1,721,205 research outputs found
Electric field periodically inverted LiNbO<sub>3</sub> for optical frequency conversion
No full textThis thesis describes work carried out at Southampton on periodic poling of LiNbO3. Periodically poled LiNbO3 (PPLN) samples have been fabricated using standard lithography processes and high voltage pulses applied via liquid electrodes. The nonlinear grating associated with the periodic domain inversion provides a means for efficient quasi-phase-matched second-order nonlinear optical processes, such as second harmonic generation (SHG) and optical parametric oscillation (OPO). Compared to birefringent phase-matching, traditionally used, quasi-phase-matching offers several advantages, such as access to higher nonlinear coefficients, non-critical interaction geometry, wider range of interaction wavelengths and reduced photorefractive damage.The fabrication of PPLN together with its optical assessment and the description of the nonlinear optical devices based on this novel material form the content of this thesis. SHG to the blue and to the green have been efficiently performed in PPLN in cw, Q-switched and mode-locked regimes. Also OPO, Q-switched and mode-locked pumped, is reported. Particular emphasis has been given to the study of the photorefractive effect in PPLN. Along the way we discovered a new type of self-organised light-induced scattering, typical of PPLN.This thesis points out that PPLN is already an effective nonlinear medium for quasi-phase-matched frequency conversion with great prospects for future improvement
Modulational instability and four-wave mixing in anisotropic χ(3) magneto-optic media
No full textNearly degenerate four-wave mixing (FWM) in anisotropic magneto-optic media is theoretically investigated. It is shown that the anisotropic nature of the nonlinear χ(3) tensor is responsible for the appearance of a novel FWM instability branch for which phase matching can be conveniently controlled by the application of an axial static magnetic field
Enhanced photosensitivity in silicate optical fibres by thermal treatment
No full textEnhanced photosensitivity using thermal treatment has been observed on several silicate optical fibers. The effect of the treatment on fibers with different dopants has been tested via Bragg grating inscription. The presence of Ge or Sn atom has been established to be fundamental for the effect to occur. To explain the main features a model involving defect dynamics is proposed
An embedded optical nanowire loop resonator refractometric sensor
No full textA novel refractometric sensor based on an embedded optical nanowire loop resonator is presented. The device sensitivity has been studied in two typical configurations and its dependence on the nanowire diameter and coating thickness determined
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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