1,720,956 research outputs found
Electro-optic effect in proton exchanged LiNbO3 and LiTaO3 waveguides
No full textElectro-optic properties of proton-exchanged (PE) waveguide layers in LiTaO3 and LiNbO3 are studied and related to their optical characteristics. The proton-exchange process induces a degradation of the electro-optic activity in both types of waveguides, PE LiNbO3 and PE LiTaO3. The measured electrooptic effect is close to the detection sensitivity even when the exchange regime is performed at low temperatures for short periods of time, The PE samples have been annealed (APE waveguides) and the changes of their tau(33) electro-optic coefficient has been followed at successively higher temperatures and periods of time, Subjected to annealing at temperatures between 265-420 degrees C, the LiTaO3 layers show a partially recovered tau(33) coefficient, the recovering being different for quick and slow cooling of the samples, In thin APE LiNbO3 waveguiding layers a restoration of tau(33) UP to 75% of the bulk value is observed due to the annealing at temperatures between 200-340 degrees C
The influence of the cooling rate on the stability of z-cut H:LiTaO3 waveguides
No full textThe influence of cooling rate on the refractive index of proton-exchanged z-cut LiTaO3 waveguides has been investigated. After short annealing at about 240 degrees C, the waveguide mode indices of as-exchanged and annealed samples are different for quick and slow cooling. A hypothesis explaining this effect by assuming a diffuse phase transition in the protonated crystal lattice is given. The possible changes in the macroscopic parameters responsible for the index differences are analysed
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
Waveguide formation mechanism generated by double doping in ferroelectric crystals
No full textPhysical mechanisms causing the refractive index increase Δn in uniaxial oxidized ferroelectric crystals subjected to double doping are discussed. The analysis shows that the index changes of the two different dopants, ΔnA and ΔnB, are additive only if the corresponding changes of the spontaneous polarization are small. Ti indiffusion and proton exchange (TIPE) waveguides in LiNbO3 are considered as an example of the case when this condition is not fulfilled. The hydrogen–lithium substitution in such waveguides leads to a strong reduction of the spontaneous polarization of the virgin crystals and the final index change Δn is not a sum of ΔnA and ΔnB. A physical model is developed to explain light waveguiding in double‐doped ferroelectric structures. Experimental data obtained for TIPE waveguides confirm the model
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
A comparative analysis of Rb : KTP and Cs : KTP optical waveguides
No full textPlanar optical waveguides were produced by exchange of Rb+ (Cs+) for K+ of Z-cut KTiOPO4 (KTP). The results of a systematic study of Rb:KTP and Cs:KTP waveguides obtained by combining measurements of the mode propagation constants, optical profiles and Raman scattering spectra are reported. The analysis of the refractive index profiles has shown that there is an exponential Rb+ (Cs+) distribution in the exchanged layers. Some dependences of the waveguide characteristics on the fabrication conditions are presented. Micro-Raman spectra have revealed a notable difference compared with the spectra of pure KTP in the range of external lattice modes as well as in the range of torsional vibrations of TiO6 octahedra. The exchanged layers exhibit lower perfection of the crystal lattice than that of pure KTP and effects of symmetry changes
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