Non-collinear interaction of photons with orbital angular momentum

We elucidate the consequences of a phase-matching theory that describes second-harmonic generation of two non-collinear incident light beams that carry orbital angular momentum (OAM). More specifically, the two incident beams generate a third that, depending on the incident OAM, may experience a significantly smaller conversion efficiency in comparison to that based on the conventional phase-matching theory. This is the case even for incident angles substantially less than those required for non-conservation of OAM in the nonlinear interaction. Experiments are performed under different conditions and are in excellent agreement with the theory. Our results have implications beyond the specific case studied here of second-harmonic generation, in particular for parametric down-conversion of photons.Comment: 6 pages, 4 figure

Coherent control of light interaction with graphene

We report the experimental observation of all-optical modulation of light in a graphene film. The graphene film is scanned across a standing wave formed by two counter-propagating laser beams in a Sagnac interferometer. Through a coherent absorption process the on-axis transmission is modulated with close to 80% efficiency. Furthermore we observe modulation of the scattered energy by mapping the off-axis scattered optical signal: scattering is minimized at a node of the standing wave pattern and maximized at an antinode. The results highlight the possibility to switch and modulate any given optical interaction with deeply sub-wavelength films.Comment: 4 pages, 4 figure

Alpha-particle formation and clustering in nuclei

The nucleonic localization function has been used for a decade to study the formation of alpha-particles in nuclei, by providing a measure of having nucleons of a given spin in a single place. However, differences in interpretation remain, compared to the nucleonic density of the nucleus. In order to better understand the respective role of the nucleonic localization function and the densities in the alpha-particle formation in cluster states or in alpha-decay mechanism, both an analytic approximation and microscopic calculations, using energy density functionals, are undertaken. The nucleonic localization function is shown to measure the anti-centrifugal effect, and is not sensitive to the level of compactness of the alpha-particle itself. It probes the purity of the spatial overlap of four nucleons in the four possible (spin, isospin) states. The density provides, in addition, information on the compactness of an alpha-particle cluster.Comment: 8 pages, 5 figure

Theoretical study of finite temperature spectroscopy in van der Waals clusters. I. Probing phase changes in CaAr_n

The photoabsorption spectra of calcium-doped argon clusters CaAr_n are investigated at thermal equilibrium using a variety of theoretical and numerical tools. The influence of temperature on the absorption spectra is estimated using the quantum superposition method for a variety of cluster sizes in the range 6<=n<=146. At the harmonic level of approximation, the absorption intensity is calculated through an extension of the Gaussian theory by Wadi and Pollak [J. Chem. Phys. vol 110, 11890 (1999)]. This theory is tested on simple, few-atom systems in both the classical and quantum regimes for which highly accurate Monte Carlo data can be obtained. By incorporating quantum anharmonic corrections to the partition functions and respective weights of the isomers, we show that the superposition method can correctly describe the finite-temperature spectroscopic properties of CaAr_n systems. The use of the absorption spectrum as a possible probe of isomerization or phase changes in the argon cluster is discussed at the light of finite-size effects.Comment: 17 pages, 9 figure

A multiscale approach to image analysis using markov random fields

The use of Markov Random Field (MRF) models within the framework of global bayesian estimation luis recently brought new powerful solutions to standard image analysis problems . These models are generally associated with greedy relaxation algorithms. This is the reason why multiresolution methods, well known in Computational Mathematics, are widely used to speed up the convergence rate of these algorithms . But for the moment there is no real mathematical framework which associates in a simple and efficient way multigrid strategies and markovian models most previous multiresolution markovian models have been defined using various heuristics, especially as far as the adjustment of parameters over scale is concerned . In this paper we present a new class of globally consistent multiscale MRF models . The models we consider here are both mathematically consistent and computationally tractable and are related to a multiscale exploration of the set of solutions . We detail the application of these new models to two basic issues in motion analysis from an image sequence motion detection and 2D-motion estimation . e show the advantages of the new approach : it allows the relaxation schemes to converge faster than those associated with standard multiresolution approaches, and toward better estimates (i.e. estimates of lower energy) .L'introduction d'une modélisation statistique par champs de Markov a récemment permis des avancées importantes dans nombre de problèmes classiques en analyse d'images. Ces modèles sont généralement associés à des algorithmes d'optimisation globale par relaxation qui restent coûteux en temps de calcul dans certaines applications. Or les techniques multigrilles, par ailleurs classiques en analyse numérique, peuvent conduire à des gains importants sur ce point. Pour l'heure il n'existe cependant pas réellement de support théorique permettant d'associer de façon simple et efficace stratégie multigrille et modélisation markovienne. Les modèles markoviens multirésolutions utilisés pour l'instant sont donc élaborés de manière heuristique, en particulier en ce qui concerne l'ajustement des différents paramètres en fonction de la résolutio

A statistical model-based approach to unsupervised texture segmentation

The general problem of unsupervised textured segmentation remains a largely unsolved issue in image analysis . Many studies proved that statistical model based texture segmentation algorithms yield good reults provided that the model parameters and the number of regions are known a priori . In this paper the problem of determining the number of regions is addressed. The segmentation algorithm relies on the analysis of second and higher order spatial statistics of the original images. The segmentation map is represented using a Markov Random Field model and a bayesian estimate of this map is computed using a deterministic relaxation algorithm . The segmentation algorithm does only require the tuning of one parameter. Results on hand-drawn images of natural textures and real textured images show the capability of the model to yield relevant segmentations when the number of regions and the texture classes are not known a priori .La segmentation des images texturées constitue une étape préliminaire cruciale dans de nombreuses applications en analyse d'images. Les approches par modélisation statistique conduisent à de bons résultats dans ce domaine, lorsque les paramètres des modèles statistiques et le nombre de régions à extraire sont connus a priori. La segmentation non supervisée d'images texturées reste, par contre, un problème délicat, auquel aucune solution complète n'a été apportée jusqu'à present. Nous contribuons à cet effort, en proposant une méthode de segmentation ne nécessitant pas de connaissance a priori sur le nombre ou le type de textures présentes dans l'imag

On the predominant mechanisms active during the high power diode laser modification of the wettability characteristics of an SiO2/Al2O3-based ceramic material

The mechanisms responsible for modifications to the wettability characteristics of a SiO2/Al2O3-based ceramic material in terms of a test liquid set comprising of human blood, human blood plasma, glycerol and 4-octonol after high power diode laser (HPDL) treatment have been elucidated. Changes in the contact angle, , and hence the wettability characteristics of the SiO2/Al2O3-based ceramic were attributed primarily to: modifications to the surface roughness of the ceramic resulting from HPDL interaction which accordingly effected reductions in ; the increase in the surface O2 content of the ceramic after HPDL treatment; since an increase in surface O2 content intrinsically brings about a decrease in , and vice versa and the increase in the polar component of the surface energy, due to the HPDL induced surface melting and resolidification which consequently created a partially vitrified microstructure that was seen to augment the wetting action. However, the degree of influence exerted by each mechanism was found to differ markedly. Isolation of each of these mechanisms permitted the magnitude of their influence to be qualitatively determined. Surface energy, by way of microstructural changes, was found to be by far the most predominant element governing the wetting characteristics of the SiO2/Al2O3-based ceramic. To a much lesser extent, surface O2 content, by way of process gas, was also seen to influence to a changes in the wettability characteristics of the SiO2/Al2O3-based ceramic, whilst surface roughness was found to play a minor role in inducing changes in the wettability characteristics

A low complexity approximation of probabilistic appearance models

Appearance models yield a compact representation of shape, pose and illumination variations. The probabilistic appearance model, proposed by Moghaddam et al. (Moghaddam and Pentland, 1997; Tipping and Bishop, 1997b), has recently shown excellent performances in pattern detection and recognition, outperforming most other linear and non-linear approaches. Unfortunately, the complexity of this model remains high. In this paper, we introduce an efficient approximation of this model, which enables fast implementations in statistical estimation-based schemes. Gains in complexity and cpu time of more than 10 have been obtained, without any loss in the quality of the results.Les modèles d'apparence permettent d'encoder les variabilités de forme, de pose, et d'illumination dans une seule représentation compacte. Le modèle d'apparence probabiliste de Moghaddam et al. (Moghaddam and Pentland, 1997; Tipping and Bishop, 1997b), reposant sur une interprétation statistique de l'Analyse en Composantes Principales (ACP) s'est récemment illustré par ses excellentes performances en détection et en reconnaissance des formes, surpassant de nombreuses autres méthodes linéaires et non linéaires. Ce modèle, performant, se heurte toutefois à une complexité calculatoire importante. Nous proposons, dans cet article, une approximation de ce modèle qui se prête à une mise en oeuvre rapide, dans le cadre de schémas d'estimation statistique. Des gains en complexité et en temps de calcul supérieurs à 10, sont obtenus, sans aucune perte de qualité dans les résultats des traitements

Exciton and negative trion dissociation by an external electric field in vertically coupled quantum dots

We study the Stark effect for an exciton confined in a pair of vertically coupled quantum dots. A single-band approximation for the hole and a parabolic lateral confinement potential are adopted which allows for the separation of the lateral center-of-mass motion and consequently for an exact numerical solution of the Schr\"odinger equation. We show that for intermediate tunnel coupling the external electric field leads to the dissociation of the exciton via an avoided crossing of bright and dark exciton energy levels which results in an atypical form of the Stark shift. The electric-field-induced dissociation of the negative trion is studied using the approximation of frozen lateral degrees of freedom. It is shown that in a symmetric system of coupled dots the trion is more stable against dissociation than the exciton. For an asymmetric system of coupled dots the trion dissociation is accompanied by a positive curvature of the recombination energy line as a function of the electric field.Comment: PRB - in prin