C. Armellini

Terbium(III) doped silica-xerogels: effect of aluminium(III) co-doping

Monolithic silica xerogels doped with diAerent concentrations of Tb 3a and Al 3a (xTbO1:5‐x6AlO1:5‐SiO2, xa 2 · 10 ˇ4 ,4 ·10 ˇ4 ,5 ·10 ˇ3 ) were prepared by a sol‐gel route: Mixtures of tetra-methoxy-silane, Tb(NO3)3 AE 5H2O, Al(NO3)3 AE 9H2O, ethanol, and water were condensed at 60∞C using HNO3 as catalyst. The samples were sintered at 950∞C for 86 h in air. The Raman spectra indicate several structural diAerences among the samples and show that there is a compositional dependence of the gel glass transformation temperature. The sample with xa 5 · 10 ˇ3 is glasslike on the outer part showing onset of crystallisation in the inner part of the sample. The samples with smallest dopant concentrations (xa 2 · 10 ˇ4 ,4 ·10 ˇ4 ) contain pores. All samples luminesce from the 5 D4 and 5 D3 states of Tb 3a with excitation at 355 nm. The 5 D3 luminescence increases with respect to the 5 D4 luminescence with increasing dopant concentration, indicating that the larger Al 3a concentrations are eAective in dispersing Tb 3a ions. This result is also confirmed by the analysis of the decay of the 5 D3 and 5 D4 luminescence. ” 1999 Elsevier Science B.V. All rights reserved.

Erbium-Activated Silica-Titania Planar Waveguides

Abstract(100 − x)SiO2-(x)TiO2-ErO3/2 planar waveguides, with 7 ≤ x ≤ 20 have been prepared by sol-gel route using the dip-coating technique. The thickness of the films was optimized to support a single propagating mode at 1550 nm, with a confinement coefficient higher than 0.75. The process of densification of the gel and the devitrification with the growth of TiO2 nanocrystals were studied by Raman scattering. Devitrification is important only for x ≥ 15, but it was not possible to obtain full densification of the samples, even at the lowest TiO2 content, without the appearance of nanocrystals.Emission in the C telecom band was observed; the spectral width of the 4I13/2 → 4I15/2 transition of Er3+ slightly increases with the titania content. For x ≤ 12 most of Er3+ ions (about 65%) decay exponentially with a lifetime of about 8 ms.

Er3+ activated silica-hafnia glass-ceramics planar waveguides

Silica-hafnia glass-ceramics waveguides activated by Er3+ ions were fabricated by sol-gel route. X ray diffraction and optical spectroscopy showed that after an adapted heat treatment, the resulting materials showed a crystalline environment. Analysis of the luminescence properties has demonstrated that erbium ions are, at least partially, trapped in a crystalline phase. Losses measurements at different wavelength highlight a very low attenuation coefficient indicating that this nanostructured material is suitable for a single band waveguide amplifier in the C band of telecommunication.

Er3+/Yb3+ Co-Activated Silica-Alumina Monolithic Xerogels

Monolithic silica xerogels doped with different concentrations of Er3+, Yb3+ and Al3+ were prepared by sol-gel route. Densification was achieved by thermal treatment in air at 950°C for 120 h with a heating rate of 0.1°C/min. We studied the luminescence properties of the 4I13/2 → 4I15/2 emission band of Er3+ as a function of the Al/Er/Yb concentration and we paid particular attention to the alumina effects. Raman spectroscopy and Vis-NIR absorption were used to monitor the degree of densification of the glasses and the residual OH content.

Erbium-activated monolithic silica xerogels and silica-titania planar waveguides: optical and spectroscopic characterization

Recent results obtained for Er2O3-SiO2 monolithic xerogels and erbium activated SiO2-TiO2 planar waveguides are presented. Monolithic erbium-activated silica xerogels with erbium content ranging from 0 up to 40000 ppm were prepared by the sol-gel technique. Samples were densified by thermal treatment in air at 950 degrees C for 120 hours. The densification degree and the relative content of hydroxyl groups were studied by NIR absorption and Raman spectroscopies. Emission at 1.5 micrometers , characteristic of the 4I13/2 yields 4I15/2 transition of Er3+ ions, was observed at room temperature for all monolithic samples upon continuous wave excitation at 980 nm. For the 5000 Er/Si ppm doped xerogel, an intense photoluminescence was observed with a lifetime of 8 ms for the metastable 4I13/2 level. Passive and erbium-activated SiO2-TiO2 planar waveguides, monomode at 632.8 nm, were prepared by a dip-coating technique. Some parameters such as H2O content, intermediate and final thermal treatments, and the molar ratio TiO2/SiO2 were modified during the preparation of the solution in order to minimize the final content of residual hydroxyl groups and the phase separation between silica and titania. Raman spectroscopy was used to check the structural properties of the waveguides. A lifetime of 7 ms was measured for the metastable 4I13/2 level in a 93SiO2-7TiO2 planar waveguide activated by 10000 ppm Er/(Si + Ti). The best value of the attenuation coefficient was of 0.5 dB/cm at 632.8 nm.

X-ray absorption and diffraction studies of Pr3+, Tb3+ and Er3+-activated silica gels

Rare-earth (Pr3+, Tb3+, Er3+) doped silica xerogels were studied by x-ray absorption spectroscopy and x-ray diffraction. A change of the local environment around rare-earth ions upon xerogel densification at 900–950°C and co-doping with aluminum ions was determined from the rare-earths L3-edge EXAFS signals. The densification process induces a decrease of the coordination number and a compression and deformation of the first coordination shell, composed of oxygen atoms. The second coordination shell, composed of silicon and/or aluminum ions, also experiences some modification, which is attributed mainly to a shortening of the shell radius. No evidence of clustering of rare-earth ions upon densification was observed. X-ray diffraction data on Tb-doped gels confirm the EXAFS results.

Sol-Gel Erbium-Doped Silica-Hafnia Planar and Channel Waveguides

Erbium activated SiO2-HfO2 planar waveguides, doped with Er3+ concentrations ranging from 0.01 to 4 mol%, were prepared by sol-gel method. The films were deposited on v-SiO2 and silica-on-silicon substrates using dip-coating technique. The waveguides show high densification degree, effective intermingling of the two film components, and uniform surface morphology. The waveguide deposited on silica-on-silicon substrates shows one single propagation mode at 1.5μm, with a confinement coefficient of 0.81 and an attenuation coefficient of 0.8 dB/cm at 632.8nm. Emission in the C-telecommunication band was observed at room temperature for all the samples upon continuous-wave excitation at 980 nm or 514.5 nm. The shape of the emission band corresponding to the 4I13/2→4I15/2 transition is found to be almost independent both on erbium content and excitation wavelength, with a FWHM between 44 and 48 nm. The 4I13/2 level decay curves presented a single-exponential profile, with a lifetime ranging between 1.1-6.6 ms, depending on the erbium concentration. Infrared to visible upconversion luminescence upon continuous-wave excitation at 980 nm was observed for all the samples. Channel waveguide in rib configuration was obtained by etching the active film in order to have a well confined mode at 1.5 μm.

Frequency converter layers based on terbium and ytterbium activated HfO2 glass-ceramics

One of the ways in which the cell efficiency of solar cells may be improved by better exploitation of the solar spectrum makes use of the down-conversion mechanism, where one high energy photon is cut into two low energy photons. When energy transfer between rare earth ions is used to activate this process, high emission and absorption cross sections as well as low cutoff phonon energy are mandatory. Glass-ceramics can be a viable system to fulfill these requirements. The main advantage of the glass-ceramic is to combine the mechanical and optical properties of the glass with a crystallike environment for the rare-earth ions, where higher cross-sections of the rare earth ion can be exploited. In the case of silica-hafnia system the glass ceramic is constituted by nanocrystals of HfO2, containing the rare earth ion, imbedded in the silica-hafnia host. Hafnia nanocrystals are characterized by a cutoff frequency of about 700 cm-1, so that nonradiative transition rates are strongly reduced, thus increasing the luminescent quantum yield of the rare-earth ions. In this work we investigated the Tb3+/Yb3+ energy transfer efficiency in a 70SiO2-30HfO2 glass-ceramic waveguide in order to convert absorbed photons at 488 nm in photons at 980 nm. The energy transfer efficiency was estimated as a function of the Tb3+/Yb3+ molar ratio as well as of the total amount of rare earth ions. A transfer efficiency of 38% was obtained for Tb3+/Yb3+ = 0.25 mol and a rare earth content [Tb+Yb]/[Si+Hf] = 5% mol.

Rare earth-activated silica-based nanocomposites

Two different kinds of rare earth-activated glass-based nanocomposite photonic materials, which allow to tailor the spectroscopic properties of rare-earth ions: (i) Er3+-activated SiO2-HfO2 waveguide glass ceramic, and (ii) core-shell-like structures of Er3+- activated silica spheres obtained by a seed growth method, are presented.

Fabrication and optical assessment of sol-gel-derived photonic bandgap dielectric structures

We present the details of the Sol-gel processing used to synthesize silica spheres, with particular attention to the conditions that permit to tailor their dimension. We have elaborated a protocol in order to obtain silica micro spheres with low polydispersivity and we have demonstrated that large well-ordered crystals of synthetic opal, that exhibit a photonic stopband, can be produced in few days by vertical deposition and evaporation-assisted sedimentation deposition methods. Scanning electron microscope was employed to characterize the samples reflectance and transmission measurements were used to put in evidence the high quality of the realized opals. Starting from the silica spheres, core-shell-like Er3+-activated silica spheres were also prepared, where the core is the silica sphere and the shell is an Er2O3-SiO2 coating. Morphologic, structural and spectroscopic properties were investigated by scanning electron microscope and luminescence spectroscopy. The emission of 4I13/2→4I15/2 of Er3+ ion transition with a 27 nm bandwidth was observed upon excitation at 514.5 nm. The 4I13/2 level decay curves presented a single-exponential profile, with a lifetime of 12.8 ms.