1,720,980 research outputs found
Application and validation of a viscosity approach to the existence of nanogratings in oxide glasses
Full text linkNanogratings are self-organized and sub-wavelength birefringent structures that are formed upon the action of high intensity ultrashort light pulses in the bulk of a transparent material. They have found interest in optics/photonics, microfluidics, optical data storage or sensing applications. However, the ability to successfully imprint 3-dimensional (3D) nanogratings in silicate glasses is a strong function of the glass composition. In this work, we investigate the role of glass viscosity on the ability to induce these nanogratings. We first study the nanogratings formation window in an energy-repetition rate laser parameter landscape for five common oxide glasses: SiO2 (Suprasil), GeO2, and Schott glasses AF32, Borofloat, and BK7. Secondly, and based on previous work, we define a domain of existence of the nanogratings using viscosity-based arguments. The lower limit corresponds to a temperature at which the viscosity is ~106.6 Pa⋅s, where nanocavitation of the glass occurs, forming the nanopores that compose the nanogratings. An upper temperature limit, set for a viscosity value of ~103.0 Pa⋅s, relates to either collapse or growth of the nanopores, resulting in the erasure of the nanopores, hence the nanogratings. The experimental results agree with the predictions made by this viscosity approach and literature data. This work opens the door to future glass viscosity engineering to maximize 3D nanogratings imprinting
On the existence of nanogratings in commercial oxide glasses
Full text linkThe ability to induce nanogratings using a femtosecond laser in common oxide glasses is investigated experimentally. A simple and general viscosity-based approach is subsequently employed to predict their existence in glass
On the Formation of Nanogratings in Commercial Oxide Glasses by Femtosecond Laser Direct Writing
Full text linkNanogratings (NGs) are self-assembled subwavelength and birefringent nanostructures created by femtosecond laser direct writing (FLDW) in glass, which are of high interest for photonics, sensing, five-dimensional (5D) optical data storage, or microfluidics applications. In this work, NG formation windows were investigated in nine commercial glasses and as a function of glass viscosity and chemical composition. The NG windows were studied in an energy—frequency laser parameter landscape and characterized by polarizing optical microscopy and scanning electron microscopy (SEM). Pure silica glass (Suprasil) exhibits the largest NG window, whereas alkali borosilicate glasses (7059 and BK7) present the smallest one. Moreover, the NG formation windows progressively reduced in the following order: ULE, GeO2, B33, AF32, and Eagle XG. The NG formation window in glasses was found to decrease with the increase of alkali and alkaline earth content and was correlated to the temperature dependence of the viscosity in these glasses. This work provides guidelines to the formation of NGs in commercial oxide glasses by FLDW
Mécanismes de formation et effacement des nanoréseaux dans les verres d'oxyde par écriture directe laser femtoseconde
No full textLes nanoréseaux sont les plus petites structures « auto-organisées » jamais créées par la lumière dans le volume d'un matériau transparent. Ils ont été observés pour la première fois en 2003 à l'intérieur d'un verre de silice, et induits au moyen d'une irradiation par laser femtoseconde (fs). Ils ont suscité un grand intérêt ces dernières années en raison de propriétés particulières. Ils sont biréfringents, avec un axe neutre orientable dans l'espace grâce à la polarisation du laser fs. Ils peuvent également induire une biréfringence circulaire et un dichroïsme, présentent une stabilité thermique extraordinaire, et permettent une attaque chimique hautement sélective. En conséquence, ces nanostructures suscitent un intérêt dans des domaines tel quel le stockage optique de données, les dispositifs optiques biréfringents 2D ou 3D, les capteurs en environnement extrême, la microfluidique, etc. Ces nanoréseaux sont constitués d'un assemblage de couches nanoporeuses principalement observées dans la silice et les verres riches en silice. Plusieurs travaux ont pu mettre en évidence l'effet des propriétés physiques du verre (température de ramolissement, diffusivité thermique, etc.) sur leur processus de formation et d'effacement au cours de l'irradiation. En outre, l'importance de comprendre les mécanismes de formation réside dans la capacité de les produire de manière contrôlée, ainsi que d'acquérir une compréhension fondamentale plus approfondie des interactions complexes lumière-matière. Cette compréhension permettrait une généralisation dans tout type de verres optiques. Dans ce contexte, l'objectif de cette thèse est de mettre en évidence le lien entre la composition chimique des verres et la fenêtre d'inscription des nanoréseaux. De plus, nous sommes intéressés à mieux comprendre les mécanismes de formation et d'effacement des nanoréseaux pendant l'irradiation, afin de contrôler les propriétés optiques anisotropes associées à ces transformations (signe, amplitude, orientation, pertes associées). Premièrement, nous discutons de l'impact de la viscosité du verre sur leur existence à l'aide de l'équation de Rayleigh-Plesset. Il est établi que la viscosité est une propriété clé à considérer, et proposons un domaine d'existence des nanoréseaux grâce à une approche basée cette dernière, en faisant une série d'hypothèses sur les conditions de confinement thermique et de pression pendant l'irradiation. L'étude de la fenêtre d'existence dans un paysage taux de répétition - énergie d'impulsion pour un grand nombre de verres d'oxyde (commerciaux et réalisés en laboratoire), valide l'approche proposée, où la fenêtre théorique est bien corrélée à son équivalent expérimental. Ensuite, nous analysons la nanostructure interne des nanoréseaux par des techniques d'imagerie en microscopie électronique à haute résolution, rendant ainsi compte de leur formation à l'intérieur de la silice. Ces investigations à l'échelle nanométrique révèlent que leur structure intrinsèque est composée de nanopores aplatis, façonnés en nanoplans, régulièrement espacés et orientés perpendiculairement à la polarisation du laser. Ces couches nanoporeuses sont organisées par la lumière sous des effets thermomécaniques via un processus de nanocavitation assisté par le plasma. D'après les observations obtenues par des expériences de recuit en conditions isochrones, l'effacement des nanopores créés lors de l'irradiation laser est également fortement lié à la dépendance de la viscosité du verre à la température. Finalement cet effacement peut être prédit pour un processus temps-température quelconque. Ainsi, il devient possible de d'anticiper la durée de vie des nanoréseaux dans différents verres commerciaux, étendant ainsi notre travail bien au-delà de la silice.Nanogratings are the smallest "self-organized" structures ever created by light inside transparent materials. They were observed for the first time in 2003 inside silica glass and induced by femtosecond (fs) laser irradiation. They have drawn considerable attention over the last decades due to their unique properties. They are birefringent, with a neutral axis orientable in space through light polarization, can induce dichroism or circular birefringence, exhibit excellent thermal stability and a highly selective chemical etching. Consequently, they have shown promise in technological areas, including optical data storage, 2D/3D birefringent optical devices, sensing in harsh environments, microfluidics, etc. These nanogratings are composed of nanoporous layers, mostly observed in silica and silicate glasses. A series of works have shed light on the role played by physical glass properties (softening temperature, thermal diffusivity, etc.) on their formation and erasure during the laser writing process. Moreover, the understanding of formation mechanism directly impacts the ability to inscribe nanogratings in a controlled way, as well as improve the fundamental physics and chemistry at play during complex light-matter interaction. This understanding could thus be generalized to all optical glasses, and in this context the main objective of this work is to decipher the relationship between glass composition and nanogratings formation window. Additionally, better control of the anisotropic properties associated with fs laser induced modifications (sign, amplitude, orientation, losses) is possible, provided the nanogratings formation and erasure mechanisms are well understood. In this thesis, we first discuss the effect of glass viscosity on nanogratings existence using the Rayleigh-Plesset equation. It is established that viscosity is a key property to consider, and we propose a related approach to determine a theoretical temperature domain of their existence provided a series of assumptions are satisfied (thermal confinement and associated pressure induced). The latter approach is experimentally validated using a large number of oxide glasses (commercial and laboratory ones) by investigating the nanogratings processing window in a repetition rate - pulse energy landscape. Following this work, the internal structuring of the nanogratings inside silica is investigated by high resolution electron microscopy techniques. Observations at the nanoscale reveal that nanoplanes are intrinsically made of regularly spaced nanoplanes, oriented perpendicular to the light polarization, and where oblate nanopores line up to form the nanoplanes. These nanoporous layers are organized by light through thermomecanical solicitations and assisted by a plasma mediated nanocavitation process. Under isochronal thermal annealing experiments, the erasure of nanopores created during the laser irradiation is strongly correlated to the temperature dependence of the glass viscosity. Finally, such an erasure can be predicted for any time-temperature process, provided the hypothesis remains valid. It becomes possible to anticipate and model nanogratings lifetime in many commercial glasses and extending it well beyond silica glass
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Molten Core Fabrication of Intrinsically Low Nonlinearity Glass Optical Fibers
No full textOptical nonlinearities limit scaling to higher output powers in modern fiber-based laser systems. Paramount amongst these parasitic phenomena are stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), and nonlinear refractive index (n2)-related wave-mixing phenomena (e.g., self-phase modulation, SPM, four-wave mixing, FWM). In order to mitigate these effects, the fiber community has largely focused on the development of micro-structured large mode area (LMA) fibers whereby the fiber geometry is engineered to spread the optical power over a larger effective area. In addition to increasing the resultant complexity and cost of these fibers, such LMA designs introduce new parasitic phenomena, such as transverse mode instability (TMI), which presently serves as the dominant limitation in power-scaling.
This dissertation explores a different approach for mitigating these nonlinearities in optical fiber lasers; namely attacking the aforementioned effects at their fundamental origin, i.e., the material through which the light propagates. Indeed, the Brillouin gain coefficient (BGC), the Raman gain coefficient (RGC), the thermo-optic coefficient (TOC) and the nonlinear refractive index (n2) are all intrinsic material properties that respectively drive SBS, SRS, TMI and wave-mixing phenomena. Though less well studied within the fiber laser community, such a materials approach offers a powerful yet simpler way to address nonlinearities.
Chapter I investigates the thermodynamic origins of light scattering and provides insight into the prime material properties that drive optical nonlinearities. Chapters II and III offer an overview of how these (and other) properties can be measured and modeled in multicomponent glass systems, considering both bulk or fiber geometries. In Chapter IV, a materials road map for binary and ternary glass material systems is provided to identify which compositions should be of specific focus for the development of intrinsically low optical nonlinearity optical fibers. These four Chapters have been adapted from a series of published journal articles1 entitled “A unified materials approach to mitigating optical nonlinearities in optical fiber” [1]–[4].
In Chapter V, the fabrication of oxyfluoride-core silica-cladding optical fibers using the molten core method is described and their core glass compositions and structures investigated. The thermodynamics and kinetics of fluoride-oxide reactions are also studied, and insights on the dominant mechanisms that drive the fluoride-oxide reactions during fiber processing are discussed. In Chapter VI, optical properties that drive optical nonlinearities are studied, and their relationships with glass compositions investigated. Oxyfluoride fibers exhibiting concomitant reductions of 6-9 dB in BGC, 0.5-1.5 dB in RGC, and 1.2-3.2 dB in TOC, relative to conventional silica fibers, as well as reduced linear and nonlinear refractive indices, are reported. Spectroscopic properties of active Yb-doped fibers are also considered, and suggest enhanced laser performance and higher lasing efficiencies for these systems compared to conventional silica fibers. Finally, Chapter VII portrays the current challenges and the future perspectives of these oxyfluoride-core silica-cladding glass optical fibers, along with approaches to overcome them
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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