1,721,104 research outputs found
Ptychographic amplitude and phase reconstruction of bichromatic vortex beams
Get PDFWe experimentally demonstrate that ptychographic coherent diffractive imaging can be used to simultaneously characterize the amplitude and phase of bichromatic orbital angular momenta-shaped vortex beams, which consist of a fundamental field, together with its copropagating second-harmonic field. In contrast to most other orbital angular momentum characterization methods, this approach solves for the complex field of a hyperspectral beam. This technique can also be used to characterize other phase-structured illumination beams, and, in the future, will be able to be extended to other complex fields in the extreme ultraviolet or X-ray spectral regions, as well as to matter waves.The NSF STROBE STC (DMR-1548924); DOE BES AMOS grant (DE-FG02-99ER14982); the NSF GRFP (DGE 1650115); 2017 Leonardo Grant for Researchers and Cultural Creators, BBVA Foundation; Junta de Castilla y León (SA046U16); Ministerio de Economía y Competitividad (FIS2016-75652-P)
Influence of the spatial confinement on the self-focusing of ultrashort pulses in hollow-core fibers
Get PDFThe collapse of a laser beam propagating inside a hollow-core fiber is investigated by numerically solving different nonlinear propagation models. We have identified that the fiber confinement favors the spatial collapse, especially in case of pulses with the input peak power close to the critical value. We have also observed that when using pulses in the femtosecond range, the temporal dynamics plays an important role, activating the spatial collapse even for pulses with input peak powers below the critical value. The complex self-focusing dynamics observed in the region below the critical power depends on the temporal evolution of the pulse and, also, on the interaction between the different spatial modes of the hollow-core fiber.The authors thank support from Spanish Ministerio de Economía y Competitividad (FIS2016-75652-P) and from Junta de Castilla y León (SA287P18). A.C. is funded by the FPI grant program of Spanish Ministerio de Economía y Competitividad and European Social Fund (BES-2017-080280)
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
Ultrashort visible energetic pulses generated by nonlinear propagation of necklace beams in capillaries
Get PDFThe generation of ultrashort visible energetic pulses is investigated numerically by the nonlinear propagation of infrared necklace beams in capillaries. We have developed a (3+1)D model that solves the nonlinear propagation equation, including the complete spatio-temporal dynamics and the azimuthal dependence of these structured beams. Due to their singular nonlinear propagation, the spectrum broadening inside the capillary extends to the visible region in a controlled way, despite the high nonlinearity, avoiding self-focusing. The results indicate that the features of these necklace beams enable the formation of visible pulses with pulse duration below 10 fs and energies of 50 μJ by soliton self-compression dynamics for different gas pressures inside the capillary.Junta de Castilla y León (SA287P18); Ministerio de Economía y Competitividad (BES-2017-080280, FIS2016-75652-P); Ministerio de Ciencia e Innovación (PID2019-106910GB-I00)
Influence of microscopic and macroscopic effects on attosecond pulse generation using two-color laser fields
Get PDFAttosecond pulses and pulse trains generated by high-order harmonic generation are finding broad applications in advanced spectroscopies and imaging, enabling sub-femtosecond electron dynamics to be probed in atomic, molecular and material systems. To date, isolated attosecond pulses have been generated either by using very short few-cycle driving pulses, or by using temporal and polarization gating, or by taking advantage of phase-matching gating. Here we show that by driving high harmonics with a two-color linearly polarized laser field, the temporal window for time-gated phase matching is shorter than for the equivalent singe-color driving laser. As a result, we can generate quasi-isolated attosecond pulses with a peak width of ∼ 450 as using relatively long 26 femtosecond laser pulses. Our experimental data are in good agreement with theoretical simulations, and show that the phase matching window decreases by a factor of 4 - from four optical cycles in the case of a single-color fundamental driving laser, to one optical cycle in the case of two-color (ω-2ω) laser drivers. Finally, we also demonstrate that by changing the relative delay between the two-color laser fields, we can control the duration of the attosecond bursts from 450 as to 1.2 fs.National Science Foundation (NSF) (1125844); Air Force Office of Scientific Research (FA9550-16-1-0121); REA (328334); Junta de Castilla y León (SA046U16); MINECO (FIS2013-44174-P, FIS2016-75652-P)
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
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