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Light-matter interactions with Flying Doughnuts
No full textThe field of toroidal electrodynamics has gained attention following the detection of toroidal dipole excitations in metamaterials in 2010. Distinct from electric and magnetic dipoles, a toroidal dipole is a localised electromagnetic excitation that corresponds to currents flowing on the surface of a torus. The electromagnetic radiation from toroidal dipole excitations can destructively interfere with the radiation from other modes of excitation, providing a new mechanism of induced transparency and scattering suppression. Toroidal electrodynamics expands with the observation of toroidal light pulses, also known as Flying Doughnuts (FD), which propagate in free space at the speed of light. FDs are solutions to Maxwell’s equations introduced by Hellwarth and Nouchi in 1996, which possess toroidal topology and exist only as short bursts of electromagnetic energy. However, it is little known about how these toroidal pulses interact with matter. This thesis reports on the interaction of toroidal light pulses with matter, focusing on toroidal excitations and non-radiating modes.I have numerically demonstrated supertoroidal anapoles, which are non-radiating charge current configurations that involve supertoroidal currents. Supertoroidal currents are fractal current configurations where each iteration replaces the previous one with a toroidal current loop, accompanied by a toroidal solenoidal current forming the smallest loop. Unlike conventional anapoles formed by the destructive interference of electric and toroidal dipoles, supertoroidal anapoles consist of the interference of a toroidal dipole, the first-order mean square radius of the toroidal dipole, and an electric dipole. I also observed (numerically) the higher-order anapoles formed from quadrupoles and octupoles of electric and toroidal type. I show numerically that under illumination with an FD pulse, scattering from a dielectric torus is substantially suppressed by supertoroidal anapoles by more than 72%. Further, I studied supertoroidal anapoles’dependence on the torus’s geometric parameters. I discovered that a dielectric torus with the largest major radius R and the smallest minor radius r is the best strategy to support supertoroidal anapoles, where R ≫ r and R+r < λ. Moreover, I show that, in contrast to plane wave illumination, FD illumination suppresses scattering by an order of magnitude due to supertoroidal anapoles.I demonstrated that by carefully tuning the geometric parameters of a dielectric disc, it is possible to engineer anapole modes (scattering suppression) over a broad bandwidth of 315nm within the wavelength range from 665nm to 980nm with the maximum suppression at 780nm, when transverse magnetic (TM) FD is an illuminating source and a disc-shaped particle is the scatterer. Further, I show that the disc radius defines the anapole excitation wavelength, and the disc height defines the bandwidth of the anapole; the taller the disc (h < λ), the broader the anapole. Additionally, I show the electric and toroidal dipoles and the anapole mode in the transient regime. I also demonstrate that even though the duration of the incident pulse is ≈ 4 fs, the excitations take up to 15 fs to dissipate their energy through electromagnetic radiation. Further, I investigated the effect of material loss on scattering suppression and observed broadly similar behaviour to lossless material, supporting broad and strong anapole modes. Moreover, unlike plane wave illumination, I report that FD illumination suppresses scattering by an order of magnitude due to the conventional anapole modes.I also investigated the interaction of FD with films and curved interfaces. I demonstrated that the reflection of transverse electric (TE) FD on dielectric film and plasmonic films is 20% and 15% stronger, respectively, compared to TM FD illumination. Moreover, the multipole expansion of displacement current within a high-index dispersive film reveals that the scattering is mediated by the combination of electric and toroidal dipoles (however, their contributions cannot be distinguished with the exact multipole expressions). Investigating FD interactions with curved interfaces shows that the radial spectra distribution of FD does not change upon reflection from the curved interface, which ensures that FD survives after interacting with a curved interface. The findings of FD interactions with curved interfaces are crucial for the experimental realisation of FD-matter interaction, as such experiments involve curved optical components such as parabolic mirrors for the tight focusing of FD.The findings outlined in this thesis contribute to the expanding field of toroidal electrodynamics, firmly establishing its potential for diverse applications, including sensing and spectroscopy using toroidal light sources, anapole nanolasers, anapole-assisted absorption engineering applications, and beyond
Light-matter interactions with Flying Doughnuts - Datas
No full textThis contains datas and the source codes that used in chapter 3-5 of the PhD thesis named Light-matter interactions with Flying Doughnuts.</span
Excitation and detection of toroidal and anapole modes
No full textSince the observations of the dynamic toroidal dipole and non-radiating anapole, the field of toroidal electrodynamics has grown rapidly paving the route to novel physical phenomena including new types of optical activity, Aharonov-Bohm effects, invisibility, and Lorentz reciprocity violations with potential applications in spectroscopy, sensing, telecommunications, and light emission. However, despite significant advances in our understanding of toroidal light-matter interactions, the excitation of toroidal and anapole modes in matter is based on heuristic approaches, which typically require complex patterning and/or illumination conditions resulting in narrowband operation and a strong background of unwanted multipole excitations. Detecting anapoles and toroidal dipoles poses an even greater challenge owing to their weak coupling to free-space radiation. Recently, Flying Doughnuts, single-cycle toroidal pulses, have been put forward as ideal probes for toroidal and anapole excitations in matter. In this talk, we will present our recent work on the light-matter interactions of Flying Doughnut pulses with simple dielectric and plasmonic systems, such as particles and films, demonstrating broadband toroidal excitations and discuss strategies for their unambiguous detection
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
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