Papers by Author: G.C. Righini

Abstract: The possibility to confine the light in optical planar structures represented the milestone for the development of integrated optical devices in different application areas, such as communications and sensing. In particular, rare-earth (RE) doped planar waveguides demonstrated to be an interesting solution in the realization of integrated optical lasers and amplifiers suitable for the generation/regeneration of the signal in metropolitan and local area networks. Nowadays, although these devices are commercially available, the major contribution of the research consists in discovering and developing better combinations of materials and fabrication processes, in order to reduce the costs and increase the performance of the aforesaid devices. In this context glass-ceramic waveguides, activated by RE ions, seem to fully respond to these requests. The aim of this paper is to offer a comprehensive review on the main results obtained in our Labs in the field of glassceramics. Fabrication and characterization of different silicate glass-ceramic thin films, doped with different percentages of RE ions, will be presented and discussed. The interesting results obtained make these systems quite promising for development of high performance integrated optical amplifiers and lasers.

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: Erbium-doped tellurite glasses show great potential for the fabrication of high-performance integrated optical amplifiers and lasers, thanks to their unique properties in terms of bandwidth and rare earth solubility. As a first step towards the development of smart multi-functional integrated optical circuits, the fabrication of multimode channel waveguides in a sodium-tungsten-tellurite glass, by using nitrogen ions implantation, has been recently demonstrated [1]. The effects of the ion implantation process, however, have not been fully clarified, and a deeper investigation would be necessary in order to optimize the process and to truly exploit the glass useful characteristics. We therefore report here the results of a broad optical, topographic, and structural characterization of tellurite samples irradiated with various doses of nitrogen ions, while keeping constant the beam energy at 1.5 MeV. Characterization techniques have included absorption and luminescence spectroscopy, modal (dark-line) spectroscopy, surface profilometry, scanning electron microscopy, cathodoluminescence spectroscopy and EDX analysis.

Abstract: The Ge+SiO2 and SiO2 alternating multilayers are prepared by the magnetron sputtering of germanium and silica targets. By controlling the substrate temperature and by subsequent thermal annealing, the self-organized germanium quantum dots in 3D rombohedral (R 3 m) superlattice are produced. The polarized low-frequency Raman scattering measurements shows the coherent effects on the symmetric and quadrupolar spheroidal vibrations of Ge nanocrystals. It has been shown that the coherence effects are dependent on degree of Ge-ordering in the superlattice.