245 research outputs found
Tunable reflection minima of nanostructured antireflective surfaces
Broadband antireflection schemes for silicon surfaces based on the moth-eye principle and comprising arrays of subwavelength-scale pillars are applicable to solar cells, photodetectors, and stealth technologies and can exhibit very low reflectances. We show that rigorous coupled wave analysis can be used to accurately model the intricate reflectance behavior of these surfaces and so can be used to explore the effects of variations in pillar height, period, and shape. Low reflectance regions are identified, the extent of which are determined by the shape of the pillars. The wavelengths over which these low reflectance regions operate can be shifted by altering the period of the array. Thus the subtle features of the reflectance spectrum of a moth-eye array can be tailored for optimum performance for the input spectrum of a specific application
Generalized scattering-matrix approach for magneto-optics in periodically patterned multilayer systems
We present here a generalization of the scattering-matrix approach for the
description of the propagation of electromagnetic waves in nanostructured
magneto-optical systems. Our formalism allows us to describe all the key
magneto-optical effects in any configuration in periodically patterned
multilayer structures. The method can also be applied to describe periodic
multilayer systems comprising materials with any type of optical anisotropy. We
illustrate the method with the analysis of a recent experiment in which the
transverse magneto-optical Kerr effect was measured in a Fe film with a
periodic array of subwavelength circular holes. We show, in agreement with the
experiments, that the excitation of surface plasmon polaritons in this system
leads to a resonant enhancement of the transverse magneto-optical Kerr effect.Comment: 12 pages, 6 figures, submitted to Physical Review
Resonances in Ferromagnetic Gratings Detected by Microwave Photoconductivity
We investigate the impact of microwave excited spin excitations on the DC
charge transport in a ferromagnetic (FM) grating. We observe both resonant and
nonresonant microwave photoresistance. Resonant features are identified as the
ferromagnetic resonance (FMR) and ferromagnetic antiresonance (FMAR). A
macroscopic model based on Maxwell and Landau-Lifschitz equations reveals the
macroscopic nature of the FMAR. The experimental approach and results provide
new insight in the interplay between photonic, spintronic, and charge effects
in FM microstructures.Comment: 4 pages, 4 figure
Gap maps and intrinsic diffraction losses in one-dimensional photonic crystal slabs
A theoretical study of photonic bands for one-dimensional (1D) lattices
embedded in planar waveguides with strong refractive index contrast is
presented. The approach relies on expanding the electromagnetic field on the
basis of guided modes of an effective waveguide, and on treating the coupling
to radiative modes by perturbation theory. Photonic mode dispersion, gap maps,
and intrinsic diffraction losses of quasi-guided modes are calculated for the
case of self-standing membranes as well as for Silicon-on-Insulator structures.
Photonic band gaps in a waveguide are found to depend strongly on the core
thickness and on polarization, so that the gaps for transverse electric and
transverse magnetic modes most often do not overlap. Radiative losses of
quasi-guided modes above the light line depend in a nontrivial way on structure
parameters, mode index and wavevector. The results of this study may be useful
for the design of integrated 1D photonic structures with low radiative losses.Comment: 9 pages, 8 figures, submitted to Physical Review
Grating theory: new equations in fourier space leading to fast converging results for tm polarization
Using theorems of Fourier factorization, a recent paper [J. Opt. Soc. Am. A 13, 1870 (1996)] has shown that the truncated Fourier series of products of discontinuous functions that were used in the differential theory of gratings during the past 30 years are not converging everywhere in TM polarization. They turn out to be converging everywhere only at the limit of infinitely low modulated gratings. We derive new truncated equations and implement them numerically. The computed efficiencies turn out to converge about as fast as in the TE-polarization case with respect to the number of Fourier harmonics used to represent the field. The fast convergence is observed on both metallic and dielectric gratings with sinusoidal, triangular, and lamellar profiles as well as with cylindrical and rectangular rods, and examples are shown on gratings with 100% modulation. The new formulation opens a new wide range of applications of the method, concerning not only gratings used in TM polarization but also conical diffraction, crossed gratings, three-dimensional problems, nonperiodic objects, rough surfaces, photonic band gaps, nonlinear optics, etc. The formulation also concerns the TE polarization case for a grating ruled on a magnetic material as well as gratings ruled on anisotropic materials. The method developed is applicable to any theory that requires the Fourier analysis of continuous products of discontinuous periodic functions; we propose to call it the fast Fourier factorization method
Identification of the material behavior of adhesive joints under dynamic multiaxial loadings
International audienceThis paper presents a numerical inverse method dedicated to the characterization of adhesive joints under multiaxial and dynamic loading conditions. The properties under scrutiny are the constitutive behavior of the joint as well as the final fracture surface. The experimental setup consists of a Split Hopkinson Pressure Bar system and local strain measurements performed by Digital Image Correlation (DIC) as well as a novel specific sandwich specimen denoted as DODECA. The direct numerical model is an original Finite Element computation combining 3D and 1D elements for an optimal handling of wave reflection and interfaces. It further provides an optimal compromise between computation time and accuracy. The identification method is based on the Finite Element Model Updating method (FEMU). Material parameters are identified for three different multiaxial loading conditions and presented as yield and fracture surfaces in the space of equivalent von Mises stress vs hydrostatic stress. As a complement of the analysis, uncertainties and confidence of the identified parameters are estimated with precise qualitative tools
Simplified open repair for anterior chest wall deformities. Analysis of results in 205 patients
SummaryIntroductionPectus deformities are the most frequently seen congenital thoracic wall anomalies. The cause of these conditions is thought to be abnormal elongation of the rib cartilages. We here report our clinical experience and the results of a sternochondroplasty procedure based on the subperichondrial resection of the elongated cartilages.HypothesisThis technique is a valuable surgical strategy to treat the wide variety of pectus deformities.Patients and methodsDuring the period from October 2001 through September 2009, 205 adult patients (171 men and 34 women) underwent pectus excavatum (181), carinatum (19) or arcuatum (5) repair. The patients’ pre and postoperative data were collected using a computerized database, and the results were assessed with a minimum 2-year follow-up.ResultsThe postoperative morbidity rate was minimal and the mortality was nil. The surgeon graded cosmetic results as excellent (72.5%), good (25%) or fair (2.5%), while patients reported better results. Patients with pectus excavatum were found to have much more patent foramen ovale (PFO) than the normal adult population, which occluded after the procedure in 61% of patients, and significant improvement was found in exercise cardiopulmonary function and exercise tolerance at the 1-year follow-up.DiscussionOur sternochondroplasty technique based on the subperichondrial resection of the elongated cartilages allows satisfactory repair of both pectus excavatum and sternal prominence. It is a safe procedure that might improve the effectiveness of surgical therapy in patients with pectus deformities.Level of evidenceLevel IV. Retrospective study
Total light absorption in graphene
We demonstrate that 100% light absorption can take place in a single
patterned sheet of doped graphene. General analysis shows that a planar array
of small lossy particles exhibits full absorption under critical-coupling
conditions provided the cross section of each individual particle is comparable
to the area of the lattice unit-cell. Specifically, arrays of doped graphene
nanodisks display full absorption when supported on a substrate under total
internal reflection, and also when lying on a dielectric layer coating a metal.
Our results are relevant for infrared light detectors and sources, which can be
made tunable via electrostatic doping of graphene.Comment: 4 figure
Robust diameter-based thickness estimation of 3D objects
International audienceWe propose a robust thickness estimation approach for 3D objects based on the Shape Diameter Function (SDF). Our method first applies a modified strategy to estimate the local diameter with increased accuracy. We then compute a scale-dependent robust thickness estimate from a point cloud, constructed using this local diameter estimation and a variant of a robust distance function. The robustness of our method is benchmarked against several operations such as remeshing, geometric noise and artifacts common in triangle soups. The experimental results show a more stable local thickness estimation than the original SDF, and consistent segmentation results on defect-laden inputs
Optical properties of photonic crystal slabs with asymmetrical unit cell
Using the unitarity and reciprocity properties of the scattering matrix, we
analyse the symmetry and resonant optical properties of the photonic crystal
slabs (PCS) with complicated unit cell. We show that the reflectivity is not
changed upon the 180deg-rotation of the sample around the normal axis, even in
PCS with asymmetrical unit cell. Whereas the transmissivity becomes
asymmetrical if the diffraction or absorption are present. The PCS reflectivity
peaks to unity near the quasiguided mode resonance for normal light incidence
in the absence of diffraction, depolarisation, and absorptive losses. For the
oblique incidence the full reflectivity is reached only in symmetrical PCS.Comment: 5 pages, 2 Postscript figure
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