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
The natures of most radiation-induced point defects in amorphous silicon dioxide (a-SiO2) 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-SiO2-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. 1. Introduction Optical fibers and metal-oxide-semiconductor (MOS) devices based on amorphous forms of SiO2 (a-SiO2) are components of many photonic devices and systems that require hardness against nuclear and/or space radiations. Development of radiation-hardened optical fibers based on glassy silica, as well as MOS devices with amorphous SiO2 gate insulators, is dependent on a fundamental understanding of radiation-induced defect formation in the a-SiO2 component. The multitude of relevant radiation damage processes are schematically illustrated in Figure 1, which is taken from [1] where these processes are discussed in considerable detail. The present paper will limit its focus to the natures of the radiation-induced point defects—also termed “color centers”—that are known to absorb light in the wavelength range ~500 to ~2000？nm in silica-based optical fibers and other photonic devices. A concise review of radiolytic trapped-oxide charges in
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