1,720,974 research outputs found
Estructuras superficiales auto-organizadas generadas por láser
Get PDFTesis doctoral inédita leída en la Universidad Autónoma de Madrid,Facultad de Ciencias, Departamento de Física Aplicada. Fecha de lectura: 29-11-2019En este trabajo se aborda la formación controlada de estructuras
periódicas superficiales inducidas por láser, LIPSS (Laser Induced Periodic
Surface Structures), producidas mediante pulsos láser de femtosegundos.
Primero, se estudia la formación de micro y nanoestructuras
amorfas en silicio cristalino. A continuación, se fabrican franjas amorfocristalinas
auto-organizadas en superficie (ac-LIPSS) desarrollando
control sobre el periodo, el espesor y las dimensiones de dichas franjas.
En especial, se descubre cómo seleccionar solo uno de los dos periodos
de franjas que se forman al irradiar con ángulo barriendo el haz sobre
la muestra en uno u otro sentido.
Se indaga asimismo sobre la formación de LIPSS ablativos en metales.
En concreto en cobre. Se logra fabricar un régimen en el que
se producen dos periodos distintos que coexisten separados espacialmente.
Además, dado que el modelo plasmónico tiende a sobrestimar
los periodos producidos en irradiaciones oblicuas, se introduce el
parámetro de la rugosidad de superficie dando forma a un modelo
plasmónico y de rugosidad. Los resultados teóricos se ajustan mucho
mejor a las franjas así obtenidas. Por otro lado, en lámina delgada de
cobre, se verifica de forma experimental la importancia del número de
barridos en la formación de LIPSS, además del fenómeno de franjas
viajeras: franjas que aparentemente se desplazan en la superficie de la
muestra durante la irradiación. Se discute su papel en la degradación
de los LIPSS formados con un único barrido.
Se continúa el trabajo para el entendimiento de la formación de
LIPSS cambiando a acero. Se descubre el efecto pluma, que permite
seleccionar de forma fina el periodo de las franjas invirtiendo el
sentido del barrido del haz. Adicionalmente, se adapta el modelo de
rugosidad-plasmónico a materiales absorbentes (como es el caso del
acero), haciendo uso de una constante dieléctrica efectiva. Por último,
se estudia la dinámica de reflectividad en acero mediante técnicas
de microscopía con resolución de femtosegundos y de medidas de
reflectividad en tiempo real.The present work addresses the controlled formation of Laser Induced
Periodic Surface Structures (LIPSS) using femtosecond laser pulses.
It started with the formation of amorphous micro- and nanostructures
on crystalline silicon. Afterwards, ripples of amorphous-crystalline
LIPSS (ac-LIPSS) were fabricated. The study is extended in order to
control the period, thickness and lateral dimensions of the fringes. In
particular, it was discovered how to select just one of the two periods
of ripples that are formed when performing oblique irradiation. The
technique consists in sweeping the beam on the sample rather in one
or the other direction due to front- or backscatter of the incident laser
beam.
In a second stage, we study ablative LIPSS in copper. We identified
a processing regime which produces two different periods coexisting
with spatial separation. In addition, we introduced a roughnessplasmonic
model given that the simple plasmonic model tends to
overestimate the periods that are fabricated when irradiations are
made with an oblique angle. This new model takes into account the
influence of the surface’s roughness on the surface plasmon polariton
(SPP) formation. The theoretical results fit adequately the ripples thus
obtained. Additionally, in thin layers of copper we verified the importance
of the number of overscans for the LIPSS formation, along with
the traveling fringes phenomenon: ripples that are apparently moving
across the surface during the irradiation. We have discussed its role in
the degradation of LIPSS that are formed with a single scan.
The final part is dedicated to the formation of LIPSS in steel. The
quill writing effect or non-reciprocal writing was discovered when
processing the surfaces at oblique angle while scanning the sample.
This phenomenon allows us to select with great finesse the ripples
period just by inverting the scan direction. The underlying mechanisms
are related to the asymmetric roughness formation at angled
incidence. Consequently, different values of roughness at different positions
generate different wavevector of the surface plasmon polariton
in function of the scan direction. In addition, we have adapted the
roughness-plasmonic model to absorbent materials such as steel, using
an effective dielectric constant. Finally, we have performed femtosecond
pump-probe imaging and real time reflectivity measurements to
study the reflectivity dynamics when the sample is irradiated by fs
pulses
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
Laser-induced periodic surface structures on germanium by femtosecond irradiations
No full textUSTS 2019 Meeting, Madrid, November 6th to 8th, 2019Laser induced ripples that are also known as Laser Induced Periodic Surface Structures (LIPSS) have gained considerable attention by researchers and industry due to their surface functionalization applications. Surface modification with LIPSS takes place in metals, semiconductors or dielectrics [1]. Their morphology depends strongly on many parameters, both on material properties and irradiation conditions (pulse fluence, wavelength, light polarization, pulse repetition rate, and processing speed). Two types of LIPSS are distinguished by their spatial periodicity, namely high-spatial frequency LIPSS (HSFL) having a periodicity significantly smaller than the laser wavelength and low-spatial frequency LIPSS (LSFL) having a periodicity close to the laser wavelength.
In this work, we have investigated the LIPSS formation on germanium by using a Ti:Sapphire (800nm, 120 fs) laser. The results show different scenarios depending on the irradiation conditions. One of these scenarios is presented in Figure 1, showing images of LIPSS formed by the same fluence but different number of pulses. For a low number of pulses, LIPSS of type HSFL were formed parallel to polarization, whereas for larger number of pulses they were of type LSFL and perpendicular to polarization. This evolution of spatial periodicity by increasing the number of pulses is fundamentally different regarding the behaviour in silicon [2], which will be discussed for comparison. On the other hand, since the spatial intensity distribution of the laser beam is Gaussian, the central disk of the irradiated spot has been exposed to the peak fluence. Suprisingly, this central disk did not present ablation but LIPSS appeared on its surroundings, as can be seen in the images. This suggests that a phase change from the crystalline phase into the amorphous one could be the origin of these LIPSS.
References
1 Bonse, J., Höhm, S., Kirner, S.V., Rosenfeld, A. & Krüger, J. IEEE J. Sel. Top. Quantum Electron 2017, 23, 9000615
2 Fuentes-Edfuf, Y., Garcia-Lechuga, M., Puerto, D., Florian, C., Garcia-Leis, A., Sanchez-Cortes, S., Solís, J., Siegel, J. Scientific Reports 2017, 7, 459
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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