Coupled Theoretical and Experimental Studies for the Characterization of Defects in Silica Under Irradiation
Résumé
Silica-based optical fiber and fiber-based sensor integration in radiative environments is more and more considered since these components and systems present unique advantages for data transport, plasma diagnostics and for temperature/strain distributed sensing in harsh environments [1]. However, ionizing radiations generate point defects inside the silica matrix leading to macroscopic changes in the optical fibers properties. Defects in amorphous silica are studied experimentally for almost fifty years (see for example [2] for E'γ center in a-SiO2) but the attribution of absorption/photoluminescence bands to an atomic structure is still a major problematic in the domain [1,3]. Indeed experimental identification of defects is still complex and requires the combination of numerous experimental characterization means such as Electronic Paramagnetic Resonance (EPR) and absorption and photoluminescence spectroscopy. But each experimental means has its own intrinsic limitations. For example, EPR can only detect paramagnetic defects. Moreover, the wide variety of the studied radiative environments in terms of dose and dose rate, applications parameters (fiber type, operating wavelength,..) implies to multiply high costs radiation tests to estimate the vulnerabilities of various commercial or prototype fibers and if necessary to investigate innovative treatments to improve their hardness. That is why, today, the development of a multi-scale simulation procedure appears mandatory to overcome the future challenges [4]. In this paper, we will present the latest results we obtained through an approach coupling experimental and theoretical characterizations of defects in pure and doped (Germanium, Fluorine, Phosphorous) amorphous silica in a multi-scale scheme [5]. On the experimental side, obtained data such as the concentration of a type of defects, coming from Electronic Paramagnetic Resonance, absorption and photoluminescence spectroscopy will be discussed in function of irradiation dose and dopants. On the theoretical side, results coming from first principles calculations will be presented. Computations using Density Functional Theory (DFT) and Many Body Perturbation Theory (MBPT) to the DFT as the GW approximation and the resolution of the Bethe Salpeter Equation have been performed to characterize the structural, electronic and optical properties of defects in pure and doped amorphous silica (see [6] for results in Germanium doped amorphous silica). [1] S. Girard et al., IEEE Trans. Nuc. Sci., 60(3), 2015 (2013). [2] R.A. Weeks, J. Appl. Phys., 27, 1376 (1956). [3] L. Skuja Journal of Non-Crystalline Solids 239, 16 (1998). [4] J. L. Bourgade et al., Rev. Sci. Instrum., 79, 10F304 (2008). [5] S. Girard et al., IEEE Trans. Nuc. Sci., 55(6), 3473 (2008); S. Girard et al., IEEE Trans. Nuc. Sci., 55(6), 3508 (2008). [6] N. Richard et al., J. Phys.: Condens. Matter, 25, 335502 (2013).