The patterns bear a strong resemblance to those from decahedra, but diffraction patterns from decahedra are similar to those from twinned FCC structures, and from a combination of shaved icosahedra and FCC structures. FCC structure is not observed, which is both interesting and surprising.
50
Edvardsson, Elisabet. "Quasicrystals : Classification, diffraction and surface studies". Thesis, Karlstads universitet, Institutionen för ingenjörsvetenskap och fysik, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kau:diva-36299.
Testo completoAbstract (sommario):
Quasicrystal is the term used for a solid that possesses an essentially discrete diffraction pattern without having translational symmetry. Compared to periodic crystals, this difference in structure gives quasicrystals new properties that make them interesting to study -- both from a mathematical and from a physical point of view. In this thesis we review a mathematical description of quasicrystals that aims at generalizing the well-established theory of periodic crystals. We see how this theory can be connected to the cohomology of groups and how we can use this connection to classify quasicrystals. We also review an experimental method, NIXSW (Normal Incidence X-ray Standing Waves), that is ordinarily used for surface structure determination of periodic crystals, and show how it can be used in the study of quasicrystal surfaces. Finally, we define the reduced lattice and show a way to plot lattices in MATLAB. We see that there is a connection between the diffraction pattern and the reduced lattice and we suggest a way to describe this connection.