Advances in unconventional optical materials via aerodynamic levitation: A review of titanate and gallate glasses

Aerodynamic levitation (ADL) is an innovative containerless melting technique that suppresses heterogenous nucleation at solid–liquid interfaces, facilitating deep undercooling and the vitrification of unconventional oxide compositions containing little or no network-forming (NWF) oxides. This review highlights the recent advances and issues in the optical properties of two representative ADL-derived glass families: TiO2- and Ga2O3-based systems. TiO2-based glasses offer ultrahigh refractive indices (2.16–2.31 at 587.56 nm) along with low visible coloration and enhanced thermal stability derived from multicomponent compositional designs. These characteristics have paved the way for scaling up bulk sample dimensions (up to 25 mm in diameter), successful glass-mold pressing into prototype lenses, active fiber drawing, and the first demonstration of laser oscillation in an ADL-derived glass. In contrast, Ga2O3-based glasses exhibit high refractive indices, low dispersions, exceptionally wide optical windows (from ≈280 nm to ≈8 µm), and intense 2.7 µm mid-infrared fluorescence facilitated by their unusually low maximum phonon energy (≈650 cm−1) for oxide glasses. Fundamental structural and electronic interpretations based on oscillator models, spectroscopic experiments, and molecular dynamics simulations revealed that their optical characteristics originate from unique atomic environments, such as densely packed oxygen ions and edge-sharing polyhedra, which differ from those of glasses based on NWF oxides.