1,721,002 research outputs found
Theory and numerical modelling of parity-time symmetric structures for photonics
No full textThis thesis presents the study of a relatively new class of photonic structures in-voking Parity-Time (PT)-symmetry. PT-symmetric structures in photonics, as a realisation of PT-symmetric Quantum Mechanics problems, are constructed by a judicious design of refractive index modulation which requires the real part of the refractive index to be an even function and the imaginary part of the refractive index to be an odd function in space. PT-symmetric structures in the form of Bragg gratings, coupled resonators and chain resonators are the main configurations studied in this thesis. These PT-symmetric structures feature a spontaneous symmetry breaking at which interesting wave behaviour such as an asymmetric response depending on the direction of the incident wave, unidirectional invisibility, simultaneous coherent-perfect absorber lasing and localised termination modes are observed; these behaviours are presented in this thesis. Theoretical and numerical studies of these PT-symmetric structures are undertaken which assume realistic material parameters,including material dispersion and material non-linearity. Moreover,in this thesis, potential applications of these PT-symmetric structures are explored.
The first part of the thesis considers PT-symmetric Bragg grating structures which are formed by introducing a PT-symmetric refractive index modulation into a Bragg grating structure. If gain/loss dispersion is considered, it is shown that dispersion limits the PT-symmetric operation to just a single frequency. As such spontaneous symmetry breaking can only be achieved by varying the gain/loss parameter. Nevertheless, it is shown that by switching the gain/loss in the system, a switching operation can be achieved by using the PT-Bragg grating at a single frequency. Subsequently,anon-linear PT-Bragg grating is investigated by using a time-domain numerical method, namely the Transmission-Line modelling (TLM) method. For the present work a TLM code is developed from scratch in order to ensure full-flexibility when modelling a dispersive and non-linear material. Using the TLM solver, it is demonstrated that gain/loss saturation is an important material property which should be considered as it may impact the practical applications of a PT-symmetry-based device. In the context of a non-linear PT-Bragg grating (NPTBG), the gain/loss saturation affects the interplay between the PT-symmetric opearation and the Kerr non-linear effect. It is further shown that gain/loss saturation plays a crucial role in securing a stable operation of non-linear PT-based devices. For practical applications, it is demonstrated that a non-linear PT-symmetric Bragg grating offers an additional degree of freedom in their operation,by modulating the gain/loss and the intensity of the input signal,compared to a passive structure which can only be manipulated by the input signal intensity. Two applications based on the interplay of PT-symmetric behaviour and Kerr non-linearity are demonstrated,namely a memory device and a logic-gate device.
The second part of the thesis studies PT-symmetric resonator structures as a coupled system and as a periodic chain system. For these studies, a semi-analytical method based on the Boundary Integral Equation (BIE) method is developed and used together with a two-dimensional TLM method. The impact of realistic material parameter on the spectral properties of the structure is again investigated. It is shown that the PT-symmetric behaviour can be observed at a single frequency. Moreover, it is shown that PT-symmetry-like behaviour is observed but with complex eigenfrequencies due to the radiation losses; this is a deviation of the strict definition of a PT-symmetric structure with balanced gain and loss. Lowering lasing threshold by increasing loss in the system is demonstrated; this occurs due to induced early symmetry breaking. The final part of the thesis studies the spectral properties of an infinite and finite chain of PT-symmetric resonators. It is shown that the type of modulation along the PT-chain affects the position of the breaking point of the PT-structure. For a finite PT-chain structure, and for a particular type of refractive index modulation, early PT-symmetry breaking is observed and shown to cause the presence of termination states which are localised at the edge of the finite-chain resulting in localised lasing and dissipative modes at each end of the chain
Theory and numerical modelling of parity-time symmetric structures for photonics
Get PDFThis thesis presents the study of a relatively new class of photonic structures in-voking Parity-Time (PT)-symmetry. PT-symmetric structures in photonics, as a realisation of PT-symmetric Quantum Mechanics problems, are constructed by a judicious design of refractive index modulation which requires the real part of the refractive index to be an even function and the imaginary part of the refractive index to be an odd function in space. PT-symmetric structures in the form of Bragg gratings, coupled resonators and chain resonators are the main configurations studied in this thesis. These PT-symmetric structures feature a spontaneous symmetry breaking at which interesting wave behaviour such as an asymmetric response depending on the direction of the incident wave, unidirectional invisibility, simultaneous coherent-perfect absorber lasing and localised termination modes are observed; these behaviours are presented in this thesis. Theoretical and numerical studies of these PT-symmetric structures are undertaken which assume realistic material parameters,including material dispersion and material non-linearity. Moreover,in this thesis, potential applications of these PT-symmetric structures are explored.
The first part of the thesis considers PT-symmetric Bragg grating structures which are formed by introducing a PT-symmetric refractive index modulation into a Bragg grating structure. If gain/loss dispersion is considered, it is shown that dispersion limits the PT-symmetric operation to just a single frequency. As such spontaneous symmetry breaking can only be achieved by varying the gain/loss parameter. Nevertheless, it is shown that by switching the gain/loss in the system, a switching operation can be achieved by using the PT-Bragg grating at a single frequency. Subsequently,anon-linear PT-Bragg grating is investigated by using a time-domain numerical method, namely the Transmission-Line modelling (TLM) method. For the present work a TLM code is developed from scratch in order to ensure full-flexibility when modelling a dispersive and non-linear material. Using the TLM solver, it is demonstrated that gain/loss saturation is an important material property which should be considered as it may impact the practical applications of a PT-symmetry-based device. In the context of a non-linear PT-Bragg grating (NPTBG), the gain/loss saturation affects the interplay between the PT-symmetric opearation and the Kerr non-linear effect. It is further shown that gain/loss saturation plays a crucial role in securing a stable operation of non-linear PT-based devices. For practical applications, it is demonstrated that a non-linear PT-symmetric Bragg grating offers an additional degree of freedom in their operation,by modulating the gain/loss and the intensity of the input signal,compared to a passive structure which can only be manipulated by the input signal intensity. Two applications based on the interplay of PT-symmetric behaviour and Kerr non-linearity are demonstrated,namely a memory device and a logic-gate device.
The second part of the thesis studies PT-symmetric resonator structures as a coupled system and as a periodic chain system. For these studies, a semi-analytical method based on the Boundary Integral Equation (BIE) method is developed and used together with a two-dimensional TLM method. The impact of realistic material parameter on the spectral properties of the structure is again investigated. It is shown that the PT-symmetric behaviour can be observed at a single frequency. Moreover, it is shown that PT-symmetry-like behaviour is observed but with complex eigenfrequencies due to the radiation losses; this is a deviation of the strict definition of a PT-symmetric structure with balanced gain and loss. Lowering lasing threshold by increasing loss in the system is demonstrated; this occurs due to induced early symmetry breaking. The final part of the thesis studies the spectral properties of an infinite and finite chain of PT-symmetric resonators. It is shown that the type of modulation along the PT-chain affects the position of the breaking point of the PT-structure. For a finite PT-chain structure, and for a particular type of refractive index modulation, early PT-symmetry breaking is observed and shown to cause the presence of termination states which are localised at the edge of the finite-chain resulting in localised lasing and dissipative modes at each end of the chain
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
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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