1,720,970 research outputs found
Suppression of dispersive broadening of light pulses with Bessel-Gauss beams
No full textWe show that a pulse can travel arbitrarily long distances without significant temporal spreading in a material with normal group velocity dispersion by endowing the pulse with a transversal Bessel-Gauss amplitude profile of suitable characteristics. Contrary to previous works. dispersion suppression is achieved with a finite-energy, transversally limited source, whose radius determines the largest dispersion-free propagation distance. (C) 2002 Elsevier Science B,V. All rights reserved
Relationship between elegant Laguerre-Gauss and Bessel-Gauss beams
No full textWe show that the elegant Laguerre-Gauss light beams of high radial order n are asymptotically equal to Bessel-Gauss light beams. The Bessel-Gauss beam equivalent to each elegant Laguerre-Gauss beam is found and shown to have almost identical propagation factors M-2. In the limit n --> infinity, elegant Laguerre-Gauss beams can be identified with Durnin's Bessel beam. Our results suggest a new experimental procedure for generating light beams with nondiffractinglike properties directly from the output of a stable resonator. (C) 2001 Optical Society of America
Few-optical-cycle Bessel-Gauss pulsed beams in free space
No full textWe introduce a new family of nonseparable, pulselike and beamlike solutions of the wave equation in the paraxial approximation with pseudonondiffracting behavior. They are the pulsed versions of the Bessel-Gauss beams by Gori et al., and encompass as particular cases the diffraction-free Bessel-X pulses, isodiffracting pulses, and, in the many-cycle limit, Bessel and Gaussian beams. Unlike Bessel-X waves, these solutions carry finite energy but retain nondiffracting behavior over a finite propagation distance, and could be physically produced with mode-locked toroidal resonators
Unified description of Bessel X waves with cone dispersion and tilted pulses
No full textWe study Bessel X waves with cone dispersion propagating in free space and dispersive media. Their propagation features find simple explanation when viewed as cylindrically symmetric versions of the so-called tilted pulses. All previously reported cases of suppression of normal material group velocity dispersion by using angular dispersion in tilted pulses, pulsed Bessel beams, and Bessel X waves are compared and presented in a unified way. We show that stationary, spatiotemporal localized Bessel X-wave transmission is also possible in the anomalous dispersion regime
Superluminality in Gaussian beams
No full textOn the basis of Rayleigh-Sommerfeld vectorial diffraction formulas, we show that the radiation from a pulsed Gaussian planar source propagates at a slightly superluminal group velocity beyond the Rayleigh distance, in agreement with a recent prediction from the approximate paraxial theory of diffraction. Moreover, superluminality can be sizably enhanced up to velocities 1.5% faster than c (speed of a plane wave in vacuum) by diminishing the diameter of the source down to one wavelength. (C) 2002 Elsevier Science B.V. All rights reserved
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
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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