Orientador: Flávio Caldas da Cruz / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-30T22:47:31Z (GMT). No. of bitstreams: 1 Depetri_WilliamIunes_M.pdf: 11907943 bytes, checksum: 4f7cedccb63e0fd32bea356921cb78f6 (MD5) Previous issue date: 2016 / Resumo: Pentes de frequência ópticos têm sido desenvolvidos para medias de precisão e metrologia, mas tabém vêm ganhando interesse para espectroscopia de precisão de banda larga. Pentes ópticos no infravermelho médio são muito atraentes para espectroscopia molecular, dado que eles permitem acesso a fortes bandas vibracionais de gases na chamada "região de digital molecular". Pentes ópticos são uma tecnologia bem desenvolvida no infravermelho próximo, dado a disponibilidade de lasers de Ti:Safira e Er-fiber, e apesar de desenvolvimentos recentes, sua extensão para o infravermelho médion depende de conversão não-linear de frequência. Aqui, pretendemos estudar a greração de pentes ópticos no infravermelho médio via geração de diferença de frequência e amplificação paramétrica óptica do espectro de um oscilador de femtosegundo no infravermelho próximo. Neste trabalho, analizamos numericamente difereça de frequência e amplificação paramétrica para a geração de pentes de frequência ópticos no infravermelho médio resolvendo no domínio do tempo as equações acopladas para o campo elétrico em mistura de três ondas para pulsos ultracurtos. Uma abordagem tradicional é a utilização de quasi-phase matching em cristais periodicamente polados, o que pode ser bastante eficiente, porém apresenta limitações na largura de banda. Nóis investigamos cristais periódicos com um período fixo, e cristais aperiódicos, ou chirped, tanto com chirp contínuo como discreto. Nós comparamos as configurações nas quais a amplificação paramétrica é otimizada ou não, explorando uma analogia entre conversãpo não-linear em sistemas de dois níveis. Nossos resultados são apresentados como gráficos temporais da propagação dos pulsos, seus respectivos espectros e potências ao longo dos cristais, e comparados com resultados experimentais, com o intuito de ajudar experimentos futuros / Abstract: Optical frequency combs (OFC) have been developed for precision measurements and metrology, but have also gained interest for broadband precision spectroscopy. OFC in the mid-infrared (MIR) are very attractive for molecular spectroscopy, since they allow to access the strong vibrational bands of gases in the so-called molecular fingerprint region. OFC are a well-developed technology in the near-infrared, due to the avilability of Ti:Saphire or Er-fiber lasers, and spite of recent developments, their extension into the MIR relies on nonlinear frequency conversion. Here we intend to study the generation of MIR OFC via difference frequency generation (DFG) and optical parametric amplification (OPA) of spectral portions of a near-infrared femtosecond oscillator. In this work, we numerically analyze DFG and OPA for the generation of optical frequency combs in the mid-infrared by solving the time-domain coupled wave equations for the electric fields in three-wave mixing for ultrashort pulses. A traditional approach is to use quasi-phase matching in periodically poled crystals, which can be quite efficient but may have bandwidth limitations.We investigate periodically poled crystals with a single-grating and aperiodic or chirped crystal with either continuous or discrete chirp. We compare the configurations in which a strong pump pulse has higher or smaller wavelength compared to the signal, cases in which the parametric amplification is enhanced or not. We also analyze these results in the context of adiabatic frequency conversion, which explores an analogy between nonlinear conversion and two-level systems. Our results are presented as time plots for the propagation of the pulses, their corresponding spectra and powers along the crystals and are compared to experimental results, and also intended to support further experiments / Mestrado / Física / Mestre em Física / 132987/2014-7 / CNPQ

Charge-carriers propagating in superlattices exhibit the related phenomena known as negative differential conductivity and Bloch oscillation. This behaviour may be utilised for the generation of tunable electromagnetic radiation. In this work, the dependence of the drift velocity and displacement of charge-carriers on external, applied electric fields is investigated. The theory is extended to incorporate a different miniband structure, with the aim of modelling a superlattice made from graphene. I predict that, for a chosen set of electric field parameters, a semiconductor superlattice will emit radiation in the terahertz range. I create an original mathematical framework within which to calculate the charge-carrier behaviour in a triangular miniband structure, while incorporating an arbitrary variable to account for the effects of corrugation or disorder, and predict the appearance of conductivity multistability. This may be of interest to further work done on the use of graphene for superlattice device construction.

The problem of two-dimensional acoustic scattering of an incident plane wave by a semi-infinite lattice is solved. The problem is first considered for sound-soft cylinders whose size is small compared to the wavelength of the incident field. In this case the formulation leads to a scalar Wiener--Hopf equation, and this in turn is solved via the discrete Wiener--Hopf technique. We then deal with a more complex case which arises either by imposing Neumann boundary condition on the cylinders' surface or by increasing their radii. This gives rise to a matrix Wiener--Hopf equation, and we present a method of solution that does not require the explicit factorisation of the kernel. In both situations, a complete description of the far field is given and a conservation of energy condition is obtained. For certain sets of parameters (`pass bands'), a portion of the incident energy propagates through the lattice in the form of a Bloch wave. For other parameters (`stop bands' or `band gaps'), no such transmission is possible, and all of the incident field energy is reflected away from the lattice.

Proefschrift--Radboud Universiteit Nijmegen, 2005. / Résumé en anglais. Bibliogr. p. 255-285.

The work described in this dissertation was motivated by a desire to better understand the cellular pathology of ischemic stroke. Two of the three bodies of research presented herein address and issue directly related to the investigation of ischemic stroke through the use of diffusion weighted magnetic resonance imaging (DWMRI) methods. The first topic concerns the development of a computationally efficient finite difference method, designed to evaluate the impact of microscopic tissue properties on the formation of DWMRI signal. For the second body of work, the effect of changing the intrinsic diffusion coefficient of a restricted sample on clinical DWMRI experiments is explored. The final body of work, while motivated by the desire to understand stroke, addresses the issue of acquiring large amounts of MRI data well suited for quantitative analysis in reduced scan time. In theory, the method could be used to generate quantitative parametric maps, including those depicting information gleaned through the use of DWMRI methods. Chapter 1 provides an introduction to several topics. A description of the use of DWMRI methods in the study of ischemic stroke is covered. An introduction to the fundamental physical principles at work in MRI is also provided. In this section the means by which magnetization is created in MRI experiments, how MRI signal is induced, as well as the influence of spin-spin and spin-lattice relaxation are discussed. Attention is also given to describing how MRI measurements can be sensitized to diffusion through the use of qualitative and quantitative descriptions of the process. Finally, the reader is given a brief introduction to the use of numerical methods for solving partial differential equations. In Chapters 2, 3 and 4, three related bodies of research are presented in terms of research papers. In Chapter 2, a novel computational method is described. The method reduces the computation resources required to simulate DWMRI experiments. In Chapter 3, a detailed study on how changes in the intrinsic intracellular diffusion coefficient may influence clinical DWMRI experiments is described. In Chapter 4, a novel, non-steady state quantitative MRI method is described.

Rapid development in nanofabrication has led to the design of new materials with very unusual properties. The exhibition of negative and zero indices of refraction are among the most striking properties of these materials, which have become the focus of intensive research worldwide. The potential for applications that is possible due to the new light manipulation capabilities of these materials has been the driving force behind this research. Most of the research in this field has primarily been experimental while the theoretical studies have mainly been limited to computer modeling, which in itself is a challenging problem. This research requires considerable computational resources and the development of new computer algorithms.The origin of the unusual properties in these materials comes from the combination of dielectric host materials with metallic nanosructures. These materials are often referred to as nanocomposite metamaterials. The plasmonic resonance in properly engineered metallic nanostructures gives rise to the resonant interaction of the incident electromagnetic field with metamaterials in such a way as to stimulate a magnetic permeability and an electric permittivity with negative real parts. The resonant nature of this phenomenon leads to considerable losses in metamaterials, which has made the study of loss compensation one of the key subjects in this field.The two techniques of loss compensation in metamaterials are considered in this dissertation. One of these techniques consists of doping the host material with active atoms. In the second technique, loss compensation is achieved by embedding these active atomic inclusions directly into the nanostructures. This dissertation presents the derivation of the systems of governing equations and studies the coherent pulse amplification for both cases.

In the present work we develop a unified way of looking at normed spaces of analytic functions (NSAF's) and their embedding into the Frechet space of analytic functions on a general domain, by requiring only that the embedding map is bounded. This is a succinct definition of NSAF and derive from it a list of interesting properties. For example Proposition 4.4 describes the behavior of point evaluations and Proposition 4.6 part (i) gives a general sufficient condition for a NSAF to be a Banach space, which as far as we know, are new results. Also, Proposition 4.5, parts (ii) and (iii) of Proposition 4.6 and Proposition 4.7 are results, which are slight generalizations of fairly standard results, which show up elsewhere in a more specific setting. Some of the facts about NSAF's are stated and proven in a more general context. In particular, a significant part of the material is dedicated to the normed space of continuous functions on a metric space. On the other hand, we provide the necessary background on differential geometry and complex analysis, which further determine the peculiarities in the context of spaces of analytic functions. At the end we illustrate our results on two specific examples of NSAF's, namely the Bergman and the Bloch Spaces over a general domain in Cd. We give a new proof of the reflexivity of the Bergman Space Ap(G, μ) for the case p>1 and of the Schur property of A1(G, μ). We also give new proofs for the equivalences of some of the definitions of the Bloch functions.

No description available.

In the present work we develop a unified way of looking at normed spaces of analytic functions (NSAF's) and their embedding into the Frechet space of analytic functions on a general domain, by requiring only that the embedding map is bounded. This is a succinct definition of NSAF and derive from it a list of interesting properties. For example Proposition 4.4 describes the behavior of point evaluations and Proposition 4.6 part (i) gives a general sufficient condition for a NSAF to be a Banach space, which as far as we know, are new results. Also, Proposition 4.5, parts (ii) and (iii) of Proposition 4.6 and Proposition 4.7 are results, which are slight generalizations of fairly standard results, which show up elsewhere in a more specific setting. Some of the facts about NSAF's are stated and proven in a more general context. In particular, a significant part of the material is dedicated to the normed space of continuous functions on a metric space. On the other hand, we provide the necessary background on differential geometry and complex analysis, which further determine the peculiarities in the context of spaces of analytic functions. At the end we illustrate our results on two specific examples of NSAF's, namely the Bergman and the Bloch Spaces over a general domain in Cd. We give a new proof of the reflexivity of the Bergman Space Ap(G, μ) for the case p>1 and of the Schur property of A1(G, μ). We also give new proofs for the equivalences of some of the definitions of the Bloch functions.

J. S. Bach se ses suites vir onbegeleide tjello word as sentrale werke in die tjelloliteratuur, en spesifiek wat die genre vir onbegeleide tjellomusiek betref, as die norm beskou. Uitgaande van bostaande beskouing vorm die bestudering van die suites dus die basis van hierdie studie. Die werke van Bach staan in die middelpunt van ’n lang ontwikkelingsgeskiedenis van onbegeleide strykersmusiek wat vanaf die 16de tot die 20ste eeu strek. ’n Algemene historiese oorsig van hierdie ontwikkeling word in Hoofstuk 2 weergegee. Die eienskappe van Bach se melodiese styl, en veral die kontraselemente, vorm die uitgangspunt van die analitiese studie van die ses suites in Hoofstuk 3. Op grond van hierdie eienskappe kan latente meerstemmigheid deurgaans in die onbegeleide suites van Bach aangetoon word. Waar Bach se suites die kernpunt van die ontwikkeling van onbegeleide strykersmusiek daarstel, vorm die werke van 20ste eeuse komponiste soos Reger, Bloch en Britten die kulminasiepunt van die hele proses. Hoewel hulle werke en die van Bach tot verskillende stylperiodes behoort, blyk dit egter duidelik uit ’n studie daarvan in Hoofstuk 4 dat daar in die skepping van hierdie werke uitgegaan is van die voorbeeld en basis wat in Bach se suites daargestel is. Die feit dat die werke tot verskillende stylperiodes behoort, bring ook mee dat daar grondliggende verskille, byvoorbeeld wat tonaliteit en vorm betref, tussen die werke van Bach aan die een kant en die werke van Reger, Bloch en Britten aan die ander kant bestaan. n Gemeenskaplike faktor tussen die werke wat in hierdie studie ondersoek is, is dat hulle almal, nieteenstaande grondliggende stylverskille, vir die tjello, wat n melodie–instrument is, geskryf is. Om in die onbegeleide werke beide die horisontale sowel as die vertikale dimensies in die musiek te kan realiseer, moes sekere tegnieke in die melodiese skryfwyse, soos byvoorbeeld meervlakkige beweging in die melodiese lyn deur onder andere pedaalpunte, kontrastering van motiewe en sekwensiële beweging aangewend word. Ooreenstemmende kenmerke in die musiek van die vier komponiste kan dikwels aangetoon word. Afgesien van die ooreenkomste is daar egter ook verskille as gevolg van die onderskeid tussen die individuele sowel as tydstyle wat as grondslag en uitgangspunt van die werke van die verskillende komponiste dien. Selfs in die werke van Reger, Bloch en Britten is daar n ontwikkeling van byvoorbeeld latente meerstemmigheid volgens die voorbeeld van Bach na n meer volwaardige meerstemmigheid soos dit veral in die werke van Britten neerslag vind. / Thesis (D.Mus)--North-West University, Potchefstroom Campus, 2012.