Active sol-gel materials, fluorescence spectra, and lifetimes

Optical spectroscopy techniques are crucial tool for the assessment of solgel- derived materials, structures, and devices of interest in several high-tech and environmental application areas, the major one concerning photonics devices. Sol-gel materials are known since long time as a convenient host for rare earths and luminescent species and have been widely used for the fabrication of photonic confined structures. Their exploitation covers a range of application possibilities and system performance that are not solely ICT oriented but also concern lighting, laser, sensing, energy, environment, and health. The aim of the present paper is to highlight the application of the spectroscopic techniques to the characterization of active sol-gel materials and to present a brief overview of the efforts and progresses made in the area. After a brief summary of the fundamentals of the photoluminescence properties and of the measurement techniques, some specific arguments are discussed. The effect of oxydril group on the luminescence quenching in rareearth- doped materials is discussed paying attention to the matrix solubility and energy-transfer mechanics. At this regard, the upconversion dynamics in erbium-activated systems is exhaustively presented. The role of europium as a structural probe and the spectroscopic techniques of site-selection spectroscopy, crucial in the assessment of sol-gel materials, are discussed looking at several experimental results. 1-D microcavities are considered as well as the effect of the presence of the cavity on the emission features of the sample and in particular on the multimodal laser action. Sol-gel technology is largely employed to develop glass-ceramics that constitute a class of nanocomposite materials which offer specific characteristics of capital importance in photonics. As tutorial examples, we mention two binary systems that have been largely investigated during the last 10 years, i.e., SiO2-SnO2 and SiO2-HfO2 glass-ceramics waveguides activated by rare-earth ions. Finally, the paragraph of this chapter is devoted to the luminescence spectroscopy in rare-earth-activated micro- and nanospheres where whispering gallery modes microresonators and monodisperse colloidal spheres are deeply discussed.