Papers by Author: Bruno Viana

Abstract: In this work, Eu3+-doped CdS nanocrystals embedded in a SiO2 matrix were synthesised by a sol-gel method since this method confers a great chemical homogeneity, and allows in-situ generation and controlled growth of CdS nanocrystals within the silica glass. The influence of the sol-gel conditions on the Eu3+ optical response has been investigated. The synthesis conditions can be varied to modify the structure of the material, and the dispersion of the rare earth ion, within the silica matrix. From the optical analysis an energy transfer appears from CdS nanoparticles to Eu3+ ions but also a back transfer Eu3+ to CdS nanoparticles is evidenced.

Abstract: Y2O3:19%Yb,1%Er nanoparticles were synthesized with the aim of imaging the luminescent cell. Coprecipitation and combustion synthesis were used to obtain particulate sizes ranging from 25 up to 140 nm. The powders showed predominant red upconversion and the emission efficiency is controlled by the particle size. A colloidal route was also followed and 2-5 nm-large agglomerated nanoparticles were obtained. In that case, the luminescence of Er3+ was only observed by direct excitation and no upconversion light has been detected on these very small particles.

Abstract: The paper presents two cerium doped lutetium silicate crystals: pyrosilicate Ce:Lu2Si2O7 (LPS) and Ce: Lu2(1-x)Y2xSiO5 (LYSO). These two crystals exhibit the expected requirements for gamma detection: high density and high atomic number, high scintillation light yield, good energy resolution and fast response. LPS and LYSO crystals doped with cerium were grown by the Czochralski process. The crystal growth parameters were studied and optimized. Development of scintillators requires good understanding of the scintillation process. The location within the forbidden band gap of the localized lanthanide energy levels is analyzed by time resolved spectroscopy and thermoluminescence studies.

Abstract: In view of the improvement of luminescent intensity in classical phosphors, different new techniques of synthesis have been developed. One of them is the sol-gel technique for nanopowder synthesis. Applied to a binary system Gd2O3-SiO2 doped with trivalent europium ion, very fine powder form samples have been obtained. In that case the emission intensity of 5D0→F2 electronic transition of 4f6 configuration is greatly enhanced compared to the bulk oxide. The lifetime measurements are given too. The size of nanoparticles is of about 10 nm, determined by dynamic light scattering and XRD analysis.

Abstract: Tungsten-phosphate glasses doped with trivalent europium ion are synthesized and their spectroscopic properties are analyzed. Following the evolution of the 5D0 → 7F1 electronic transition in a series of various glass compositions via two rank crystal field parameters (cfp’s), a slight modification of the local symmetry is evidenced in the series. Decay lifetimes of 5D1 and 5D0 levels are also analyzed. Optical properties clearly correspond to Eu3+ in the glassy phase. At room as well as at liquid nitrogen temperatures, the emission spectra display comparable features with prominent 5D0 → 7F2 transition. However, at 77 K, the maximum 7F1 splitting exhibits an increase of ≈ 20-23 % for all compounds. This increase is directly connected to a change in local environment around the rare earth ion. The optical properties and the crystal field variation could be correlated with the shortness of the metal-oxygen distances produced by the pressure when cooling.

Abstract: The emergence of new laser crystals has allowed progress in ultrashort-pulsed laser technology and high power lasers. Extensive researches on new Yb3+ doped crystals enable development of all-solid-state femtosecond lasers. Thermo-mechanical properties should be taken into account. The review of the state of the art on the Yb doped laser material for high power and ultrafast pulses generation is presented.