Ultrafast laser-induced refractive index tuning inside GeSbSNa glasses for mid-IR applications

Femtosecond laser direct writing (FLDW) enables precise, three-dimensional (3D) structuring within transparent materials, offering a versatile platform for integrated photonics. In this work, we investigate the photo-response of sodium-doped GeSbS (GeSbSNa) chalcogenide glasses under femtosecond laser irradiation, a system of interest for mid-infrared (MIR) applications yet largely unexplored. Through quantitative phase microscopy, we extract refractive index changes as a function of composition and laser pulse energy, achieving phase shifts up to 62 radians and refractive index contrasts (Δn) exceeding 0.05-among the highest reported for chalcogenide systems. Raman spectroscopy reveals localized structural reorganizations, notably a transition from corner- to edge-sharing GeS₄ tetrahedra, consistent with laser-induced densification. We also evaluate the thermal stability of the modifications via isochronal annealing and compare them with commercial glass benchmarks. Overall, these results provide a quantitative basis for assessing GeSbSNa glasses as prospective materials for MIR photonic integration, particularly in contexts where both high photosensitivity and moderate thermal durability are desirable, even though no specific devices are demonstrated here.