G. Chadeyron

Spectroscopy and upconversion processes in YAlO3:Ho3+ crystals

Abstract Emission from the high lying excited states, energy transfer, and upconversion processes are investigated in YAlO 3 :Ho 3+ . Selectively excited emission spectra in the range from 300 to 800 nm starting from the 3 D 3 , 3 G 5 , 5 F 3 , 5 S 2 and 5 F 5 multiplets were measured at 15 K. This, together with the detailed absorption and excitation measurements at 15 K allowed determination of the Stark energy levels of Ho 3+ ions in YAlO 3 up to UV energies. The 5 S 2 fluorescence decays were recorded as a function of temperature and Ho 3+ concentration in order to investigate the process of quenching of fluorescence due to cross relaxation among two ions. Conversion of red and infrared laser radiation to green 5 S 2 and blue 5 F 3 emission is reported. Under pulsed resonant excitation of the 5 F 5 or 5 I 5 levels the upconversion was found to be due to energy transfer process between two excited ions. The photon avalanche effect was observed under cw excitation around 585 nm.

Comparative study of physico-mechanical and antioxidant properties of edible gelatin films from the skin of cuttlefish.

Physicochemical properties of edible films based on cuttlefish skin gelatin extracted without (G0) or with different concentrations of pepsins (5 (G5), 10 (G10) and 15 (G15) U/g of skin) were investigated. Edible films prepared with partially hydrolyzed gelatins had lower tensile strength (TS) and elongation at break (EAB), but higher water vapour permeability (WVP) and water solubility than the control film. FTIR spectra of obtained gelatin films revealed a significant loss of molecular order of the triple helix. In addition, differential scanning calorimetric (DSC) analysis indicated that partially hydrolyzed gelatine films exhibited lower transition temperature and enthalpy compared with those of control film. The properties of the films were related to their microstructure, which was observed by scanning electron microscopy. Films with G0 and G5 had a smooth surface and a more compact structure, while films prepared with G10 and G15 had coarser surface. Thus, the chain length of extracted gelatin directly affected the properties of corresponding films.

Infra-red to visible up-conversion in holmium-doped materials

Infra-red to visible wavelength up-conversion is reported for trivalent holmium ion (Ho3+) doped: YAP, YAG, YLF, SrLaGa3O7 and SrLaGaO4 crystals and ZBLAN glass. The spectra and possible mechanisms of up-conversion are discussed. Changing of the holmium luminescence colour between red and green was observed, after excitation in the 870- to 930-nm band. Wavelength-dependent ground state absorption and excited state absorption resonances were investigated and explained. For discussing the up-conversion pathways, we determined the cross-sections for the excited state absorption and for the non-resonant side-band absorption.

Optical properties upon vacuum ultraviolet excitation of sol-gel based Y3Al5O12:Tb3+, Ce3+ powders

Optical properties in the vacuum ultraviolet (VUV) region of cerium and terbium activated Y3Al5O12 powders obtained by the sol-gel process were investigated at room and low temperatures. Both Tb3+ and Ce3+ doped samples exhibited a broad absorption band within the conventional Xe-Ne plasma emission range (140–190 nm). Optimal doping rates have been determined for both rare-earth ions. Emission spectra from cerium doped samples were recorded at 8.2 K and uncovered the splitting of the ground state of Ce3+ ions. Decay times of Ce3+ fluorescence can be fitted by double exponential curves, which can be ascribed to a retrapping phenomenon. Eventually, optical characteristics of codoped Y3Al5O12:(Tb,Ce) and Y3Al5O12:(Tb,Gd) were studied.

Room temperature photon avalanche in Ho3+ doped YAG, YAP, YLF and ZBLAN

Abstract Upconverted, green emission from the 5 S 2 level of Ho 3+ ion in two oxides, YAG and YAP, and two fluorides, YLF and ZBLAN, was observed at room temperature under cw 580–590 nm excitation. Under these excitation wavelengths, which are off resonance with any ground state absorption, characteristic features of photon avalanche process are observed. A time-dependent rate equation model was employed to describe this behavior using experimentally determined parameters.

Modifications induced by acetylacetone in properties of sol–gel derived Y3Al5O12 : Tb3+ – I: structural and morphological organizations

Acetylacetone has been used as a chemical modifier for the synthesis of undoped and Tb(3+)-doped Y(3)Al(5)O(12) powders. A systematic investigation concerning its influence on the structural and morphological properties of amorphous and crystallized samples has been carried out. These properties have been comparatively studied by means of X-ray diffraction, infrared spectroscopy, SEM, XAS and SAXS. (27)Al NMR and EPR experiments have been performed to complete the study. The combined results have evidenced that acetylacetone promotes organic groups departure during calcination, entailing a better structural organization at lower temperatures compared with unmodified powders. Structuration has been proven to occur at short-scale range until a 600 degrees C heating treatment before being extended by coalescence at higher temperatures. Finally, the presence of acac ligands on the alkoxides leads to a monomer-cluster aggregation process, and thus to a more open network.

Investigations on PVP/Y3BO6:Eu3+, a red luminescent composite for lighting devices based on near UV-LEDs

This work deals with a red phosphor, Y3BO6:Eu3+, and its corresponding poly(N-vinylpyrrolidone) (PVP)/Y3BO6:Eu3+ luminescent composite film suitable for applications in the next generation of Hg-free lamps based on near ultraviolet (UV) light emitting diodes (LEDs). Well crystallized samples of Y3BO6 powders with the Eu3+ content up to 20 mol% were prepared by the Pechini method. After structural, morphological and optical characterization, the best doping rate of Eu3+ in the matrix was determined to be 15 mol%. This optimal powder, which is highly friable, was easily ground into fine particles and homogeneously dispersed into a PVP polymer solution to give rise to a polymer phosphor composite. Structural and optical features of the composite film have been studied and compared to those of a pristine PVP film and Y3BO6:Eu3+ powder. All the characterization (XRD, SAXS, luminescence…) proved that the red phosphor particles are well incorporated into the polymer composite film which exhibited the characteristic red emission of Eu3+ under UV light excitation. Furthermore, photostability of the polymer/phosphor composite film under UV-LED irradiation was evaluated from exposure to accelerated artificial photoageing at wavelengths above 300 nm.

A promising way to obtain large, luminescent and transparent thick films suitable for optical devices

This work focuses on the formation of large, luminescent and transparent thick films of polyvinylpyrrolidone (PVP) embedding phosphors. Characterization of these films was made using UV-Visible absorption spectroscopy and cw-selective excitation induced photoluminescence. Results presented here demonstrate the preparation polymer films incorporating luminescent materials in order to obtain large, transparent and thick surfaces of well-dispersed phosphors suitable for use in optical devices. This technique allows the preparation of self-standing, flexible and handy films that could be easily cut or shaped and then used for various purposes.

Synthesis of yttrium orthoborate powders

Yttrium orthoborate YBO3 is synthesized by calcining precursors precipitated with aqueous ammonium hydroxide from yttrium nitrate or yttrium chloride solutions (with concentrations ranging from 4.8 × 10−3 to 0.0165 mol/L) and with a more than tenfold excess of boric acid. Single-phase YBO3 is obtained at pH 5–6. Higher pHs result in the formation of mixtures of yttrium orthoborate and yttrium hydroxide phases. Dehydration occurs up to 288°C as shown by differential thermal analysis. Further heating induces crystallization. Addition of surfactants (polyvinylpyrrolidone (PVP) or ammonium polyacrylate (APA)) to the starting solution in an amount of 1 wt % of the yttrium salt affects the sizes and shapes of the precipitated particles. YBO3 platelets with nanometer thicknesses are obtained. The temperature of the low-temperature ⇄ high-temperature vaterite phase transition in YBO3 is 977°C upon heating and 640°C upon cooling.

Effects of densification atmosphere on optical properties of ionic copper-activated sol–gel silica glass: towards an efficient radiation dosimeter

Ionic copper-doped pure silica glass, with high photoluminescence (PL) quantum yield (QY), has been prepared using the sol–gel technique and densification of the porous xerogel under inert atmosphere. The optical properties of the obtained glass, and especially the green light emission due to Cu+ ions under UV excitation, have been studied and compared to those of the glass produced in air atmosphere conditions. We have shown that the sintering process under helium increases the quantum efficiency of this emission by a factor 40. Moreover, the resulting copper-doped silica rod has been used in the fabrication of a high numerical aperture microstructured optical fiber, which shows similar luminescent properties to the sol–gel preform. The obtained fiber exhibits promising characteristics to be employed in an all-fibred UV sensor. In particular, the high emission efficiency and the specific air-clad geometry of the fiber made it possible to perform measurements without removing the polymer coating.