1,720,961 research outputs found
Light propagation and localisation on periodic dieletric and metallic nanostructures
No full textThis thesis presents firstly a study in the light propagation in dielectric photonic crystal waveguides and secondly light propagation and localisation on arrays of metallic inverted pyramidal pits. As well as introducing the concept and properties of photonic crystals, the first part of the thesis concentrates on 2D photonic crystals and in particular the superprism effect. Experimental results demonstrate the angular dispersion experienced by wavelength close to the bandgap. Reflectivity measurements allowed the acquisition of dispersion diagram as a function of azimuthal angles, therefore permitting the experimental observation of dispersion surfaces which are then compared with results from plane wave simulation. The same reflectivity technique as been employed to measure arrays of gold coated inverted pyramidal pits with square apertures. These nano-patterned gold structures show clear evidence of propagating and localised plasmon as standing wave localised in the pits. Using a sample graded in depth, we showed that the localised plasmon resonance follows a simple interference model. This model is then confirmed using reflectivity data from a sample graded in depth and pitch, which also shows that the dip in reflectivity observed at normal incidence is independent of pitch. Repeating the measurements for different coating and comparing them with simulation results leads to an intuitive understanding of the coupling mechanism of the light to the pit. Surface enhanced Raman scattering is then used to probe the field localised in the pit and the surface enhanced Raman scattered signal from the pit array is found to be in agreement with the electric field enhancement predicted by a plasmon cavity model. Further correlation with simulation results paves the way to optimised plasmon cavity for applications such as surface enhanced Raman scattering or to enhance the interaction between light and matter.</p
The self-orientation of mammalian cells in optical tweezers - the importance of the nucleus
No full textHere we present the first evidence showing that eukaryotic cells can be stably trapped in a single focused Gaussian beam with an orientation that is defined by the nucleus. A mammalian eukaryotic cell (in suspension) is trapped and is re-oriented in the focus of a linearly polarized Gaussian beam with a waist of dimension smaller than the radius of the nucleus. The cell reaches a position relative to the focus that is dictated by the nucleus and nuclear components. Our studies illustrate that the force exerted by the optical tweezers at locations within the cell can be predicted theoretically; the data obtained in this way is consistent with the experimental observations
Self-phase modulation induced spectral broadening of ultrashort laser pulses in tantalum pentoxide (Ta<sub>2</sub>O<sub>5</sub>) rib waveguide
No full textSelf-phase modulation induced spectral broadening has been observed for ultrashort pulses propagating through Ta2O5 rib waveguide. The associated nonlinear refractive index was estimated to be 7.23 x10-19 m2/W, which is higher by one order of magnitude than silica glass
Optical spectroscopy of neodymium-doped tantalum pentoxide slab waveguides
No full textNeodymium doped tantalum pentoxide (Nd:Ta2O5) waveguides were fabricated by RF sputtering from a Nd doped Ta2O5 target. Waveguide losses, absorption spectra, fluorescence spectra and excited-state lifetime were measured, and show promise for realisation of waveguide lasers
Fabrication of submicrometer high refractive index tantalum pentoxide waveguides for optical propulsion of microparticles
No full textDesign, fabrication, and optimization of tantalum pentoxide (Ta2O5) waveguides to obtain low-loss guidance at a wavelength of 1070 nm are reported. The high-refractive index contrast (Δn ~ 0.65, compared to silicon oxide) of Ta2O5 allows strong confinement of light in waveguides of submicrometer thickness (200 nm), with enhanced intensity in the evanescent field. We have employed the strong evanescent field from the waveguide to propel micro-particles with higher velocity than previously reported. An optical propelling velocity of 50 µm/s was obtained for 8 µm polystyrene particles with guided power of only 20 mW
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
Sharp-cornered liquid drops by wetting of nanoscale features
No full textSerrated-edge droplets: The wetting of metallic microstructures with nanoscale features, used as substrates for surface-enhanced Raman scattering, can result in sharp bending of the droplet edges. Tethering of the liquid/air/substrate contact line induces flows of analyte to particular locations as the droplet recedes during evaporation (see SEM image). Molecules are preferentially deposited in the pyramidal pits on these structures
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