A comparison between different propagative schemes for the simulation of tapered step index slab waveguides

The performance and accuracy of a number of propagative algorithms are compared for the simulation of tapered high contrast step index slab waveguides. The considered methods include paraxial as well as nonparaxial formulations of optical field propagation. In particular attention is paid to the validity of the paraxial approximation. To test the internal consistency of the various methods the property of reciprocity is verified and it is shown that for the paraxial algorithms the reciprocity can only be fulfilled if the paraxial approximation of the power flux expression using the Poynting vector is considered. Finally, modeling results are compared with measured fiber coupling losses for an experimentally realized taper structure

Placental lactogens induce serotonin biosynthesis in a subset of mouse beta cells during pregnancy

AIMS/HYPOTHESIS: Upregulation of the functional beta cell mass is required to match the physiological demands of mother and fetus during pregnancy. This increase is dependent on placental lactogens (PLs) and prolactin receptors, but the mechanisms underlying these events are only partially understood. We studied the mRNA expression profile of mouse islets during pregnancy to gain a better insight into these changes. METHODS: RNA expression was measured ex vivo via microarrays and quantitative RT-PCR. In vivo observations were extended by in vitro models in which ovine PL was added to cultured mouse islets and MIN6 cells. RESULTS: mRNA encoding both isoforms of the rate-limiting enzyme of serotonin biosynthesis, tryptophan hydroxylase (TPH), i.e. Tph1 and Tph2, were strongly induced (fold change 25- to 200-fold) during pregnancy. This induction was mimicked by exposing islets or MIN6 cells to ovine PLs for 24 h and was dependent on janus kinase 2 and signal transducer and activator of transcription 5. Parallel to Tph1 mRNA and protein induction, islet serotonin content increased to a peak level that was 200-fold higher than basal. Interestingly, only a subpopulation of the beta cells was serotonin-positive in vitro and in vivo. The stored serotonin pool in pregnant islets and PL-treated MIN6 cells was rapidly released (turnover once every 2 h). CONCLUSIONS/INTERPRETATION: A very strong lactogen-dependent upregulation of serotonin biosynthesis occurs in a subpopulation of mouse islet beta cells during pregnancy. Since the newly formed serotonin is rapidly released, this lactogen-induced beta cell function may serve local or endocrine tasks, the nature of which remains to be identified

PARAMETRIC STUDIES OF GaAs GROWTH BY METALORGANIC MOLECULAR BEAM EPITAXY

L'association d'un bâti d'épitaxie par jets moléculaires aux sources gazeuses d'organométalliques employées en croissance en phase vapeur (MOCVD) du GaAs est décrite. La décomposition thermique de l'arsine (AsH3) et du trimethylgallium (TMG) ont été étudiées par spectrométrie de masse. L'énergie d'activation apparente de dissociation de l'arsine est évaluée à 35 kcal/mole. La possibilité de croissance épitaxiale de GaAs à partir de TMG pur et d'arsenic élémentaire (As4) sous ultra vide est démontrée. Les cinétiques de croissance dans les systèmes (TMG, As4) et (TMG, AsH3) sont analysées. En particulier le type de mécanisme de croissance mis en jeu a permis de développer une méthode pour effectuer des épitaxies localisées.The combination of the gaseous species of a conventional MOCVD system with a UHV chamber is described. Thermal decomposition of arsine (AsH3) and trimethylgallium (TMG) has been analyzed by mass spectrometry. An activation energy of 35 kcal/mole is found to be associated with arsine decomposition. The possibility of GaAs growth in high vacuum from pure TMG and elemental arsenïc (As4) source is demonstrated and growth kinetics in both (TMG, As4) and (TMG, AsH3) systems is investigated. In particular, the growth mechanisms involved let the possibility for localized epitaxy

An InP-based photonic integrated beamformer for phased-array antennas

An InP-based integrated beamformer for beamsteering of a four-element Phased-Array Antenna has been designed, fabricated and characterized. This beamformer consists of an AWG, 14 high-speed MZI switches, and 9delaylines integrated on a single compact chip