H+ implanted channel waveguides in buried epitaxial crystalline YAG:Nd, Tm layers and infrared-to-blue upconversion characterization

Nd, Tm:YAG codoped single crystal waveguides were studied in order to discover if the presence of Nd3+ ions favors blue luminescence at 486 nm. Innovative implantation techniques were applied to locally change Delta n and form varied H+ implanted channel structures in Nd, Tm: YAG buried epitaxial waveguiding layers. The guided blue luminescence due to the Tm3+ (1)G(4)-> H-3(6) transition was studied under infrared excitation at 785 nm (Tm3+ absorption) and 808 nm (Nd3+ absorption) for the epitaxial planar waveguides of different Tm3+ and Nd3+ concentrations for all the implanted channel waveguide structures

Large expert-curated database for benchmarking document similarity detection in biomedical literature search

Document recommendation systems for locating relevant literature have mostly relied on methods developed a decade ago. This is largely due to the lack of a large offline gold-standard benchmark of relevant documents that cover a variety of research fields such that newly developed literature search techniques can be compared, improved and translated into practice. To overcome this bottleneck, we have established the RElevant LIterature SearcH consortium consisting of more than 1500 scientists from 84 countries, who have collectively annotated the relevance of over 180 000 PubMed-listed articles with regard to their respective seed (input) article/s. The majority of annotations were contributed by highly experienced, original authors of the seed articles. The collected data cover 76% of all unique PubMed Medical Subject Headings descriptors. No systematic biases were observed across different experience levels, research fields or time spent on annotations. More importantly, annotations of the same document pairs contributed by different scientists were highly concordant. We further show that the three representative baseline methods used to generate recommended articles for evaluation (Okapi Best Matching 25, Term Frequency-Inverse Document Frequency and PubMed Related Articles) had similar overall performances. Additionally, we found that these methods each tend to produce distinct collections of recommended articles, suggesting that a hybrid method may be required to completely capture all relevant articles. The established database server located at https://relishdb.ict.griffith.edu.au is freely available for the downloading of annotation data and the blind testing of new methods. We expect that this benchmark will be useful for stimulating the development of new powerful techniques for title and title/abstract-based search engines for relevant articles in biomedical research.Peer reviewe

Spectroscopie de cristaux dopés par l'ion Nd3+ et étude de leurs potentialités pour la réalisation de sources laser à solide UV accordables

Ce travail est consacré à l'étude spectroscopique des transitions interconfigurationnelles 4f3 4f25d de l'ion Nd3+ introduit à l'état de dopant dans des cristaux. Des émissions laser dans l'UV ont été démontrées par le passé à partir de ces transitions; leurs faibles efficacités liées en partie à l'utilisation de photons très énergétiques pour le pompage direct des états de la configuration excitée 4f25d nous conduisent à envisager des schémas d'excitation à deux photons. Une étude spectroscopique expérimentale approfondie réalisée sur des cristaux de LiYF4:Nd3+, BaY2F8:Nd3+, Na0.4Y0.6F2.2:Nd3+ et Cs2NaYCl6:Nd3+ permet de définir des schémas d'excitation et d'émission optimisés. Les spectres expérimentaux obtenus pour LiYF4:Nd3+ et BaY2F8:Nd3+ sont ensuite simulés par un calcul de champ cristallin. Les tests de gain réalisés sur les fluorures, bien que négatifs, ont permis de mettre en évidence des processus d'absorption à partir des états de la configuration excitée 4f25dLYON1-BU.Sciences (692662101) / SudocSudocFranceF

Spectroscopie de matériaux dopés par l'ion Pr3+ pour la réalisation de sources laser à solide accordables dans le domaine spectral ultraviolet

Dans le cadre de la recherche de nouveaux lasers à solide ultraviolets accordables, l'ion Pr3+ introduit comme dopant dans des matériaux est intéressant puisque ses états 4f5d, émetteurs dans le domaine spectral UV, peuvent être efficacement peuplés par un pompage à 2 étapes via les niveaux métastables 3Pj, 1I6 ou 1D2. Grâce à l'utilisation de photons excitateurs moins énergétiques que dans le cas d'un pompage direct des états 4f5d, les risques de solarisation sont réduits ce qui devrait être plus favorable à l'obtention de l'émission laser. Pour l'étude des potentialités d'une telle émission laser avec des matrices dopées Pr3+, le choix du matériau hôte est un élément primordial. Les fluorures et les oxydes semblent les plus intéressants, du fait notamment de leur large bande interdite. Un ensemble des mesures spectroscopiques (absorption, fluorescence, excitation, déclin de fluorescence) a été effectué sur les cristaux sélectionnés permettant de déterminer les longueurs d'onde d'excitation de la première et de la deuxième étape du pompage par up-conversion, de définir les délais temporels entre ces 2 pompes, etc? Ensuite, pour analyser l'efficacité du pompage à 2 étapes utilisant les niveaux intermédiaires 3Pj, 1I6 ou 1D2, des expériences d'absorption dans l'état excité (AEE) ont été réalisées (entre 430 et 240 nm), indiquant des sections efficaces d'AEE élevées de l'ordre de 10-18 cm2. Les mesures de gain ont utilisé au mieux l'ensemble des résultats précédents. Malgré un pompage à 2 étapes, les tests ont montré des pertes depuis les états émetteurs empêchant l'amplification, pertes dues à la photoionisation des ions Pr3+ et à la formation de centres colorés. Néanmoins, ces pertes sont moindres dans les fluorures. Dans des échantillons codopés par Pr3+ et Ce3+, des transferts d'énergie efficaces depuis les états 4f5d du Pr3+ vers les états 5d du Ce3+ permettent le pompage de l'émission UV du Ce3+ par up-conversion. Les pertes sont nettement réduites par rapport aux cristaux dopés Pr3+Within research of new ultraviolet tuneable solid state lasers, Pr3+ ion as dopant in materials is interesting insofar as its 4f5d levels emitting in the near UV spectral domain can be efficiently populated by a two step pumping using 3Pj, 1I6 or 1D2 metastable levels. The use of less energetic exciting photons than in the case of direct pumping should help to reduce solarisation effects and, consequently, should improve the possibility of laser emission achievement. To study such potentialities with Pr3+ doped crystals, the choice of the most suitable hosts is important. Fluoride and oxide materials seem to be quite interesting as they presents wide forbidden band gap. Spectroscopic measurements (absorption, fluorescence, excitation, fluorescence dynamics) were performed on selected crystals in order to, for example, get wavelengths of first and second steps up-conversion pumping or determine delays between these both pumps? Moreover, to evaluate the efficiency of two step excitation processes during 1D2 or 3Pj, 1I6 intermediate levels, excited state absorption (ESA) spectra were recorded from 430 to 240 nm. Then ESA cross sections were found in the order of 10-18 cm2. Gain measurements used as well as possible previous results. In spite of a two step 4f5d levels excitation, losses from these levels prevent amplification due to Pr3+ ions photoionisation and colour centres formation. However, these losses are weaker in fluoride crystals. In codoped Pr3+ and CE3+ samples, efficient energy transfers from Pr3+ 4f5d states to Ce3+ 5d states enable UV emission of Ce3+ after two step excitation of Pr3+ 4f5d states. Losses are reduced compared to losses in Pr3+-doped crystalsST ETIENNE-BU Sciences (422182103) / SudocSudocFranceF

Étude de la photoconductivité dans les matériaux luminescents isolants dopés terres rares par la méthode de la cavité résonnante micro-ondes

Many technological applications are based on the optical properties of rare-earth ions-doped dielectric crystals. Indeed, the 4fn->4fn-15d interconfigurationnal transitions of such ions give rise to broad, intense emission bands from the visible to the UV range. However, the energy levels of excited configurations of these optically active ions are usually located close to the host material conduction band, which can result in electronic transfers between the two systems, followed by electronic delocalization processes, thus reducing the overall efficiency of optical emission. In this context, the main objective of this work was to develop a new experimental method for photoconductivity measurements in these isolating materials, to gather information on the delocalization processes affecting the efficiency of optical materials and allow a precise positioning of the excited levels of rare-earth ions within the band gap of the host crystals. This new technique, so called the Microwave Resonant Cavity Technique, is based on the use of a microwave field as a probe of the transient modifications affecting the dielectric properties of a sample, while submitted to laser pulses. Among other benefits over pre-existing techniques, it gives access to the dynamics of the delocalization processes and to the nature of the charged species involved, and it can be applied to various types of samples, from bulk crystals to nanometric powders. The main results presented here were obtained on Ce3+-doped scintillating crystals, as well as on rare-earth-doped fluoride crystals which were well suited for the study of photoionization and solarization mechanisms affecting solid-state laser materialsDe nombreuses applications technologiques sont basées sur les propriétés de luminescence des ions de terres rares insérés en tant que dopants dans des matériaux cristallins isolants. Les transitions interconfigurationnelles 4fn->4fn-15d des ces ions donnent lieu à des bandes d émission larges et intenses dans un domaine proche de l UV. Cependant, les niveaux d énergie de la configuration excitée des ions dopants étant généralement situés proches de la bande de conduction du matériau hôte, l interaction entre ces deux systèmes peut engendrer des processus de photoionisation des ions optiquement actifs puis de délocalisation électronique, limitant ainsi les rendements lumineux. Dans ce contexte, l objectif de notre travail a été de développer une technique expérimentale novatrice pour réaliser des mesures de photoconductivité. La méthode proposée, dite méthode de la cavité résonante micro-ondes, s appuie sur l utilisation d un champ hyperfréquences comme sonde des modifications transitoires des propriétés diélectriques d un matériau soumis à une brève irradiation laser. Cette technique a l avantage majeur de permettre l observation de la dynamique de la photoconductivité, en donnant accès à la nature et au devenir des porteurs de charges impliqués, et permet en outre d élargir la gamme des matériaux étudiés, des cristaux massifs aux poudres nanométriques. Les résultats présentés concernent des scintillateurs dopés Ce3+, largement utilisés dans le domaine de l imagerie médicale, et des fluorures alcalino-terreux dopés par divers ions terres rares, matériaux d école pour la compréhension des mécanismes de photoionisation et de solarisation affectant les matériaux laserLYON1-BU.Sciences (692662101) / SudocSudocFranceF

Estimation of the Electron Thermalization Length in Ionic Materials

We report estimations of the thermalization length and the diffusion coefficient of photogenerated carriers in the insulator LiYF₄ as a function of their initial energy. Combining modeling of electron–phonon interaction and the detailed analysis of the kinetic response of fluorescent center Ce³⁺ under vacuum ultraviolet excitation, the thermalization length is obtained as a function of the initial kinetic energy of the electron. This parameter is essential for the description of the carrier recombination in the case of nonideal plasma conditions, where electrons and holes are strongly correlated. This approach also demonstrates the effect of a complicated structure of electronic band on the thermalization process, which impacts the complex nonproportionality response of materials under ionizing radiation excitation

Crosslinking of PET through solid state functionalization with alkoxysilane derivatives

International audienceA new way for crosslinking poly(ethylene terephthalate) (PET) films and fibers is described using solid state functionalization of the PET end groups (alcohol and acid) with two reagents, respectively, 3-isocyanatopropyltriethoxysilane (IPTESI) or/and 3-glycidoxypropyltrimethoxysilane (GOPTMSI). This functionalization is then followed by hydrolysis-condensation reactions of PET-alkoxysilane end groups leading to the PET crosslinking. First of all, the functionalization reactions were investigated on model compounds by 1H NMR spectroscopy in a range of temperature 80-160 °C. Furthermore, the diffusion of reagents in solid PET, depending on the initial degree of PET crystallinity, was characterized in the same temperature range through the variation of sample mass. On the other hand, this method allowed us to determine the diffusion coefficients and the solubility of the reagents in solid PET at different temperatures and initial crystallinity degrees. End groups functionalized PET films and fibers by alkoxysilane were then crosslinked by immersion of the samples in hot water. The crosslinking density was characterized by measuring the insoluble fraction of PET in good solvent constituted by a mixture of trifluoroacetic acid and dichloromethane (50/50 vol.). An insoluble fraction close to 70% was obtained by the functionalization treatment of amorphous PET film (8% crystallinity) by a mixture of GOPTMSI + IPTESI (50/50 M) at 155 °C for 1 h followed by hydrolysis-condensation reactions at 80 °C for 72 h. Thermomechanical and thermal properties of films and fibers were observed and found to be considerably enhanced in comparison to the untreated samples. The tensile properties of these partially crosslinked samples were maintained up to 320 °C

Quest to enhance up-conversion efficiency: a comparison of anhydrous vs. hydrous synthesis of NaGdF4: Yb3+ and Tm3+ nanoparticles

International audienceA major challenge in the field of up-converting (UC) nanomaterials is to enhance their efficiencies. The –OH defects on the surface of the nanoparticles are thought to be the main cause of luminescence quenching, but there are no comparative studies in the literature showing the impact of anhydrous vs. hydrous synthesis on up-conversion efficiency. In this article, we present the synthesis of up-converting NaGdF4: Yb+3, Tm+3 nanoparticles by two different methods: thermal decomposition of single source metal-organic anhydrous precursors [NaLn(TFA)4(diglyme)] (Ln = Gd, Tm, Yb; TFA = trifluoroacetate) and room temperature co-precipitation using hydrated inorganic salts Ln(NO3)3·5H2O (Ln = Gd, Tm, Yb), NaNO3 and NH4F in ethylene glycol. After a detailed study on the influence of solvents and the percentage of lanthanide dopant on the crystal phase of the up-converting nanoparticles (NPs) and their complete characterization, a comparative up-conversion study was carried out which revealed that the uniform nanospheres (av. size ∼13 nm) obtained from the anhydrous SSP had significantly higher up-conversion efficiency than agglomerated nanorods (∼197 nm in length and ∼95 nm in width) produced from hydrated inorganic salts. An enhanced up-conversion quantum yield of 1.8% for the anhydrous sample validates the anhydrous precursor approach as a strategy to obtain small but highly emitting up-converting particles without requiring a silica or undoped matrix surface passivation layer