1,720,987 research outputs found
Quantitative measurement of the optical cross-sections of single nano-objects by correlative transmission and scattering micro-spectroscopy
Full text linkThe scattering and absorption of light by nano-objects is a key physical property exploited in many applications, including biosensing and photovoltaics. Yet, its quantification at the single object level is challenging, and often requires expensive and complicated techniques. We report a method based on a commercial transmission microscope to measure the optical scattering and absorption cross-sections of individual nano-objects. The method applies to micro-spectroscopy and wide-field image analysis, offering fine spectral information and high throughput sample characterization. Accurate cross-section determination requires a detailed modeling of the measurement, which we develop, accounting for the geometry of the illumination and detection, as well as for the presence of a sample substrate. We demonstrate the method on three model systems (gold spheres, gold rods, and polystyrene spheres), which include metallic and dielectric particles, spherical and elongated, placed in a homogeneous medium or on a dielectric substrate. Furthermore, by comparing the measured cross-sections with numerical simulations, we are able to determine structural parameters of the studied system, such as the particle diameter and aspect ratio. Our method therefore holds the potential to complement electron microscopy as a simpler and cost-effective tool for structural characterization of single nano-objects
Tailoring core size, shell thickness, and aluminium doping of Au@ZnO core@shell nanoparticles
Get PDFPlasmonic materials, such as gold nanoparticles (AuNPs), exhibit significant extinction and near-field enhancement across the visible and near-infrared spectrum, attributable to localized surface plasmon resonances (LSPRs). Epsilon-near-zero (ENZ) materials, such as aluminium doped zinc oxide (AZO) are known in non-linear optics for their ability to generate and manipulate light-matter interactions through processes like higher harmonic generation. Combining doped ZnO with plasmonic materials therefore holds promise for enhancing non-linear efficiencies and tuning their operational wavelengths. To date, however, only top-down structures based on plasmonically decorated thin ENZ films have been realized, and no colloidal and scalable route to obtain these hybrid materials has been reported yet. Here, we introduce a novel colloidal synthesis approach for fabricating Au@AZO core@shell nanoparticles with tunable core size, shell thickness, and dopant concentration, allowing for the spectral alignment of the LSPRs of the AuNPs with the non-linear optical properties of the AZO shells. Our method involves the colloidal synthesis of gold cores followed by an ascorbic acid-assisted process to deposit polycristalline ZnO and AZO shells, resulting in core diameters ranging from 25 to 69 nm, shell thicknesses from 16 to 47 nm, and aluminium doping levels between 0 and 4 at%. Our procedure widens the range of hybrid plasmonic nanostructures that can be colloidally synthesised, opening new possibilities for the large scale fabrication of high-performance nanomaterials for integration in photonic, photocatalytic, and sensing applications.</p
Standoff CARS spectroscopy and imaging using an ytterbium-based laser system
Full text linkA laser system for standoff coherent anti-Stokes Raman scattering (CARS) spectroscopy of various materials under ambient light conditions is presented. The system is based on an ytterbium laser and an ultrafast optical parametric amplifier for the generation of a broadband pump tunable from 880 to 930 nm, a Stokes at 1025 nm, and a narrowband probe at 512.5 nm. High-resolution Raman spectra encompassing the fingerprint region (400-1800 cm-1) are obtained in 5 ms for toluene, and 100 ms for two types of sugars, glucose and fructose, at a distance of 1 m. As a demonstration of the potential of the setup, hyperspectral images of a 2×2-cm2 target area are collected for a toluene cuvette and a glucose/fructose pressed disk. Our approach is suitable for implementation of a portable system for standoff CARS imaging of chemical and biological materials
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
Far-field mapping and efficient beaming of second harmonic by a plasmonic metagrating
Full text linkWe study numerically and experimentally the second-harmonic generation (SHG) from rectangular meta-
gratings of V-shaped gold nanoantennas. We show that by carefully engineering the array pitch to steer the diffraction orders toward the single antenna emission, the extracted signal is maximized. This enhancement is attributed to the angular overlap between the radiation pattern and array factor and is comparable to the improvement yielded by the coupling of surface lattice resonances (SLRs) with local modes. Moreover, we demonstrate a simple technique to experimentally reconstruct the emission diagram of an antenna from measurements of the collective grating response as a function of the excitation angle. Excellent agreement is obtained with simulations when the sample is immersed either in air or in water, which is crucial in view of future sensing application. Thanks to the high signal-to-noise ratio and low dependence on the statistical particle dispersity, this method constitutes an effective alternative to back-focal plane imaging when very weak signals such as SHG are involved
Quantitative optical microspectroscopy, electron microscopy, and modelling of individual silver nanocubes reveals surface compositional changes at the nanoscale
Full text linkThe optical response of metal nanoparticles is governed by plasmonic resonances, which depend often intricately on the geometry and composition of the particle and its environment. In this work we describe a method and analysis pipeline unravelling these relations at the single nanoparticle level through a quantitative characterization of the optical and structural properties. It is based on correlating electron microscopy with micro-spectroscopy measurements of the same particle immersed in media of different refractive index. The optical measurements quantify the magnitude of both the scattering and the absorption cross sections, while the geometry measured in electron microscopy is used for numerical simulations of the cross section spectra accounting for the experimental conditions. We showcase the method on silver nanocubes of nominal 75nm edge size. The large amount of information afforded by the quantitative cross section spectra, and measuring the same particle in two environments, allows us to identify a specific degradation of the cube surface. We find a layer of tarnish, only a few nanometers thick, a fine surface compositional change of the studied system which would be hardly quantifiable otherwise
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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