Inhomogeneities in glass: From defects to functional nanostructures

Glass inhomogeneities represent variations in the structural or compositional uniformity of glass, traditionally associated with process-related defects such as striae, bubbles, stones, and inclusions that impair transparency and mechanical stability. These “technological” inhomogeneities emerge during melting, forming, or annealing, and have long been the focus of industrial elimination strategies. However, recent developments in glass science and nanotechnology have reframed inhomogeneity as a potential asset. When precisely engineered at the nanoscale, inhomogeneities, such as nanocrystals, metal or semiconductor nanoparticles, and nanopores, can enhance glass with tailored optical and photonic functionalities, including upconversion luminescence, plasmonic response, nonlinear refractive behavior, and sensing capabilities. This entry provides an integrated perspective on the evolution of glass inhomogeneities, tracing the shift from defect suppression to functional nanostructuring. It discusses both the traditional classification and mitigation of detrimental defects, and the design principles enabling the intentional incorporation of beneficial nanoinhomogeneities, particularly in the context of optics and photonics. The utilization of engineered inhomogeneities in nuclear waste glasses is also discussed.