In Situ SrF ₂ Crystallization in Transparent Germanate Glass‐Ceramics Driven by Erbium for NIR and MIR Applications

ABSTRACT

Transparent oxyfluoride glass‐ceramics combining broad ultraviolet‐to‐mid‐infrared transmission with enhanced rare‐earth luminescence are promising materials for mid‐infrared photonic applications. In this work, erbium‐doped indium‐germanate oxyfluoride glasses exhibiting good glass‐forming ability under conventional melt–quenching conditions were synthesized and converted into transparent glass‐ceramics through controlled heat treatment near the glass transition temperature. All compositions exhibit weak hydroxyl absorption (<0.3 cm ⁻¹ ) and wide optical transparency from the ultraviolet to the mid‐infrared. Ceramization selectively induces the precipitation of SrF ₂ nanocrystals without significantly altering the glass transition temperature or refractive index. Structural analyses reveal uniformly distributed SrF ₂ nanocrystals with characteristic sizes below ∼40 nm and suggest partial incorporation of Er ³⁺ ions into fluorine‐rich crystalline environments. Based on Vegard‐law analysis of the X‐ray diffraction lattice parameters using orthorhombic ErF ₃ (Pnma) as reference, the erbium concentration within the SrF ₂ nanocrystals is estimated to reach up to ∼8 at.%. The associated visible transmission losses are attributed to Rayleigh scattering from the nanocrystalline domains. Spectroscopic measurements show that nanocrystallization modifies the local environment of Er ³⁺ ions, leading to reduced inhomogeneous broadening, resolved Stark splitting, and enhanced emission intensity and lifetimes at both 1.5 and 2.7 µm. In particular, part of the 2.7 µm emission exhibits lifetimes extending from a few hundred microseconds in the precursor glass up to 1.77 ms in the glass‐ceramics. The luminescence enhancement is attributed to the presence of fluorine‐rich low‐phonon‐energy environments associated with the SrF ₂ nanocrystals, which reduce multiphonon relaxation rates. Time‐resolved studies indicate the coexistence of two erbium environments in the glass‐ceramics, corresponding to ions located in the oxide glass matrix and in fluorine‐rich and/or crystalline surroundings. Despite crystalline fractions remaining below ∼6.2%, millisecond‐scale lifetime components reaching 1.77 ms are observed at 2.7 µm, demonstrating that redistribution of only a small fraction of Er ³⁺ ions toward fluorine‐rich/crystalline environments is sufficient to induce pronounced changes in the luminescence response. These results demonstrate the effectiveness of controlled SrF ₂ nanocrystal precipitation for enhancing erbium luminescence and highlight the potential of this material system for 2.7 µm erbium‐doped mid‐infrared fiber lasers.