1,720,996 research outputs found
Photonic band structure of periodically etched high index waveguide layer:mode suppression in multimode waveguides
No full textThere is considerable current interest in using photonic band gap structures to control spontaneous emission and mode density in miniature laser resonators. In this paper, the photonic band structure and possible applications of resonant modes that occur within strongly modulated periodic dielectric films are reported. The structure analysed consists of a planar waveguide formed between planar sheets of perfect metal. Within the waveguide are alternating lines of high index dielectric and air. The guided modes in this structure (solid lines on figure) can be compared with those that would occur in a multimode waveguide with a homogeneous guiding layer of the same average index (dashed lines). At the Bragg condition a bandgap appears and this can be used to reduce the number of guided modes. From the figure it can be seen that at a normalised frequency of 4.2 there is a 13:1 mode reduction. In this way a fairly thick multi-mode waveguide can be made in which all but the highest order mode are suppressed, rendering the structure single-mode. The field microstructure of this mode shows that most of its intensity is a concentrated in the high index regions. This mode could be excited by rare-earth doping in the high index regions alone. This could have applications in single-mode waveguide lasers where an increased mode volume could be very beneficial
Photonic band structure of guided block modes in high index films fully etched through with periodic microstructure
No full textBy adapting the well-known 'zigzag' ray model for use with a periodic waveguide (i.e. replacing the plane wave rays with Bloch wave rays), we show that thin films of high refractive index, supported by a low index substrate and fully etched through with a periodic pattern, can support guided modes. From the dispersion relation of these guided Bloch modes, it is shown that the in-plane modal group velocity can be zero, suggesting applications in enhanced dipole-field interactions and control of spontaneous emission in waveguide lasers
Full 2D photonic band gaps in silica/air structures
No full textFull 2-D photonic bandgaps are demonstrated for all polarisations in structures with refractive index contrast as small as that of silica and air. They occur for light propagating out of the transverse plane, with a longitudinal component of wave vector. A new type of optical fibre based on these structures is proposed
Two-dimensional photonic band-gap structures as quasi-metals
No full textBy considering waves that propagate out of the transverse plane, we show that common high index materials (eg GaAs) with a 2D array of air holes can act in some ways like a 3D photonic band-gap structure. In particular, we describe a dielectric "quasi-metal" that reflects all propagating light incident from free space
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
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