Enhancement of Nonlinear Optical Properties in Nb ₂ O ₅ -Modified Fluorophosphate Glasses

Fluorophosphate glasses combine the beneficial features of both phosphate and fluoride networks. They exhibit high thermal and chemical stability, low hygroscopicity, low phonon energy, and a broad transparency window from ultraviolet UVI to near-infrared (NIR), making them attractive for advanced photonic applications. However, their nonlinear optical behavior has remained poorly explored, limiting their applicability for integrated photonics. The incorporation of heavy metal oxides (HMOs), such as Nb2O5, enables tailored structural and optical properties, enhancing third-order nonlinearities. In this work, the nonlinear refractive index (n2) of Nb2O5-modified fluorophosphate glasses was investigated using nonlinear elliptic rotation (NER) measurements, and the results were interpreted with the structural analyses obtained from X-ray photoelectron spectroscopy (XPS). A significant increase in n2 was observed with Nb2O5 content, reaching 1.7 × 10–19 m2/W compared to 0.56 × 10–19 m2/W for samples with 20 and 5% content, respectively. XPS confirmed the progressive formation of nonbridging oxygen species (NBOs) over bridging oxygens (BOs), directly correlating with enhanced polarizability and nonlinear response. Thus, Nb2O5-modified fluorophosphate glasses emerge as promising materials for nonlinear photonic applications, particularly in all-optical switching platforms, due to their good third-order optical figure of merit for photonic applications and favorable structural properties.