Spatio-temporal dynamics of quantum-well excitons

We investigate the lateral transport of excitons in ZnSe quantum wells by using time-resolved micro-photoluminescence enhanced by the introduction of a solid immersion lens. The spatial and temporal resolutions are 200 nm and 5 ps, respectively. Strong deviation from classical diffusion is observed up to 400 ps. This feature is attributed to the hot-exciton effects, consistent with previous experiments under cw excitation. The coupled transport-relaxation process of hot excitons is modelled by Monte Carlo simulation. We prove that two basic assumptions typically accepted in photoluminescence investigations on excitonic transport, namely (i) the classical diffusion model as well as (ii) the equivalence between the temporal and spatial evolution of the exciton population and of the measured photoluminescence, are not valid for low-temperature experiments.Comment: 8 pages, 6 figure

The FuturICT education accelerator

Education is a major force for economic and social wellbeing. Despite high aspirations, education at all levels can be expensive and ineffective. Three Grand Challenges are identified: (1) enable people to learn orders of magnitude more effectively, (2) enable people to learn at orders of magnitude less cost, and (3) demonstrate success by exemplary interdisciplinary education in complex systems science. A ten year ‘man-on-the-moon’ project is proposed in which FuturICT’s unique combination of Complexity, Social and Computing Sciences could provide an urgently needed transdisciplinary language for making sense of educational systems. In close dialogue with educational theory and practice, and grounded in the emerging data science and learning analytics paradigms, this will translate into practical tools (both analytical and computational) for researchers, practitioners and leaders; generative principles for resilient educational ecosystems; and innovation for radically scalable, yet personalised, learner engagement and assessment. The proposed Education Accelerator will serve as a ‘wind tunnel’ for testing these ideas in the context of real educational programmes, with an international virtual campus delivering complex systems education exploiting the new understanding of complex, social, computationally enhanced organisational structure developed within FuturICT

The FuturICT education accelerator

Education is a major force for economic and social wellbeing. Despite high aspirations, education at all levels can be expensive and ineffective. Three Grand Challenges are identified: (1) enable people to learn orders of magnitude more effectively, (2) enable people to learn at orders of magnitude less cost, and (3) demonstrate success by exemplary interdisciplinary education in complex systems science. A ten year ‘man-on-the-moon’ project is proposed in which FuturICT’s unique combination of Complexity, Social and Computing Sciences could provide an urgently needed transdisciplinary language for making sense of educational systems. In close dialogue with educational theory and practice, and grounded in the emerging data science and learning analytics paradigms, this will translate into practical tools (both analytical and computational) for researchers, practitioners and leaders; generative principles for resilient educational ecosystems; and innovation for radically scalable, yet personalised, learner engagement and assessment. The proposed Education Accelerator will serve as a ‘wind tunnel’ for testing these ideas in the context of real educational programmes, with an international virtual campus delivering complex systems education exploiting the new understanding of complex, social, computationally enhanced organisational structure developed within FuturICT

PICOSECOND CASCADABLE NOR GATE ON CdSe AND GaAs USING FREQUENCY DOUBLING

Nous étudions la mise en série de portes Non-Ou picosecondes en décrivant le fonctionnement d'un inverseur tout optique où l'on utilise le doublage de fréquence du signal de sortie. Le dispositif ne comporte pas de cavité Fabry-Pérot. Une porte optique picoseconde cascadable est donc possible en mode monochromatique. Des expériences sur CdSe sont réalisées à 77 K et à la longueur d'onde 673 nm. L'énergie de commande de la porte Non-Ou est de 30 µJ/cm2/impulsion à 337 nm. Des résultats préliminaires sur une porte Non-Ou en GaAs, fonctionnant à la température ambiante sont décrits ; la longueur d'onde du signal modulé est 1,57 µm.We report a first step toward cascadable picosecond NOR gates by demonstrating the possible operation of an all-optical inverter gate based on the frequency doubling of the output signal between NOR stages. No Fabry-Perot cavity is required. This implies that a cascadable all-optical NOR gate operating in an effective one-wavelength mode is possible. Experiments were carried out on CdSe at liquid Nitrogen temperature. The operation wavelength is around 673 nm in CdSe. The switching energy of the NOR gate is 30 µJ/cm2/pulse at 337 nm. Preliminary experiments on room temperature GaAs NOR gate without Fabry-Perot cavity are also reported. The modulation wavelength is around 1.57 µm

Concevoir des stratégies de gestion de la végétation forestière à partir des mécanismes et dynamiques de la compétition entre les arbres et la végétation les entourant

International audiencePlant interactions can be defined as the ways plants act upon the growth, fitness, survival and reproduction of other plants, largely by modifying their environment. These interactions can be positive (facilitation) or negative (competition or exploitation). During plantation establishment or natural forest regeneration after a disturbance, high light levels and sometimes increased availability of water and nutrients favor the development of opportunistic, fast-growing herbaceous and woody species which capture resources at the expense of crop trees. As a consequence, the growth and survival of crop trees can be dramatically reduced. Although the effects of this competition are well documented, the physical and physiological mechanisms of competition are not. Moreover, the competition process is never constant in time or space. We present a conceptual competition model based on plant growth forms common in global forests, i.e., graminoids, forbs, small shrubs, large shrubs and midstorey trees, and main storey trees. Their competitive attributes and successional dynamics are examined. An overview is presented on the way forest vegetation management (FVM) treatments influence these components and outcomes regarding crop tree performance and diversity conservation. Finally, a synthesis of literature yields FVM guidelines for efficiently optimizing crop tree performance and safeguarding diversity. Future research needs to further sustainable FVM are presented

Enhanced self-configurability and yield in multicore grids

ISBN 978-1-4244-4596-7International audienceAs we move deeper in the nanotechnology era, computer architecture is solicited to manipulate tremendous numbers of devices per chip with high defect densities. These trends provide new computing opportunities but efficiently exploiting them will require a shift towards novel, highly parallel architectures. Fault tolerant mechanisms will have to be integrated to the design to deal with the low yield of future nanofabrication processes. In this paper we consider multi processor grid (MPG) architectures that assure scalability beyond hundreds of cores per chip. We study self-diagnosis and self-configuration methods at the architectural level and propose an enhanced self-configuration methodology that enables usage of a maximum percentage of available fault-free cores in MPGs with high defect densities. We show that our approach achieves usability of all fault-free cores for the case of fault-free routers whereas previous work was efficient for defect densities of up to 20-25% of defective cores. We also address the case of faulty routers, achieving usability of almost all fault-free nodes (fault-free cores having a fault-free router) for very high defect densities both in the cores and in the routers

The cooperative royal road: Avoiding Hitchhiking

We propose using the so called Royal Road functions as test functions for cooperative co-evolutionary algorithms (CCEAs). The Royal Road functions were created in the early 90's with the aim of demonstrating the superiority of genetic algorithms over local search methods. Unexpectedly, the opposite was found to be true. The research deepened our understanding of the phenomenon of hitchhiking where unfavorable alleles may become established in the population following an early association with an instance of a highly fit schema. Here, we take advantage of the modular and hierarchical structure of the Royal Road functions to adapt them to a co-evolutionary setting. Using a multiple population approach, we show that a CCEA easily outperforms a standard genetic algorithm on the Royal Road functions, by naturally overcoming the hitchhiking effect. Moreover, we found that the optimal number of sub-populations for the CCEA is not the same as the number of components that the function can be linearly separated into, and propose an explanation for this behavior. We argue that this class of functions may serve in foundational studies of cooperative co-evolution