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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
41

Holographic Sculpting of Electron Beams with Diffraction Gratings

Pierce, Jordan 11 January 2019 (has links)
Electron microscopes offer scientists an invaluable tool in probing matter at a very small scale. Rapid advancements over the past several decades has allowed electron microscopes to routinely image samples at the atomic scale. These advancements have been in all aspects of electron microscope design – such as more stable control voltages and currents, brighter and more coherent sources, beam aberration correction, and direct electron detectors, to name a few. One very recent advancement is in shaping the electron beam to provide an almost arbitrary set of possible beam profiles. Following the demonstration of electron vortex beams in 2010, there has been a surge of interest in the potential shaping electron beams. Utilizing holographic electron diffraction gratings, an almost arbitrary set of electron beams can be generated. These diffraction gratings are challenging to create due their tiny size and the precision with which they must be fabricated. We present a comprehensive study on the fabrication and design of electron diffraction gratings with the aim of being able to produce optimal gratings that result in bright, well separated beams which closely match a desired beam profile. We have developed and optimized fabrication of these gratings with focused ion beam milling, and have been able to use the fabricated gratings in a number of important experiments. These electron diffraction gratings have allowed us to perform various experiments such as aberration correction, electron helical dichroism, advanced phase-contrast imaging, and multi-beam interferometric techniques. Holographic beam shaping will continue to be an important tool for electron microscopists.
42

Application of the Maximum Entropy Method to X-Ray Profile Analysis

January 1999 (has links)
The analysis of broadened x-ray diffraction profiles provides a useful insight into the structural properties of materials, including crystallite size and inhomogenous strain. In this work a general method for analysing broadened x-ray diffraction profiles is developed. The proposed method consists of a two-fold maximum entropy (MaxEnt) approach. Conventional deconvolution/inversion methods presently in common use are analysed and shown not to preserve the positivity of the specimen profile; these methods usually result in ill-conditioning of the solution profile. It is shown that the MaxEnt method preserves the positivity of the specimen profile and the underlying size and strain distributions, while determining the maximally noncommital solution. Moreover, the MaxEnt method incorporates any available a priori information and quantifies the uncertainties of the specimen profile and the size and strain distributions. Numerical simulations are used to demonstrate that the MaxEnt method can be applied at two levels: firstly, to determine the specimen profile, and secondly to calculate the size or strain distribution, as well as their average values. The simulations include both sizeand strain-broadened specimen profiles. The experimental conditions under which the data is recorded are also simulated by introducing instrumental broadening, a background level and statistical noise to produce the observed profile. The integrity of the MaxEnt results is checked by comparing them with the traditional results and examining problems such as deconvolving in the presence of noisy data, using non-ideal instrument profiles, and the effects of truncation and background estimation in the observed profile. The MaxEnt analysis is also applied to alumina x-ray diffraction data. It is found that the problems of determining the specimen profile, column-length and strain distributions can be solved using the MaxEnt method, with superior results compared with traditional methods. Finally, the issues of defining the a priori information in each problem and correctly characterising the instrument profile are shown to be critically important in profile analysis.
43

Diffraction inverse par des petites inclusions.

Iakovleva, Ekaterina 19 November 2004 (has links) (PDF)
Des inclusions de petits encombrements sont sources de perturbations pour les champs électromagnétiques ambiants (ceux qui existeraient en leurs absences, par exemple). Il est facilement imaginable que la mesure de ces perturbations puisse fournir des informations permettant l'identification des inclusions, où par iden- tification l'on signifie au minimum leur localisation, mais où l'on pourrait aussi signifier quantifocation de leurs paramètres électriques, voire dans la meilleure des hypothèses, caractérisation de leurs encombrements et formes. Récemment, une théorie mathématique a été développée pour préciser de petites inclusions à partir de mesures de frontière, voir [7] et références citées. Cette thèse porte principalement sur l'identication d'inclusions homogènes (de nombre inconnu a priori) d'un milieu donné à partir de mesures d'amplitudes de diraction lors de l'éclairement approprié de ce milieu. Premièrement, nous four- nissons de nouvelles formules asymptotiques, tant robustes que précises, des champs électromagnétiques résultant du phénomène de diffraction. Ensuite, nous les ex- ploitons pour la construction d'algorithmes d'identification non itératifs pertinents. Le problème est traité en trois grandes parties, chacune étant dédiée à une géométrie spécique : 1) Le milieu d'enfouissement de la collection est homogène, l'espace libre. 2) Le milieu est constitué de deux demi-espaces séparés par une interface plane, la collection étudiée se situant dans le demi-espace inférieur et sources et capteurs se situant dans le demi-espace supérieur. 3) Le milieu est un guide d'ondes, et la collection est dans le coeur de ce guide d'ondes.
44

The fabrication of fiber Bragg gratings

Stump, Kurt 30 June 2000 (has links)
Fiber Bragg gratings have become an important element in the fields of telecommunications and fiber optical sensing. Their small size, light weight and high tunability have made them ideal for many unique applications. In this paper the fabrication of these devices is investigated. Following a review of current literature and a discussion of the mechanisms underlying the photosensitivity of optical fibers, a new technique for writing fiber Bragg gratings is presented. This technique uses a diode-pumped Nd:YAG laser operating at 266 nm with pulse energies up to 137 ��J per pulse to write high quality gratings in standard optical fiber. This technique allows for the writing of variable wavelength gratings using a standard diffractive optical element (phase mask). The new technique has much lower setup and laser stability requirements than the conventional methods of writing variable wavelength fiber Bragg gratings. Furthermore, it is found to be very tunable and extremely robust, allowing for extended writing times. A preliminary study of the time evolution of the writing process is also presented. It is evident that the process of grating inscription is very complicated and is not always adequately described by current models. In addition. it appears that the process is strongly dependent upon pulse energy and is accelerated by simultaneous heating of the fiber with a C0��� laser and by heating due to the writing laser. A brief study of the thermal stability of different gratings is then given to determine some of the thermal stability properties of the fiber Bragg gratings written at OSU. / Graduation date: 2001
45

The three-grating optical interferometer used as a monitoring and stabilization device for an atomic interferometer

Lopez, Peggy A. 19 February 1997 (has links)
The three-grating optical interferometer is studied to identify restrictions on alignment and improve stability. A description of the way a three-grating interferometer works is given as well as a method for proper set up. The overall power loss through the three gratings is measured and can be used to estimate the amount of atoms that will be detected at the output of the atomic interferometer. Criteria are developed for misalignment affects. Equations are presented for spacing and rotation limitations which can then be applied to the atomic interferometer. A stabilization technique is developed using a servo system. The elimination of low frequency inertial noise is accomplished. / Graduation date: 1997
46

Fundamental aspects of the enhanced transmission phenomenon and its application to photon sorting

Laux, Eric Ebbesen, Thomas W.. January 2009 (has links)
Thèse de doctorat : Physique : Strasbourg : 2009. / Titre provenant de l'écran-titre. Bibliogr. 13 p.
47

Underwater acoustic backscatter from a model of Arctic ice open leads and pressure ridges

Browne, Michael Joseph. January 1987 (has links) (PDF)
Thesis (M.S. in Engineering Acoustics)--Naval Postgraduate School, June 1987. / Thesis Advisor(s): Medwin, Herman. Second Reader: Bourke, Robert H. "June 1987." Description based on title screen as viewed on March 16, 2010. DTIC Identifier(s): Mode conversion. Author(s) subject terms: Underwater Acoustics, Arctic Ice, Backscatter, Mode Conversion, Diffraction. Includes bibliographical references (p. 156-157). Also available in print.
48

Development and analysis of image reconstruction algorithms in diffraction tomography /

Anastasio, Mark Anthony. January 2001 (has links)
Thesis (Ph. D.)--University of Chicago, Department of Radiology, June 2001. / Includes bibliographical references. Also available on the Internet.
49

IMAGE SAMPLING AND MULTIPLEXING WITH TWO-DIMENSIONAL PHASE GRATINGS

Scott, Paul Walter January 1978 (has links)
No description available.
50

The (3x3) reconstruction of SIC(0001): a low energy electron diffraction study

何永健, Ho, Wing-kin. January 1998 (has links)
published_or_final_version / Physics / Master / Master of Philosophy

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