Papers by Author: Yoann Jestin

Abstract: We report experimental results obtained in our laboratories in the development of Er3+- doped glass microspherical cavities for the fabrication of compact and low threshold laser sources at 1.55 μm. We investigate three different approaches in order to fabricate the microspheres including direct melting of Er3+-doped glass powders, coating of silica microspheres with an Er3+- doped sol-gel layer, and synthesis of Er3+-doped monolithic microspheres using the sol-gel route in acid catalysis. Details of the different fabrication processes are presented together with the photoluminescence characterization in free space configuration of the microspheres and of the glass precursor. We analyse the photoluminescence spectra of the whispering gallery modes of the microspheres exited using evanescent coupling and we demonstrate laser action in a wide range of wavelengths around 1.55 μm.

Abstract: We present the details of the sol-gel processing used to realize inverse silica opal, where the silica was activated with 0.3 mol% of Er3+ ions. The template (direct opal) was obtained assembling polystyrene spheres of the dimensions of 260 nm by means of a vertical deposition technique. The Er3+-activated silica inverse opal was obtained infiltrating, into the void of the template, the silica sol doped with Er3+ ions and subsequently removing the polystyrene spheres by means of calcinations. Scanning electron microscope showed that the inverse opals possess an fcc structure with a air hollow of about 210 nm. A photonic band gap in the visible range was observed from reflectance measurements. Spectroscopic properties of Er3+activated silica inverse opal were investigated by photoluminescence spectroscopy. A bandwidth of 21 nm was measured for the 4I13/2 → 4I15/2 transition of Er3+ ions upon excitation at 514.5 nm. The luminescence decay curve of the 4I13/2 metastable state of the Er3+ ions presents a lifetime τ = 16.8 ± 0.1 ms giving a very high quantum efficiency of the fabricated system. Core-shell Er3+-activated silica spheres, where the core is the silica sphere and the shell is an Er2O3-SiO2 coating is proposed as a possible route for opal fabrication. For core-shell system a quantum efficiency of about 70% was estimated.

Abstract: Two series of xHfO2 - (100-x) SiO2 (x=10, 20, 30 mol%) glass-ceramics planar waveguides doped with 0.3 mol% Er3+ ions were prepared by the sol-gel route. A thermal treatment at 1000°C was applied to the second series of samples to nucleate HfO2 crystals. The waveguides were analyzed by X-ray photoelectron spectroscopy to study the effect of the Hf concentration and of the annealing on the material structure. XPS shows that in the first series of samples a Hf concentration threshold exists. Above this threshold the material undergoes a spinodal decomposition with formation of HfO2 rich domains. In the second series of samples the presence of thermal treatment lowers the concentration threshold so that the phase separation occurs also at a Hf concentration of 10%mol. In the waveguides where spinodal decomposition in present, the emission spectra from the Er3+ ions reveal a sensible narrowing of the 4I13/2 → 4I15/2 line. This demonstrates the presence of a crystalline environment for the Er3+ ions since the inhomogeneous broadening due to the disordered glassy network is suppressed. These results may have important implications for the fabrication of photonic devices with increased efficiency.

Abstract: The Raman light to vibrations coupling coefficients C(ν) of quadrupolar and symmetrical vibrational modes of spherical nanoparticles embedded in a matrix are calculated. In contrast to the symmetrical mode, the C(ν) of the quadrupolar modes consists of the longitudinal and transversal sound velocity contributions. It is shown, that depending on the ratio of longitudinal and transverse sound velocity, these two contributions can interfere constructively or destructively resulting in enhancing or vanishing of some radial modes. Different peaks in the C(ν) spectrum were attributed to transverse and longitudinal spheroidal modes and the longitudinal spheroidal modes were found to have a higher Raman intensity than the transverse modes. The theoretical model was tested on a sample of HfO2 nanoparticles in a silica matrix.

Abstract: Yb3+ doped SnO2 nanocrystals were synthesized, using the hydrolytic route in the presence of starch as the size stabilizer. Starting from salt precursors, the nanopowders obtained with various Yb3+ loads are thermally stable up to 600 °C. They were characterized by XRD, solid state NMR, TEM, ICP and TG-MS techniques. The nanocrystal Cassiterite structure, with a size of a few nanometers (<6 nm), was identified by XRD, NMR and Raman measurements confirming a homogeneous dispersion of Yb3+ ions in the lattice. The preliminary assessment of the spectroscopic features of nanocrystals was performed by absorption and photoluminescence spectroscopy. The typical Yb3+ absorption peak, centred at 977 nm, and an intense 2F5/2→ 2F7/2 Yb3+ emission band were observed.