Enhanced transmission of slit arrays in an extremely thin metallic film

Horizontal resonances of slit arrays are studied. They can lead to an enhanced transmission that cannot be explained using the single-mode approximation. A new type of cavity resonance is found when the slits are narrow for a wavelength very close to the period. It can be excited for very low thicknesses. Optimization shows these structures could constitute interesting monochromatic filters

Photon recycling in Fabry-Perot micro-cavities based on Si3_3N4_4 waveguides

We present a numerical analysis and preliminary experimental results on one-dimensional Fabry-Perot micro-cavities in Si$_3$N$_4$ waveguides. The Fabry-Perot micro-cavities are formed by two distributed Bragg reflectors separated by a straight portion of waveguide. The Bragg reflectors are composed by a few air slits produced within the Si$_3$N$_4$ waveguides. In order to increase the quality factor of the micro-cavities, we have minimized, with a multiparametric optimization tool, the insertion loss of the reflectors by varying the length of their first periods (those facing the cavity). To explain the simulation results the coupling of the fundamental waveguide mode with radiative modes in the Fabry-Perot micro-cavities is needed. This effect is described as a recycling of radiative modes in the waveguide. To support the modelling, preliminary experimental results of micro-cavities in Si$_3$N$_4$ waveguides realized with Focused Ion Beam technique are reported.Comment: 5 pages, 5 figure

Modélisation des échanges thermiques et radiatifs en environnement urbain à très haute résolution spatiale : aide à l’interprétation des mesures par télédétection infrarouge

The main origin of the energy consumption in France is in the residential and the commercial sectors. In urban environment, housing is mostly old, which means high heat losses. The improvement of energy performances requires the quantification of heat losses. This quantification may be based on a global measurement by infrared thermography at high spatial resolution.The infrared images interpretation requires a description of the radiative terms that make up the IR signal. For that purpose, a new simulator was developed. The temperature field is calculated from a meshed 3D scene composed of 1D walls and 2D thermal bridges (i.e, the thermal bridges located at floor slabs and at building corners). This is achieved with SUSHI, a thermal software we developed. It uses as input environmental data measured in the field, the solar irradiance pre-computed by SOLENE (CERMA) and the 2D unitary responses pre-computed by CASTEM (CEA). The infrared sensor signal is then simulated by the 3D radiative software MOHICANS we adapted for the purpose. The originality of this software chain is to provide an efficient fusion of the dynamic response simulations of temperature and radiance, for areas with 1D heat transfer and for areas with 2D heat transfer. SUSHI software allows evaluating the sensitivity of the temperature on a façade or on a roof to environmental measured data and it allows identifying the critical parameters therefrom.The experimental campaign BATIR was set-up for measuring the thermal behavior of a building façade and its convective and radiative environment. A local validation of the temperature calculation by SUSHI was achieved through a comparison with thermocouple measurements. Infrared cameras were operated in order to collect the radiance coming from the analyzed façade in band II and in band III. The radiances calculated by MOHICANS were compared with these acquisitions in order to validate the software chain at this level. As a result, the computation of a temperature field is possible when all parameters are measured with precision because estimation biases are prejudicial. The images in band III confirm this statement and the images in band II highlight the need, for this particular spectral band, for a better evaluation of the radiative environment.La consommation énergétique en France a pour origine principale les secteurs résidentiel et tertiaire. En environnement urbain, l’habitat est encore principalement ancien, ce qui implique des déperditions importantes. L’amélioration des performances énergétiques des bâtiments passe par une quantification des pertes thermiques, ce qui peut être basé sur une mesure globale par thermographie infrarouge à haute résolution spatiale.L’interprétation des images infrarouges acquises nécessite une description des termes radiatifs composant le signal IR, pour cela un simulateur original a été mis au point. A partir d’une scène 3D maillée, le champ de température est calculé pour les parois à transfert 1D et pour les ponts thermiques à transfert 2D (ex : ponts thermiques au droit des planchers et des arêtes de bâtiments) par un code thermique développé à cet effet, SUSHI, qui s’appuie sur un pré-calcul d’éclairement solaire par SOLENE (CERMA) et un pré-calcul de réponses indicielles 2D par CASTEM (CEA). Le calcul s’effectue à partir des données environnementales acquises sur le terrain. Le signal du capteur infrarouge est ensuite simulé par le code radiatif 3D MOHICANS que nous avons adapté pour le besoin.Cette chaîne logicielle a l’originalité de proposer, pour une scène donnée, une fusion efficace des simulations de la réponse dynamique en température et en luminance pour les zones 1D et pour les zones 2D. L’utilisation du code SUSHI permet d’évaluer la sensibilité de la température externe d’une paroi de bâtiment aux données environnementales mesurées et d’en spécifier les paramètres critiques.La mise en œuvre de la campagne expérimentale BATIR a permis de mesurer le comportement thermique d’une façade de bâtiment et de son environnement radiato-convectif. Une validation ponctuelle des températures calculées par SUSHI a été réalisée par confrontation à des mesures par thermocouple. Des caméras infrarouges ont été mises en œuvre afin de collecter la luminance issue de la façade étudiée en bande II et en bande III. Les luminances calculées par MOHICANS sont comparées à ces acquisitions et valident la chaîne logicielle à ce niveau. On en déduit que le calcul d’un champ de température est possible lorsque tous les paramètres sont mesurés avec précision car les biais d’estimation sont pénalisants. Les images en bande III illustrent cette observation et les images en bande II mettent en évidence la nécessité, pour cette bande particulière, d’améliorer l’évaluation de l’environnement radiatif

Ac conductivity and dielectric properties of CuFe1−xCrxO2 : Mg delafossite

The electrical and dielectric properties of CuFe(1−x)Cr(x)O(2) (0 ≤ x ≤ 1) powders, doped with 3% of Mg and prepared by solid-state reaction, were studied by broadband dielectric spectroscopy in the temperature range from −100 to 150 °C. The frequency-dependent electrical and dielectric data have been discussed in the framework of a power law conductivity and complex impedance and dielectric modulus. At room temperature, the ac conductivity behaviour is characteristic of the charge transport in CuFe1−xCrxO2 powders. The substitution of Fe3+ by Cr3+ results in an increase in dc conductivity and a decrease in the Cu+–Cu+ distance. Dc conductivity, characteristic onset frequency and Havriliak–Negami characteristics relaxation times are thermally activated above −40 °C for x = 0.835. The associated activation energies obtained from dc and ac conductivity and from impedance and modulus losses are similar and show that CuFe1−xCrxO2 delafossite powders satisfy the BNN relation. Dc and ac conductivities have the same transport mechanism, namely thermally activated nearest neighbour hopping and tunnelling hopping above and below −40 °C, respectively

Museum DNA reveals the demographic history of the endangered Seychelles warbler

The importance of evolutionary conservation – how understanding evolutionary forces can help guide conservation decisions – is widely recognized. However, the historical demography of many endangered species is unknown, despite the fact that this can have important implications for contemporary ecological processes and for extinction risk. Here, we reconstruct the population history of the Seychelles warbler (Acrocephalus sechellensis) – an ecological model species. By the 1960s, this species was on the brink of extinction, but its previous history is unknown. We used DNA samples from contemporary and museum specimens spanning 140 years to reconstruct bottleneck history. We found a 25% reduction in genetic diversity between museum and contemporary populations, and strong genetic structure. Simulations indicate that the Seychelles warbler was bottlenecked from a large population, with an ancestral Ne of several thousands falling to <50 within the last century. Such a rapid decline, due to anthropogenic factors, has important implications for extinction risk in the Seychelles warbler, and our results will inform conservation practices. Reconstructing the population history of this species also allows us to better understand patterns of genetic diversity, inbreeding and promiscuity in the contemporary populations. Our approaches can be applied across species to test ecological hypotheses and inform conservation

Enhancement of spatial coherence by surface plasmons

We report on a method to generate a stationary interference pattern from two independent optical sources, each illuminating a single slit in Young's interference experiment. The pattern arises as a result of the action of surface plasmons traveling between subwavelength slits milled in a metal film. The visibility of the interference pattern can be manipulated by tuning the wavelength of one of the optical sources. © 2007 Optical. Society of America

Development and validation of a numerical tool for the simulation of the temperature field and infrared radiance rendering in an urban scene

International audienceWe present a numerical tool aimed at simulating infrared images of an urban environment, by solving the direct heat transfer problem, and then computing the radiance rendering at the sensor level. SOLENE (Cerma, Nantes) was coupled with two software packages developed at ONERA: SUSHI (Simulation in Urban Scene of Heat dIffusion) and MOHICANS (MOdélisation Hyperspectrale d'Images en entrée Capteur pour l'ANalyse et l'inversion du Signal) for realizing this task. SUSHI is also used for computing the surface temperatures: either a 1D model or a 2D model is used. We present the whole software chain, its validation by software and experimental analysis

Characterizations of a KHz Pulsed Laser Detection System

A KHz Pulsed Laser Detection System was developed employing the concept of charge integration with an electrometer, in the NASA Goddard Space Flight Center, Code 618 Calibration Lab for the purpose of using the pulsed lasers for radiometric calibration. Comparing with traditional trans-impedance (current-voltage conversion) detection systems, the prototype of this system consists of a UV-Enhanced Si detector head, a computer controlled shutter system and a synchronized electrometer. The preliminary characterization work employs light sources running in either CW or pulsed mode. We believe this system is able to overcome the saturation issue when a traditional trans-impedance detection system is used with the pulsed laser light source, especially with high peak-power pulsed lasers operating at kilohertz repetition rates (e.g. Ekspla laser or KHz OPO). The charge integration mechanism is also expected to improve the stability of measurements for a pulsed laser light source overcoming the issue of peak-to-peak stability. We will present the system characterizations including signal-to-noise ratio and uncertainty analysis and compare results against traditional trans-impedance detection systems

Inversionless light amplification and optical switching controlled by state-dependent alignment of molecules

We propose a method to achieve amplification without population inversion by anisotropic molecules whose orientation by an external electric field is state-dependent. It is based on decoupling of the lower-state molecules from the resonant light while the excited ones remain emitting. The suitable class of molecules is discussed, the equation for the gain factor is derived, and the magnitude of the inversionless amplification is estimated for the typical experimental conditions. Such switching of the sample from absorbing to amplifying via transparent state is shown to be possible both with the aid of dc and ac control electric fields.Comment: AMS-LaTeX v1.2, 4 pages with 4 figure