Electron acceleration in vacuum by ultrashort and tightly focused radially polarized laser pulses

Exact closed-form solutions to Maxwell's equations are used to investigate electron acceleration driven by radially polarized laser beams in the nonparaxial and ultrashort pulse regime. Besides allowing for higher energy gains, such beams could generate synchronized counterpropagating electron bunches.Comment: 3 pages, 3 figures. To appear in the proceedings of the Ultrafast Phenomena XVIII conferenc

Resonator with variable-reflectivity output coupler

A novel resonator is proposed, in which the polarizing properties of standard grating and polarizing components (such as Brewster angle windows) inside the cavity are used to vary continuously the output coupling over a wide range of values. An analysis of the polarization properties of the modes of such a resonator is presented, and practical suggestions are made about the choice of the polarizing components. The results of an experiment involving a TEA CO2 laser are shown to verify some of the conclusions of the analysis

La situation démographique du Québec à l’heure du référendum

L’article présente la situation démographique du Québec à l’heure du référendum en resituant son évolution dans le contexte démographique du Canada et des pays industrialisés.Le Québec et le reste du Canada constituent deux entités démo-linguistiques qui se différencient de plus en plus et qui se réfèrent chacune à un territoire bien délimité. Le premier, sous l’effet de la Charte de la langue française (mesure gouvernementale favorisant l’épanouissement de cette langue au Québec), devrait devenir de plus en plus français dans la composition de sa population comme dans sa langue de travail, des communications et de l’affichage. Le second devrait devenir, quant à lui, de plus en plus anglais par suite de l’assimilation grandissante de ses minorités francophones et de l’apport d’une immigration internationale importante qui vient grossir sa majorité anglophone.La population du Québec, comme celle de la plupart des pays industrialisés, est appelée à connaître une croissance lente et un vieillissement de sa structure par âge, situation qui ne présente pas que des désavantages mais qui implique certaines reconversions. Plus inquiétante sans doute est la perte relative de poids démographique du Québec dans l’ensemble canadien et ses conséquences politiques

Electron acceleration driven by ultrashort and nonparaxial radially polarized laser pulses

Exact closed-form solutions to Maxwell's equations are used to investigate the acceleration of electrons in vacuum driven by ultrashort and nonparaxial radially polarized laser pulses. We show that the threshold power above which significant acceleration takes place is greatly reduced by using a tighter focus. Moreover, electrons accelerated by tightly focused single-cycle laser pulses may reach around 80% of the theoretical energy gain limit, about twice the value previously reported with few-cycle paraxial pulses. Our results demonstrate that the direct acceleration of electrons in vacuum is well within reach of the current laser technology.Comment: 3 pages. Accepted for publication in Optics Letter

From Sea to Sea: Canada's Three Oceans of Biodiversity

Evaluating and understanding biodiversity in marine ecosystems are both necessary and challenging for conservation. This paper compiles and summarizes current knowledge of the diversity of marine taxa in Canada's three oceans while recognizing that this compilation is incomplete and will change in the future. That Canada has the longest coastline in the world and incorporates distinctly different biogeographic provinces and ecoregions (e.g., temperate through ice-covered areas) constrains this analysis. The taxonomic groups presented here include microbes, phytoplankton, macroalgae, zooplankton, benthic infauna, fishes, and marine mammals. The minimum number of species or taxa compiled here is 15,988 for the three Canadian oceans. However, this number clearly underestimates in several ways the total number of taxa present. First, there are significant gaps in the published literature. Second, the diversity of many habitats has not been compiled for all taxonomic groups (e.g., intertidal rocky shores, deep sea), and data compilations are based on short-term, directed research programs or longer-term monitoring activities with limited spatial resolution. Third, the biodiversity of large organisms is well known, but this is not true of smaller organisms. Finally, the greatest constraint on this summary is the willingness and capacity of those who collected the data to make it available to those interested in biodiversity meta-analyses. Confirmation of identities and intercomparison of studies are also constrained by the disturbing rate of decline in the number of taxonomists and systematists specializing on marine taxa in Canada. This decline is mostly the result of retirements of current specialists and to a lack of training and employment opportunities for new ones. Considering the difficulties encountered in compiling an overview of biogeographic data and the diversity of species or taxa in Canada's three oceans, this synthesis is intended to serve as a biodiversity baseline for a new program on marine biodiversity, the Canadian Healthy Ocean Network. A major effort needs to be undertaken to establish a complete baseline of Canadian marine biodiversity of all taxonomic groups, especially if we are to understand and conserve this part of Canada's natural heritage

Focusing properties of an axicon pair

[en] The focusing properties of a so-called reflaxicon (a combination of a diverging and a converging axicon) are studied both theoretically and experimentally. Calculations of intensity distributions produced by this system are made by evaluating the Kirchhoff-Fresnel diffraction integral, first. by means of an approximate technique, the stationary phase method, then by a more exact numerical method. The calculations are presented for various planes along the axis of the axicons. The effects of the presence of the supporting mount of the axicons and of some important misalignments of the system on the distributions is also investigated. Experimental results of actual intensity distributions produced by focusing a near-fundamental Gaussian beam by such a system are also presented and are seen to be in fair agreement with numerical calculations. Such calculations would be valuable in many applications for predicting important characteristics (e.g., peak intensity, length of the focal line, degree of asymmetry) of the intensity distributions formed by optical systems containing an axicon pair as the focusing component._______ [fr] Les propriétés de focalisation d'un système optique formé d'un axicon convergent suivi d'un axicon divergent font l'objet d'une étude théorique et expérimentale. Les distributions d'intensité théoriques produites par ce système sont déterminées en résolvant l'intégrale de diffraction de Kirchhoff–Fresnel, d'abord de façon approximative, par la méthode de la phase stationnaire, puis, d'une manière plus exacte, par le recours au calcul numérique. Ces distributions sont illustrées pour divers plans situés le long de l'axe des axicons; on examine également l'effet de la présence de la monture des axicons et des principale erreurs d'alignement du système. On présente ensuite le résultat de mesures de distributions d'intensité produites en focalisant un faisceau gaussien quasi-fondamental à l'aide d'un système semblable; on constate un bon accord avec les résultats des calculs numériques. De tels calculs présentent un intérêt pour nombre d'applications : ils permettraient de prédire certaines caractéristiques importantes (par exemple, l'intensité maximale, la longueur de la ligne focale, le degré d'asymétrie) des distributions d'intensité produites par des systèmes optiques utilisant une paire d'axicons comme éléments focalisants