465 research outputs found
Engineering aperiodic spiral order for photonic-plasmonic device applications
Thesis (Ph.D.)--Boston UniversityDeterministic arrays of metal (i.e., Au) nanoparticles and dielectric nanopillars (i.e., Si and SiN) arranged in aperiodic spiral geometries (Vogel's spirals) are proposed as a novel platform for engineering enhanced photonic-plasmonic coupling and increased light-matter interaction over broad frequency and angular spectra for planar optical devices. Vogel's spirals lack both translational and orientational symmetry in real space, while displaying continuous circular symmetry (i.e., rotational symmetry of infinite order) in reciprocal Fourier space. The novel regime of "circular multiple light scattering" in finite-size deterministic structures will be investigated. The distinctive geometrical structure of Vogel spirals will be studied by a multifractal analysis, Fourier-Bessel decomposition, and Delaunay tessellation methods, leading to spiral structure optimization for novel localized optical states with broadband fluctuations in their photonic mode density. Experimentally, a number of designed passive and active spiral structures will be fabricated and characterized using dark-field optical spectroscopy, ellipsometry, and Fourier space imaging. Polarization-insensitive planar omnidirectional diffraction will be demonstrated and engineered over a large and controllable range of frequencies. Device applications to enhanced LEDs, novel lasers, and thin-film solar cells with enhanced absorption will be specifically targeted. Additionally, using Vogel spirals we investigate the direct (i.e. free space) generation of optical vortices, with well-defined and controllable values of orbital angular momentum, paving the way to the engineering and control of novel types of phase discontinuities (i.e., phase dislocation loops) in compact, chip-scale optical devices. Finally, we report on the design, modeling, and experimental demonstration of array-enhanced nanoantennas for polarization-controlled multispectral nanofocusing, nanoantennas for resonant near-field optical concentration of radiation to individual nanowires, and aperiodic double resonance surface enhanced Raman scattering substrates
Recommended from our members
Quasicrystalline optical lattices for ultracold atoms
Quasicrystals are long-range ordered and yet non-periodic. This interplay results in a wealth
of intriguing physical phenomena, such as the inheritance of topological properties from higher
dimensions, self-similarity, and the presence of non-trivial structure on all scales. The concept of
aperiodic order has been extensively studied in mathematics and geometry, exemplified by the
celebrated Penrose tiling. However, the understanding of physical quasicrystals (the vast majority
of them are intermetallic compounds) is still incomplete owing to their complexity, regarding
both growth processes and stability.
Ultracold atoms in optical lattices offer an ideal, yet untested environment for investigating
quasicrystals. Optical lattices, i.e. standing waves of light, allow the defect-free formation of a
large variety of potential landscapes, including quasiperiodic geometries. In recent years, optical
lattices have become one of the most successful tools in the large-scale quantum simulation of
condensed-matter problems.
This study presents the first experimental realisation of a two-dimensional quasicrystalline potential
for ultracold atoms, based on an eightfold symmetric optical lattice. It is aimed at bringing
together the fields of ultracold atoms and quasicrystals – and the more general concept of
aperiodic order. The first part of this thesis introduces the theoretical aspects of aperiodic order
and quasicrystalline structure. The second part comprises a detailed account of the newly designed
apparatus that has been used to produce quantum-degenerate gases in quasicrystalline
lattices. The third and final part summarises the matter-wave diffraction experiments that have
been performed in various lattice geometries. These include one- and two-dimensional simple
cubic lattices, one-dimensional quasiperiodic lattices, as well as two-dimensional quasicrystalline
lattices. The striking self-similarity of this quasicrystalline structure has been directly
observed, in close analogy to Shechtman’s very first discovery of quasicrystals using electron
diffraction. In addition, an in-depth study of the diffraction dynamics reveals the fundamental
differences between periodic and quasicrystalline lattices, in excellent agreement with ab initio
theory. The diffraction dynamics on short timescales constitutes a continuous-time quantum
walk on a homogeneous four-dimensional tight-binding lattice.
On the one hand, these measurements establish a novel experimental platform for investigating
quasicrystals proper. On the other hand, ultracold atoms in quasicrystalline optical lattices are
worth studying in their own right: Possible avenues include the observation many-body localisation
and Bose glasses, as well as the creation of topologically non-trivial systems in higher
dimensions.Funded in part by a Vice-Chancellor's Award from the Cambridge Commonwealth, European, and International Trus
DISCOVERING PECULIAR NANOSTRUCTURES IN TWO-DIMENSIONAL MATERIALS BY SCANNING TUNNELING MICROSCOPY
Ph.DDOCTOR OF PHILOSOPH
Subshifts with Simple Cellular Automata
A subshift is a set of infinite one- or two-way sequences over a fixed finite set, defined by a set of forbidden patterns. In this thesis, we study subshifts in the topological setting, where the natural morphisms between them are ones defined by a (spatially uniform) local rule. Endomorphisms of subshifts are called cellular automata, and we call the set of cellular automata on a subshift its endomorphism monoid. It is known that the set of all sequences (the full shift) allows cellular automata with complex dynamical and computational properties. We are interested in subshifts that do not support such cellular automata. In particular, we study countable subshifts, minimal subshifts and subshifts with additional universal algebraic structure that cellular automata need to respect, and investigate certain criteria of ‘simplicity’ of the endomorphism monoid, for each of them. In the case of countable subshifts, we concentrate on countable sofic shifts, that is, countable subshifts defined by a finite state automaton. We develop some general tools for studying cellular automata on such subshifts, and show that nilpotency and periodicity of cellular automata are decidable properties, and positive expansivity is impossible. Nevertheless, we also prove various undecidability results, by simulating counter machines with cellular automata. We prove that minimal subshifts generated by primitive Pisot substitutions only support virtually cyclic automorphism groups, and give an example of a Toeplitz subshift whose automorphism group is not finitely generated. In the algebraic setting, we study the centralizers of CA, and group and lattice homomorphic CA. In particular, we obtain results about centralizers of symbol permutations and bipermutive CA, and their connections with group structures.Siirretty Doriast
A model of sonority based on pitch intelligibility
Synopsis:
Sonority is a central notion in phonetics and phonology and it is essential for generalizations related to syllabic organization. However, to date there is no clear consensus on the phonetic basis of sonority, neither in perception nor in production. The widely used Sonority Sequencing Principle (SSP) represents the speech signal as a sequence of discrete units, where phonological processes are modeled as symbol manipulating rules that lack a temporal dimension and are devoid of inherent links to perceptual, motoric or cognitive processes. The current work aims to change this by outlining a novel approach for the extraction of continuous entities from acoustic space in order to model dynamic aspects of phonological perception. It is used here to advance a functional understanding of sonority as a universal aspect of prosody that requires pitch-bearing syllables as the building blocks of speech.
This book argues that sonority is best understood as a measurement of pitch intelligibility in perception, which is closely linked to periodic energy in acoustics. It presents a novel principle for sonority-based determinations of well-formedness – the Nucleus Attraction Principle (NAP). Two complementary NAP models independently account for symbolic and continuous representations and they mostly outperform SSP-based models, demonstrated here with experimental perception studies and with a corpus study of Modern Hebrew nouns.
This work also includes a description of ProPer (Prosodic Analysis with Periodic Energy). The ProPer toolbox further exploits the proposal that periodic energy reflects sonority in order to cover major topics in prosodic research, such as prominence, intonation and speech rate. The book is finally concluded with brief discussions on selected topics: (i) the phonotactic division of labor with respect to /s/-stop clusters; (ii) the debate about the universality of sonority; and (iii) the fate of the classic phonetics–phonology dichotomy as it relates to continuity and dynamics in phonology
Sobre a aplicação de técnicas de controlo em redes industriais com falhas
Doutoramento em Engenharia EletrotécnicaThe performance of real-time networks is under continuous improvement as
a result of several trends in the digital world. However, these tendencies not
only cause improvements, but also exacerbates a series of unideal aspects
of real-time networks such as communication latency, jitter of the latency
and packet drop rate.
This Thesis focuses on the communication errors that appear on such realtime
networks, from the point-of-view of automatic control. Specifically, it
investigates the effects of packet drops in automatic control over fieldbuses,
as well as the architectures and optimal techniques for their compensation.
Firstly, a new approach to address the problems that rise in virtue of such
packet drops, is proposed. This novel approach is based on the simultaneous
transmission of several values in a single message. Such messages can
be from sensor to controller, in which case they are comprised of several
past sensor readings, or from controller to actuator in which case they are
comprised of estimates of several future control values. A series of tests
reveal the advantages of this approach.
The above-explained approach is then expanded as to accommodate the
techniques of contemporary optimal control. However, unlike the aforementioned
approach, that deliberately does not send certain messages in
order to make a more efficient use of network resources; in the second case,
the techniques are used to reduce the effects of packet losses.
After these two approaches that are based on data aggregation, it is also
studied the optimal control in packet dropping fieldbuses, using generalized
actuator output functions. This study ends with the development of a new
optimal controller, as well as the function, among the generalized functions
that dictate the actuator’s behaviour in the absence of a new control
message, that leads to the optimal performance.
The Thesis also presents a different line of research, related with the output
oscillations that take place as a consequence of the use of classic co-design
techniques of networked control. The proposed algorithm has the goal of
allowing the execution of such classical co-design algorithms without causing
an output oscillation that increases the value of the cost function. Such
increases may, under certain circumstances, negate the advantages of the
application of the classical co-design techniques. A yet another line of research, investigated algorithms, more efficient than
contemporary ones, to generate task execution sequences that guarantee
that at least a given number of activated jobs will be executed out of
every set composed by a predetermined number of contiguous activations.
This algorithm may, in the future, be applied to the generation of message
transmission patterns in the above-mentioned techniques for the efficient
use of network resources. The proposed task generation algorithm is better
than its predecessors in the sense that it is capable of scheduling systems
that cannot be scheduled by its predecessor algorithms.
The Thesis also presents a mechanism that allows to perform multi-path
routing in wireless sensor networks, while ensuring that no value will be
counted in duplicate. Thereby, this technique improves the performance of
wireless sensor networks, rendering them more suitable for control applications.
As mentioned before, this Thesis is centered around techniques for the
improvement of performance of distributed control systems in which several
elements are connected through a fieldbus that may be subject to packet
drops. The first three approaches are directly related to this topic, with the
first two approaching the problem from an architectural standpoint, whereas
the third one does so from more theoretical grounds. The fourth approach
ensures that the approaches to this and similar problems that can be found
in the literature that try to achieve goals similar to objectives of this Thesis,
can do so without causing other problems that may invalidate the solutions
in question. Then, the thesis presents an approach to the problem dealt
with in it, which is centered in the efficient generation of the transmission
patterns that are used in the aforementioned approaches.Em resultado de várias tendências que têm afetado o mundo digital, o desempenho
das redes de comunicação em tempo-real está continuamente a
ser melhorado. No entanto, tais tendências não só introduzem melhorias,
como também introduzem uma série de não idealidades, tais como a
latência, o jitter da latência de comunicação e uma maior probabilidade de
perda de pacotes.
Esta tese tem o seu cerne em falhas de comunicação que surgem em tais
redes, sob o ponto de vista do controlo automático. Concretamente, são
estudados os efeitos das perdas de pacotes em redes de controlo, bem como
arquitecturas e técnicas óptimas de compensação das mesmas.
Primeiramente, ´e proposta uma nova abordagem para colmatar os problemas
que surgem em virtude de tais perdas. Essa nova abordagem ´e baseada
no envio simultâneo de vários valores numa única mensagem. Tais mensagens
podem ser de sensor para controlador, caso em que as mesmas são
constituídas por um conjunto de valores passados, ou de controlador para
actuador, caso em que tais mensagens contˆem estimativas de futuros valores
de controlo. Uma série de testes revela as vantagens de tal abordagem.
A abordagem acima explanada ´e seguidamente expandida de modo a acomodar
o controlo óptimo. Contudo, ao contrário da abordagem acima
apresentada, que passa pelo não envio deliberado de certas mensagens com
vista a alcançar um uso mais eficiente dos recursos de rede; no presente
caso, as técnicas são usadas para reduzir os efeitos da perda de pacotes.
Em seguida são estudadas abordagens de controlo óptimo que em situações
de perda de pacotes empregam formas generalizadas da aplicação de valores
de saída. Este estudo culmina com o desenvolvimento de um novo
controlador óptimo, bem como a função, entre as funções generalizadas
do funcionamento do actuador, que conduz o sistema a um desempenho óptimo. É
também apresentada uma linha de investigação diferente, relacionada com a oscilação da saída que ocorre em consequência da utilização de técnicas e algoritmos clássicos de co-desenho de controlo e redes industriais. O algoritmo
proposto tem como finalidade permitir que tais algoritmos clássicos
possam ser executados sem causar oscilações de saída, oscilações que por
sua vez aumentam o valor da função de custo. Tais aumentos da função
do custo, podem, em certas circunstâncias, por em causa os benefícios da
aplicação das técnicas de co-desenho clássico. Numa outra linha de investigação, foram estudadas formas, mais eficientes
que as contemporâneas, de geração de sequências de execuções de tarefas
que garantam que pelo menos um dado número de tarefas activadas
serão executadas por cada conjunto contíguo composto por um número
predefinido de activações. Esta técnica poderá, no futuro, ser aplicada na
geração dos padrões de envio de mensagens que ´e empregue na abordagem
de utilização eficiente dos recursos de rede acima referida. A técnica proposta
de geração de tarefas é melhor que as anteriores no sentido em que
a mesma é capaz de escalonar sistemas que não são escalonáveis pelas
técnicas clássicas.
A tese também apresenta um mecanismo que permite fazer o encaminhamento
multi-caminho em redes de sensores sem fios com falhas sem
causar a contagem em duplicado. Assim sendo a mesma técnica melhora
o desempenho das redes de sensores sem fios, tornando as mesmas mais
maleável as necessidades do controlo aum´atico em redes sem fios.
Como foi referido acima, a tese foca-se em t´ecnicas de melhoria de desempenho
de sistemas de controlo distribu´ıdo em que os v´arios elementos
de controlo encontram-se interligados por meio de uma rede industrial que
pode estar sujeita a perda de pacotes. As primeiras três abordagens cingemse
a este tema, sendo que primeiras duas olham para o problema sob um
ponto de vista arquitetural, enquanto que a terceira olha sob um ponto de
vista mais teórico. A quarta abordagem garante que outras propostas que
podem ser encontradas na literatura e que visam atingir resultados semelhantes
aos que se pretendem atingir nesta tese, possam fazˆe-lo sem causar
outros problemas que invalidem as soluções em questão. Seguidamente, é
apresenta-se uma abordagem ao problema proposto nesta tese que foca-se
na geração eficiente de padrões para subsequente utilização nas abordagens
acima referidas. E por fim, apresentar-se-a uma técnica de optimização do
funcionamento de redes sem fios que promete melhorar o controlo em tais
redes
Aeronautical Engineering, a continuing bibliography with indexes, supplement 173
This bibliography lists 704 reports, articles and other documents introduced into the NASA scientific and technical information system in March 1984
- …