1,720,964 research outputs found
High-order harmonic generation in a microfluidic glass device
Get PDFWe report on the efficient generation of high-order harmonics in helium gas inside complex glass micro-devices fabricated by femtosecond laser micromachining. By exploiting the three-dimensional capabilities and extreme flexibility of this fabrication technique we developed fluidic micro-structures in a fused-silica substrate that allowed us to achieve accurate control of the gas density inside a micrometer-sized microchannel. As a result, we achieved a broadband spectrum of extreme ultraviolet (XUV) radiation which extends up to 200 eV and we observed a considerable increase in the harmonics generation efficiency if compared with traditional harmonic generation in gas jets. We foresee that the application of femosecond-laser-micromachined glass devices to high-order harmonics generation can be extended to more complex on-chip systems including different functionalities, thus opening the possibility to future miniaturization of XUV and Attosecond beamlines
Development of ultrafast Mid-IR sources and their applications to high-order harmonic generation spectroscopy
No full textLa generazione di armoniche di ordine elevato (HHG) è un effetto non lineare che si verifica in caso di interazione di impulsi ultracorti con campioni atomici o solidi. La generazione di armoniche nel caso atomico è ben consolidata e costituisce la base della scienza agli attosecondi. Il processo non lineare avviato dalla ionizzazione tunnel e completato dalla ricombinazione dell'elettrone ionizzato con il suo ione parente codifica numerose informazioni sulla struttura interna del campione in esame. In questo contesto, abbiamo sviluppato una tecnica per la ricostruzione dell'orbitale molecolare più esterno di una molecola. La tecnica prevede l'utilizzo di un primo impulso laser che permette di allineare le molecole nello spazio e di un secondo impulso per generare gli spettri di armoniche necessari alla ricostruzione dell'orbitale. In questo contesto, verrano mostrati i risultati ottenuti nella molecola di CO2.
Nel caso dei solidi, l'osservazione di armoniche di ordine elevato costituisce uno degli ambiti di frontiera della scienza ultraveloce e agli attosecondi. Lo sviluppo di impulsi intensi a medio infrarosso ha aperto recentemente l'indagine sperimentale del processo HHG nei semiconduttori. Nei solidi, in analogia con il caso atomico, l'emissione di armoniche ha origine dal processo di ricombinazione degli elettroni con le lacune associate. Queste coppie elettrone-lacuna vengono generate a seguito del processo di ionizzazione tunnel, e la loro propagazione prima della ricombinazione produce due correnti indipendenti nelle bande di valenza e conduzione del semiconduttore. La seconda parte di questa tesi mira ad approfondire questo processo. Verranno introdotti i fondamenti che regolano i fenomeni di campo forte a seguito dell'interazione laser-materia in sistemi periodici (materiali semiconduttori) a confronto con il caso atomico. Verranno poi discussi i risultati sperimentali ottenuti in cristalli di seleniuro di diamante e zinco.High-order harmonic generation (HHG) is a nonlinear effect occurring upon interaction of intense ultrashort pulses with atomic or solid targets. Harmonic generation in the atomic case has been well established and has formed the basis of attosecond science. The nonlinear process initiated by tunnel ionization and completed by the recombination of the ionized electron with its parent ion encodes needed information regarding the intimate structure of the species under consideration. In this framework, we have developed a technique for the tomographic reconstruction of the Highest Occupied Molecular Orbital of a molecule, in which a laser pulse aligns the molecule in space, and another one generates the harmonic spectra needed for the reconstruction. In this context, we performed experiments for the tomographic reconstruction of the CO2 molecule. In the case of solids, the advent of potential mid-infrared pulses has opened up the experimental investigation of HHG process in semiconductors. In solids, harmonic emission originates from the recollision of electrons with their associated holes, similarly to atomic case, while tunnel ionization is termed as excitation of electron-hole pairs that accelerate within the material. The second part of the thesis aims at providing this insight. An introduction to the basic foundation of HHG in solids along with comparison to the atomic case is discussed. This will be followed by an experimental demonstration of HHG from Diamond and Zinc Selenide crystals.DIPARTIMENTO DI FISICA31DE SILVESTRI, SANDROFINAZZI, MARC
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“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
Get PDFWe 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
Get PDFWe 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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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