43,759 research outputs found
Surface plasmon enhanced light-emitting diode
A method for enhancing the emission properties of light-emitting diodes, by coupling to surface plasmons, is analyzed both theoretically and experimentally. The analyzed structure consists of a semiconductor emitter layer thinner than λ/2 sandwiched between two metal films. If a periodic pattern is defined in the top semitransparent metal layer by lithography, it is possible to efficiently couple out the light emitted from the semiconductor and to simultaneously enhance the spontaneous emission rate. For the analyzed designs, we theoretically estimate extraction efficiencies as high as 37% and Purcell factors of up to 4.5. We have experimentally measured photoluminescence intensities of up to 46 times higher in fabricated structures compared to unprocessed wafers. The increased light emission is due to an increase in the efficiency and an increase in the pumping intensity resulting from trapping of pump photons within the microcavity
Punctuated vortex coalescence and discrete scale invariance in two-dimensional turbulence
We present experimental evidence and theoretical arguments showing that the
time-evolution of freely decaying 2-d turbulence is governed by a {\it
discrete} time scale invariance rather than a continuous time scale invariance.
Physically, this reflects that the time-evolution of the merging of vortices is
not smooth but punctuated, leading to a prefered scale factor and as a
consequence to log-periodic oscillations. From a thorough analysis of freely
decaying 2-d turbulence experiments, we show that the number of vortices, their
radius and separation display log-periodic oscillations as a function of time
with an average log-frequency of ~ 4-5 corresponding to a prefered scaling
ratio of ~ 1.2-1.3Comment: 22 pages and 38 figures. Submitted to Physica
Multiscale Surrogate Modeling and Uncertainty Quantification for Periodic Composite Structures
Computational modeling of the structural behavior of continuous fiber
composite materials often takes into account the periodicity of the underlying
micro-structure. A well established method dealing with the structural behavior
of periodic micro-structures is the so- called Asymptotic Expansion
Homogenization (AEH). By considering a periodic perturbation of the material
displacement, scale bridging functions, also referred to as elastic correctors,
can be derived in order to connect the strains at the level of the
macro-structure with micro- structural strains. For complicated inhomogeneous
micro-structures, the derivation of such functions is usually performed by the
numerical solution of a PDE problem - typically with the Finite Element Method.
Moreover, when dealing with uncertain micro-structural geometry and material
parameters, there is considerable uncertainty introduced in the actual stresses
experienced by the materials. Due to the high computational cost of computing
the elastic correctors, the choice of a pure Monte-Carlo approach for dealing
with the inevitable material and geometric uncertainties is clearly
computationally intractable. This problem is even more pronounced when the
effect of damage in the micro-scale is considered, where re-evaluation of the
micro-structural representative volume element is necessary for every occurring
damage. The novelty in this paper is that a non-intrusive surrogate modeling
approach is employed with the purpose of directly bridging the macro-scale
behavior of the structure with the material behavior in the micro-scale,
therefore reducing the number of costly evaluations of corrector functions,
allowing for future developments on the incorporation of fatigue or static
damage in the analysis of composite structural components.Comment: Appeared in UNCECOMP 201
A domain decomposition strategy to efficiently solve structures containing repeated patterns
This paper presents a strategy for the computation of structures with
repeated patterns based on domain decomposition and block Krylov solvers. It
can be seen as a special variant of the FETI method. We propose using the
presence of repeated domains in the problem to compute the solution by
minimizing the interface error on several directions simultaneously. The method
not only drastically decreases the size of the problems to solve but also
accelerates the convergence of interface problem for nearly no additional
computational cost and minimizes expensive memory accesses. The numerical
performances are illustrated on some thermal and elastic academic problems
On the evaluation of quasi-periodic Green functions and wave-scattering at and around Rayleigh-Wood anomalies
This article presents full-spectrum, well-conditioned, Green-function methodologies for evaluation of scattering by general periodic structures, which remains applicable on a set of challenging singular configurations, usually called Rayleigh-Wood (RW) anomalies (at which the quasi-periodic Green function ceases to exist), where most existing quasi-periodic solvers break down. After reviewing a variety of existing fast-converging numerical procedures commonly used to compute the classical quasi-periodic Green-function, the present work explores the difficulties they present around RW-anomalies and introduces the concept of hybrid âspatial/spectralâ representations. Such expressions allow both the modification of existing methods to obtain convergence at RW-anomalies as well as the application of a slight generalization of the Woodbury-Sherman-Morrison formulae together with a limiting procedure to bypass the singularities. (Although, for definiteness, the overall approach is applied to the scalar (acoustic) wave-scattering problem in the frequency domain, the approach can be extended in a straightforward manner to the harmonic Maxwell's and elasticity equations.) Ultimately, this thorough study of RW-anomalies yields fast and highly-accurate solvers, which are demonstrated with a variety of simulations of wave-scattering phenomena by arrays of particles, crossed impenetrable and penetrable diffraction gratings and other related structures. In particular, the methods developed in this article can be used to âupgradeâ classical approaches, resulting in algorithms that are applicable throughout the spectrum, and it provides new methods for cases where previous approaches are either costly or fail altogether. In particular, it is suggested that the proposed shifted Green function approach may provide the only viable alternative for treatment of three-dimensional high-frequency configurations with either one or two directions of periodicity. A variety of computational examples are presented which demonstrate the flexibility of the overall approach
Predictability of catastrophic events: material rupture, earthquakes, turbulence, financial crashes and human birth
We propose that catastrophic events are "outliers" with statistically
different properties than the rest of the population and result from mechanisms
involving amplifying critical cascades. Applications and the potential for
prediction are discussed in relation to the rupture of composite materials,
great earthquakes, turbulence and abrupt changes of weather regimes, financial
crashes and human parturition (birth).Comment: Latex document of 22 pages including 6 ps figures, in press in PNA
Surface Plasmon Excitation of Second Harmonic light: Emission and Absorption
We aim to clarify the role that absorption plays in nonlinear optical
processes in a variety of metallic nanostructures and show how it relates to
emission and conversion efficiency. We define a figure of merit that
establishes the structure's ability to either favor or impede second harmonic
generation. Our findings suggest that, despite the best efforts embarked upon
to enhance local fields and light coupling via plasmon excitation, nearly
always the absorbed harmonic energy far surpasses the harmonic energy emitted
in the far field. Qualitative and quantitative understanding of absorption
processes is crucial in the evaluation of practical designs of plasmonic
nanostructures for the purpose of frequency mixing
Three-dimensional quasi-periodic shifted Green function throughout the spectrum--including Wood anomalies
This work presents an efficient method for evaluation of wave scattering by
doubly periodic diffraction gratings at or near "Wood anomaly frequencies". At
these frequencies, one or more grazing Rayleigh waves exist, and the lattice
sum for the quasi-periodic Green function ceases to exist. We present a
modification of this sum by adding two types of terms to it. The first type
adds linear combinations of "shifted" Green functions, ensuring that the
spatial singularities introduced by these terms are located below the grating
and therefore outside of the physical domain. With suitable coefficient choices
these terms annihilate the growing contributions in the original lattice sum
and yield algebraic convergence. Convergence of arbitrarily high order can be
obtained by including sufficiently many shifts. The second type of added terms
are quasi-periodic plane wave solutions of the Helmholtz equation which
reinstate certain necessary grazing modes without leading to blow-up at Wood
anomalies. Using the new quasi-periodic Green function, we establish, for the
first time, that the Dirichlet problem of scattering by a smooth doubly
periodic scattering surface at a Wood frequency is uniquely solvable. We also
present an efficient high-order numerical method based on the this new Green
function for the problem of scattering by doubly periodic three-dimensional
surfaces at and around Wood frequencies. We believe this is the first solver in
existence that is applicable to Wood-frequency doubly periodic scattering
problems. We demonstrate the proposed approach by means of applications to
problems of acoustic scattering by doubly periodic gratings at various
frequencies, including frequencies away from, at, and near Wood anomalies
- âŠ