Energy transfer from Ag species to Nd3+ in Ga–fluoride–phosphate glasses: near-infrared emission enhancement via controlled heat treatment and femtosecond laser inscription

Abstract

Gallium fluoride–phosphate glasses are promising materials with wide optical transmission window, high volumetric density, and the ability to accommodate high concentrations of rare earth dopant ions within a tailored fluoride-rich coordination environment, resulting in high emission cross sections. In this work, the compositional system 25Ga(PO₃)₃–20ZnF₂–30BaF₂–(25–x–y)SrF₂–xAgNO₃–yNdF₃ (x = 0–10 mol%, y = 0 or 1 mol%) was studied to understand how silver species affect the near-infrared (NIR) emission of Nd³⁺ ions, when the glasses are subjected to controlled heat treatment and to femtosecond direct laser writing (DLW). The glasses were obtained via the melt-quenching technique and characterized by DSC, XRD, UV-Vis-NIR absorption, and PL spectroscopy. The as-prepared glasses show broad UV-Vis excitation and emission bands arising from the coexistence of Ag⁺ ions and ionic Ag pairs. In samples with 10 mol% Ag⁺, brownish coloration and modified emission profiles indicated Ag nanoparticle formation at the surface. Heat treatment promoted the conversion of isolated Ag⁺ into ionic pairs, producing broadband emissions tunable by excitation wavelength and Ag⁺ concentration. In co-doped samples, Nd³⁺ introduced absorption dips in the Ag-related UV-Vis bands, consistent with energy transfer, which was further confirmed by shortened Ag excited-state lifetimes and increased Nd³⁺ NIR emission under UV-Vis excitation. In order to control the spatial distribution and size of Ag aggregates (nanoclusters, NCs) and to increase the energy transfer efficiency to Nd³⁺, femtosecond direct laser writing (DLW) was employed to co-doped glasses with 3 and 5 mol% Ag⁺. This approach enabled three-dimensional localized growth of Ag NCs with sub-micron spatial control. In the laser processed regions, the NIR emissions of Nd³⁺ at 900 and 1060 nm were significantly enhanced, clearly evidencing enhanced energy transfer from the localized laser-induced Ag-NCs to the Nd³⁺ ions. These findings suggest the possibility of tailoring high optical contrast near-IR emissions in glasses, enabling progress in advanced photonic applications.