Modelization of the whispering gallery mode in microgear resonators using the Floquet-Bloch formalism

In this paper, a two-dimensional (2-D) method describing the whispering gallery mode in a microgear resonator is presented. The microgear is a microdisk surrounded by a circular grating. The method, which is based on the Floquet-Bloch formalism, analytically describes the field within the disk and outside the grating. On the other hand, the field within the grating is calculated using a finite-difference scheme in polar coordinates. Matching the boundary conditions, it is possible to work in a forced oscillation regime or in a free oscillation regime (laser mode). The resonant wavelength and quality factor can then be deduced. Compared to the coupled mode theory and to 2-D finite-difference time-domain computations, the method is faster and more accurate. Moreover, a polarization effect of the microgear is demonstrated. The TE polarization experiences a Q-factor improvement contrary to TM polarization. Finally, microgear structures prove to be more efficient than micro flowers.     

Free radical grafting of glycidyl methacrylate onto polypropylene in a co-rotating twin screw extruder

Free radical grafting of glycidyl methacrylate (GMA) onto molten polypropylene (PP) was studied in a co-rotating twin screw extruder. Grafting yields of GMA obtained under various experimental conditions along the screw length allowed for a good appreciation of the effects of chemical parameters (the presence of styrene and the concentrations of peroxide and monomers) and those of processing parameters (feed rate, screw speed, and specific throughput). Similar to the results obtained in a batch mixer,¹ free radical grafting of GMA carried out in the extruder in the presence or absence of styrene proceeded rapidly, as it was virtually completed half-way down stream of the extruder. Additionally, the presence of styrene as a second monomer increased the GMA grafting yield reatly with reduced PP chain degradation. The ultimate GMA grafting yield increased with increasing concentration of peroxide, 1,3-bis(tert-butylperoxyisopropyl)benzene. This similarity between the batch mixer and the extruder is related to the fact that in both cases it is the concentration of the peroxide and its half lifetime that determine the grafting rate and the ultimate grafting yield. On the othe hand, the GMA grafting yield decreased with increasing screw speed or feed rate. For a particular specific throughput (the ratio of throughput to screw speed), an increase in throughput with a concomitant increase in screw speed brought about a decrease in GMA grafting yield. It was concluded that the GMA grafting yield is affected primarily by the residence time in the zone in which free radicals are not depleted. The effects of screw speed, feed rate, and specific throughput manifest mainly through this local residence time distribution. Specific energy is not a good measure of the performance of the extruder with respect to the free radical rafting of GMA onto PP. © 1995 John Wiley & Sons, Inc.     

Toxicokinetics of 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) in the European corn borer,Ostrinia nubilalis (Hübner)

2,4-Dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), the major hydroxamic acid present in corn, and its tritiated derivative, were prepared synthetically for use in the determination of the toxicokinetics of this insect deterrent in the European corn borer (ECB),Ostrinia nubilalis. In growth studies with DIMBOA (0, 0.05, 0.2, and 0.5 mg/g diet), the mean time to pupation and adult emergence were significantly lengthened by an increase in concentration. Pupal and adult weights, for both female and male, decreased with an increase in concentration. Increased larval and pupal mortality occurred at the highest concentration of DIMBOA. DIMBOA, at concentrations of 0.2 and 0.5 mg/g diet, resulted in a decrease in the number of egg masses produced per female, and at 0.5 mg/g diet, in a decrease in the number of eggs per egg mass. Larvae fed from the neonate stage on a diet containing 0.2 mg [³H]- + [¹H]DIMBOA/g diet showed an increase in the content of label from fourth to fifth instar, but levels declined at pupation and emergence. A large amount of the labeled compounds was excreted by the insect in the pupal case. In dose-related studies, both uptake and excretion increased with an increase in concentration of DIMBOA (0.05, 0.2, 0.4 mg/g diet), while a body burden (concentration in the tissues/concentration in the frass) of approximately 0.25 was maintained for all concentrations. At the highest dose of DIMBOA (0.4 mg/g), the ECB increased consumption, possibly to compensate for the toxic effects of the compound. In topical application studies, elimination of the labeled compound in the frass was rapid, reaching 65% by 4 hr and 88% by 48 hr. Accumulation of label in tissues other than hemolymph was low. The results show that the ECB does possess adaptive mechanisms to deal with the effects of this host-derived compound.     

Dispersed fringe tracking with the multi-r(0) apertures of the Grand Interféromètre à 2 Télescopes

A new fringe tracker based on photon-counting detectors and real-time image processing has been implemented on the Grand Interféromètre à 2 Télescopes at the Observatoire de la Cote d'Azur. Fringe visibilities have been recorded on P Cygni and other stars across the Hαemission line with optical path differences stabilized to between 4 and 7 μm rms (1% of the coherence length). We present our initial results and describe the principle, implementation, and performance of the fringe tracker.     

How Brittany and Florida coasts cope with green tides

Although the direct involvement of nitrogen and phosphorus has been shown, eutrophication remains poorly managed to this day. The excessive growth of some opportunist seaweeds is the consequence in coastal ecosystems close to agricultural or strongly urbanized and industrialized zones. In Brittany, a leading tourist region of France, green tides set down on the beaches big quantities of Ulva, of which some 100,000 m³ are harvested annually, with significant ecological and economic consequences. In Florida, although the macroalgae (notably Codium) drift about with the currents, they are of sufficient mass and spread over such areal extent to inhibit penetration of sunlight through the water column to the coral reef surface. They strand also on the beaches. Stabilization of algae by composting and methanization of hydrolyzed and pressed algae juice are two methods which have been studied in Brittany and can be used to enhance the value of the harvested seaweed, depending on local conditions and on evolution of needs in energy, basic materials or organic products. But the parallel made here between two very different ecosystems, both disturbed, leads above all to the question of what course to follow now to avoid a worldwide disaster.

Bien que le rôle de l’azote et du phosphore dans les processus d’eutrophication ait été montré, celle‐ci reste mal contrôlée à ce jour. La croissance excessive d’algues opportunistes en est la conséquence dans des écosystèmes côtiers proches de zones agricoles ou fortement urbanisées et industrialisées. En Bretagne, région touristique de France, des marées vertes déversent sur les plages de grandes quantités d’ulves, dont environ 100,000 m³ sont récoltés annuellement, avec d’importantes conséquences écologiques et économiques. En Floride, bien que les macroalgues (notamment Codium) dérivent avec les courants, elles sont en masse suffisante et suffisamment réparties pour empêcher la pénétration de la lumière dans la colonne d’eau jusqu’au récif de corail. Elles s’échouent également sur les plages. La stabilisation par compostage et la méthanisation de jus d’algues hydrolysées et pressées sont deux méthodes qui ont été étudiées en Bretagne afin de valoriser l’algue récoltée, en fonction des conditions locales et de l’évolution des besoins en énergie, matières premières ou produits organiques. Mais le parallèle fait ici entre deux écosystèmes très différents, tous deux perturbés, mène surtout à se demander quelle voie suivre maintenant pour éviter un désastre mondial.     

Photoanodic reactions occurring at nanostructured titanium dioxide films

The reasons of unusually large differences observed in photocurrent efficiencies for the oxidation of various organic and inorganic substrates at nanostructured TiO₂ photoelectrodes are discussed. The “redox cycling”, where a product of the hole transfer acts, in turn, as scavenger for the photogenerated electrons, appears as a frequent cause of weak photocurrents. Such a recombination mechanism operates not only under open-circuit conditions but also in the presence of large anodic bias applied to the conducting support of the TiO₂ electrode. Experiments conducted in the presence of both, an efficient hole scavenger, formate ion, and an electron acceptor, methyl viologen dication, showed that, in a major part of the anodically polarized nanostructured TiO₂ film, the quasi-Fermi level of electrons remains actually close to the conduction band edge potential. Importantly, addition to the solution of an electron acceptor causes large drop of the photocurrent both under weak and intense UV illumination generated by an argon-ion laser. On the other hand, a large number of organic molecules undergoing essentially irreversible photooxidation (e.g., to form CO₂ and H₂O) generate high photocurrents at the nanostructured TiO₂ electrodes. Such reactions, when occurring in the presence of large concentrations of the reactants, may involve direct hole transfer.