GeO₂-based novel optical glasses with high refractive index, large mid-infrared transmission, and low thermal expansion

Glasses with a high refractive index and wide transparency in the infrared (IR) region were successfully synthesized. We prepared 20BaO–15TiO₂–5Ga₂O₃–xTeO₂-(40-x)GeO₂ (x = 20–30 mol%) and xBaO–15TiO₂–10Ga₂O₃–20TeO₂-(40-x)GeO₂ (x = 15–25 mol%) glasses and analyzed the relationship between their compositions and physical, thermal, and optical properties. The effects of substituting TeO₂ and BaO for GeO₂ on the refractive index and Abbe number are discussed based on the electronic polarizability, density of the oscillators, and resonance wavelength of the oscillators. The refractive indices increased with increasing TeO₂ and BaO concentrations, owing to the increase in the electronic polarizability of oxygen and the density of the oscillators. The Abbe number decreased with increasing TeO₂ concentration because the resonance wavelength of the oscillator shifted to a longer wavelength, indicating a decrease in the optical bandgap energy. In addition, we observed that the Abbe numbers were almost constant with increasing refractive index when BaO was substituted with GeO₂, and this trend was similar to that of the ultraviolet absorption edge. By substituting TeO₂ and BaO for GeO₂, the cutoff wavelength of the IR region shifted to a longer wavelength. Furthermore, the thermal-expansion coefficient values of the glass samples developed in this study (9.45–11.58 × 10^–6/K) were lower than those of commercial optical glasses.