Effect of processing on the structure and properties of glasses and glass-ceramics for photonic applications

• Autores: Giulio Gorni
• Directores de la Tesis: M. J. Pascual Francisco (dir. tes.), María Yolanda Castro Martín (dir. tes.)
• Lectura: En la Universidad Autónoma de Madrid ( España ) en 2019
• Idioma: inglés
• Número de páginas: 357
• Tribunal Calificador de la Tesis: Maurizio Ferrari (presid.), María de la O Ramírez Herrero (secret.), David Salomon Levy Cohen (voc.)
• Programa de doctorado: Programa de Doctorado en Materiales Avanzados y Nanotecnología por la Universidad Autónoma de Madrid
• Materias:
  • Física
• Enlaces
  • Tesis en acceso abierto en: Biblos-e Archivo
• Resumen

  • Active optical materials based on transparent glass-ceramics doped with rare-earth (RE) ions are considered promising materials for photonic applications. The incorporation of RE ions in low phonon energy fluoride nanocrystals (NCs) enhances the optical emission along with the energy transfer processes between RE ions. The combination of specific crystal properties and glass processing methods allows novel materials to be developed, such as glass-ceramic optical fibres and thin films. However, the correlation between processing method, structure and optical properties is an essential step for a better understanding and optimisation of performance.

    Materials have been prepared by melt quenching (MQ) and sol gel (SG), providing a comparison with respect to crystallisation mechanism, structure, dopant incorporation in the NCs and optical properties. Transparent oxyfluoride glass-ceramics based on aluminosilicate compositions have been prepared by MQ. The crystallisation of LaF3, alpha and beta-KLaF4, and a solid solution of NaLuF4 NCs after heat treatment at Tg +20-100 ºC has been achieved by controlling the base glass composition. Glass-ceramic optical fibres with LaF3 and NaLuF4 NCs have also been prepared using direct melting process and the rod-in-tube method. Sol-gel materials of compositions (100-x)SiO2-x, with up to 20 mol % of active phase x, have been obtained for the first time. LaF3, GdF3 and KLaF4 NCs have been precipitated by controlling the synthesis process and after heat treatment at 350-750 ºC.

    The crystallisation mechanism and structure of bulk materials, films and fibres have been extensively studied, comparing and establishing the differences according to the processing method. The crystallisation of MQ materials is based on a diffusion-controlled process that may or may not start from initial phase-separation regions. The depletion of crystal formers induces a compositional change in the glass matrix causing a self-limited crystal growth. The effect of RE dopants on the crystallisation has also been studied by DTA and XRD. Moreover, specific doping levels favour the nucleation of certain phases in polymorphous crystals. In contrast, the crystallisation of sol-gel materials occurs via chemical reactions followed by crystal precipitation. In the case that all polymorphous phases are obtained, the principal phase is the most stable after long thermal treatments. NCs homogeneously distributed in the SiO2 glass matrix are observed after fast heat treatment, compared to MQ materials.

    The incorporation of RE dopants in the fluoride NCs has been studied using X-ray Absorption Spectroscopy (XAS) and Electron Paramagnetic Resonance Spectroscopy (EPR). The mechanism of RE incorporation in the NCs was elucidated from evidence of a fluorine-rich environment for RE ions in the initial glass and xerogel. The quantification of crystal fraction, using Rietveld refinement, and RE-ion incorporation in the fluoride NCs have also been determined, demonstrating how low doping levels (0.1 mol%) produce much higher effective concentrations in the NCs. Relevant differences in the crystallised fraction have been observed for MQ and SG materials.

    The optical characterisation of RE-doped materials showed the photoluminescence (PL) enhancement in the glass-ceramics compared to the untreated glass and xerogel samples. Stronger energy-transfer processes take place in the glass-ceramic. Crystallike optical features have been reproduced in bulk and fibre materials prepared by SG and MQ, respectively. PL spectra of LaF3:Nd3+ single crystals in glass-ceramic optical fibres have been obtained for the first time. However, for SG materials, notable differences in the PL of bulk samples and thin films have been observed and correlated to the structure.