The possibility of obtaining permanent photoinduced refractive index changes, up to -10^-3, in nanostructured silica-based composites has been demonstrated in SiO₂:SnO₂ optical-grade glass ceramics exposed to ultraviolet radiation. Optical and electron paramagnetic resonance data suggest that the negative refractive index change involves a modification at the surface of the SnO₂ nanoclusters, leading to a reduction of their crystalline size.
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Chiodini et al. (2003) studied this question.
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