A Minireview of the Natures of Radiation-Induced Point Defects in Pure and Doped Silica Glasses and Their Visible/Near-IR Absorption Bands, with Emphasis on Self-Trapped Holes and How They Can Be Controlled

Abstract

The natures of most radiation-induced point defects in amorphous silicon dioxide (a-SiO 2 ) are well known on the basis of 56 years of electron spin resonance (ESR) and optical studies of pure and doped silica glass in bulk, thin-film, and fiber-optic forms. Many of the radiation-induced defects intrinsic to pure and B-, Al-, Ge-, and P-doped silicas are at least briefly described here and references are provided to allow the reader to learn still more about these, as well as some of those defects not mentioned. The metastable self-trapped holes (STHs), intrinsic to both doped and undoped silicas, are argued here to be responsible for most transient red/near-IR optical absorption bands induced in low-OH silica-based optical fibers by ionizing radiations at ambient temperatures. However, accelerated testing of a-SiO 2 -based optical devices slated for space applications must take into account the highly supralinear dependence on ionizing-dose-rate of the initial STH creation rate, which if not recognized would lead to false negatives. Fortunately, however, it is possible to permanently reduce the numbers of environmentally or operationally created STHs by long-term preirradiation at relatively low dose rates. Finally, emphasis is placed on the importance and utility of rigorously derived fractal-kinetic formalisms that facilitate reliable extrapolation of radiation-induced optical attenuations in silica-based photonics recorded as functions of dose rate backward into time domains unreachable in practical laboratory times and forward into dose-rate regimes for which there are no present-day laboratory sources.

Nature of intrinsic and extrinsic electron trapping in SiO2

“…1,2 Hole trapping in silica has been relatively well understood with the models of self-trapped holes [3][4][5][6] and several hole trapping defects well established. [7][8][9] However, identifying sites responsible for electron trapping in silica, bulk and surface, has proved particularly challenging. This is because of a large number of possible charge redistribution channels and presence of water and impurities in most samples.…”

Systematic investigation of composition effects on the radiation-induced attenuation mechanisms of aluminosilicate, Yb-doped silicate, Yb- and Yb,Ce-doped aluminosilicate fiber preforms [Invited]

“…1 together with a Gaussian decomposition attempt. Gaussian components can be assigned to the so-called SiE' and Non-Bridging Oxygen Hole Centers (NBOHCs) [13][14][15]. Centers at 5.75-5.83 eV and full width at half maximum (FWHM) of 0.62-1.05 eV have been indeed widely admitted for the OA band of SiE' centers [13,[22][23][24].…”

“…SiE' centers correspond to threefold oxygen coordinated silicon atoms having an unpaired electron (≡Si•). Almost all variants of SiE' centers have been described as holes trapped at neutral oxygen vacancies [14,15]. NBOHCs correspond to oxygen dangling bonds (≡Si-O•) that mainly result from the radiolysis of O-H bonds [14].…”

“…Finally, an additional OA band is required around 2.7 eV. AlOHC correspond to holes trapped at negatively-charged fourfold oxygen coordinated aluminum units [≡Al-O-Si≡] [15,32]. AlE' centers are formed by electrons trapped at neutral oxygen vacancies shared by two Al atoms [≡Al Al≡] (see [32]) or more probably at positively charged oxygen 'pseudo vacancies' [≡Al + Si≡] [14,15].…”

“…Almost all intrinsic and dopant-related CCs that have been identified so far in silica-based glasses have OA bands in the VIS and UV ranges (only one phosphorus-related color center, the P1 center, has a peak absorption at 1570 nm) [4,5,13]. For most of them, structural models have been debated, generation processes have been proposed in terms of photoionization or carrier trapping, and thermal stability has been assessed by isochronal annealing experiments [13][14][15]. Based on this rich and key knowledge, a much-refined understanding of the RIA or PD mechanisms is now expected which should reveal on which defect free carriers trap preferably, how trapping centers compete, how they transform over time or temperature, how they anneal.…”