1,720,962 research outputs found
Photonics in the ferroelectric super-crystal phase
No full textNanodisordered ferroelectric perovskites belong to the family of relaxor ferroelectrics, and have long been attracting considerable attention in view of their unique physical properties. The introduction of compositional disorder on the nanoscale leads to the appearance of a broad temperature and frequency dependent peak in the dielectric susceptibility that manifests thermal, electric field, and strain hysteresis and is associated with anomalous relaxation. The presence of different compounds introduces, for specific composition concentrations, competing structural phases leading to unique polarization properties, such as the anomalous large capacitance and the giant piezoelectric effect. Recently, a new ferroelectric phase of matter, the spontaneous super-crystal phase (SC), has been discovered in bulk solid-solution of nanodisordered ferroelectric perovskite, several degrees below the Curie point. In this phase, domains, instead of locking into a disorganized pattern of clusters, form a 3D regular lattice of spontaneous polarization with micrometer lattice constant across macroscopic samples.
This phase mimics standard solid-state structures but on scales that are thousands of times larger. The work presented in this thesis is an experimental investigation, through several photonics techniques, of the SC phase.
In order to investigate the properties of the underlying ferroelectric domains, we first analyze the light-polarization dynamics which emerge from the interplay of mesoscopic domain ordering and anisotropy. Results indicate that polarized light propagating through the SC spatially separates in its polarization components, of mutually orthogonal linear polarization states. Furthermore, performing diffraction and refraction experiments, we discover that the SC phase is also accompanied by a broadband giant refraction (GR). Here the effective index of refraction is greater than 26 across the entire visible spectrum, even though no optical resonance is in place. The result is a material with no chromatic aberration and no diffraction. The discovery of GR opens up a wholly new realm of study, allowing us to expand our investigation to the field of nonlinear optics. Enhanced response causes wavelength conversion to occur in the form of bulk Cherenkov radiation with an arbitrarily wide spectral acceptance, more than 100 nm in the near infrared spectrum, an ultra-wide angular acceptance, up to , with no polarization selectivity.
From a more fundamental point of view, trying to understand the behavior and physics of complexity-driven GR, in particular the role played by ferroelectric clusters, using a 3D orthographic cross-polarizer projection technique, we provide for the first time, direct imaging of fractal cluster percolation. We also study the effect that the SC, of micrometer-scale, has on the average atomic structure, using several results, obtained through different experimental techniques, from X-ray diffraction, to calorimetry. What we have found, is that the emergence of the SC is accompanied by a large scale and coherent anomalous lattice deformation.
Alongside the investigation of the SC phase, we have exploited the strong nonlinear optical response of disordered ferroelectric crystals at the phase transition,
which makes these materials suitable to study the physics of nonlinear waves.
In our study, we focus principally on the exploration of applications in electro-optic integrated circuits, based on linear and nonlinear waves, and on the analysis of the physical origin of so-called soliton rogue waves
Using Bessel Beams to Induce Optical Waveguides
No full textOptical fabrication of waveguides in a volume is limited by diffraction in the writing beams. We
demonstrate the use of nondiffracting waves in the form of Bessel beams to fabricate scalable optical wiring through direct writing in a photosensitive volume. Experiments are performed in paraelectric
potassium-lithium-tantalate-niobate (KLTN), where writing occurs through photogenerated space charge
while guiding and electro-optic functionality are supported by the quadratic electro-optic effect. The
method allows components to be integrated sequentially without interfering with each other during fabrication, an intrinsic superposition property that is used to realize single, double, and multiple waveguides,
and 1 × 2, 1 × 3, and 1 × 4 splitters, and electrically controlled optical switching
Observation of extreme nonreciprocal wave amplification from single soliton-soliton collisions
Full text linkWe report the observation of strong nonreciprocal soliton amplification mediated by a Raman-scattering-like effect in single isolated collisions. A pump soliton is found to lose the greater part of its energy to a signal soliton, irrespective of the pump-signal relative amplitude. The result is an efficient rectifying mechanism able to accumulate energy into extreme waves. Experiments are carried out through photorefractive soliton two-wave mixing with a gain coefficient of up to 80 cm-1 that emerges in conditions of nonlinear response, leading to the formation of rogue waves
Evidence of 1+1D photorefractive stripe solitons deep in the Kerr limit
No full textThe Kerr nonlinearity allows for exact analytic soliton solutions in 1 +1D. While nothing excludes that these solitons form in naturally occurring real-world 3D settings as solitary walls or stripes, their observation had previously been considered unfeasible because of the strong transverse instability intrinsic to the extended nonlinear perturbation. We report the observation of solitons that are fully compatible with the 1 +1D Kerr paradigm limit hosted in a 2 +1D system. The waves are stripe spatial solitons in bulk copper doped potassium-lithium-tantalate-niobate (KLTN) supported by unsaturated photorefractive screening nonlinearity. The parameters of the stripe solitons fit well, in the whole existence domain, with the 1 +1D existence curve that we derive for the first time in closed form starting from the saturable model of propagation. Transverse instability, that accompanies the solitons embedded in the 3D system, is found to have a gain length much longer than the crystal. Findings establish our system as a versatile platform for investigating exact soliton solutions in bulk settings and in exploring the role of dimensionality at the transition from integrable to nonintegrable regimes of propagation
Observation of polarization-maintaining light propagation in depoled compositionally disordered ferroelectrics
No full textWe investigate the evolution of the state of polarization of light propagating through bulk depoled composite ferroelectrics below the Curie temperature. In contrast to standard depoled ferroelectrics, where random birefringence causes depolarization and scattering, light is observed to suffer varying degrees of depolarization and remains fully polarized when linearly polarized along the crystal principal axes. The effect is found to be supported by the formation
of polarized speckles organized into a spatial lattice and occurs as the ferroelectric settles into a spontaneous supercrystal, a three-dimensional coherent mosaic of ferroelectric clusters. The polarization lattices gradually disappear as the ferroelectric state reduces to a disordered distribution of polar nanoregions above the critical point
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
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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