Mechanisms of Femtosecond Laser-Induced Structural Reconfiguration in Fluoride Glasses

Femtosecond laser direct writing enables precise three-dimensional structuring of transparent materials. However, in fluoride glasses-key platforms for mid-infrared photonics-the pathway linking ultrafast energy deposition to permanent refractive-index modification remains mechanistically unclear. Here, we decode this multiscale pathway from femtosecond excitation to microscale structural modification in fluoroindate glass by integrating time-resolved pump-probe shadowgraphy, birefringence imaging, microscopic elemental analysis, and physics-based modelling. Plasma evolution dynamics reveal peak electron densities approaching 5 × 10²⁰ cm^-3 at 1.2 ps, accompanied by gigapascal-level stress waves propagating at ~4.3 μm/ns. These transient processes generate steep thermal-pressure gradients that drive ion-selective redistribution, producing polarizability-dependent refractive index changes (Δn ≈ 10^-3-10^-2). By correlating plasma dynamics, stress evolution, and compositional redistribution, we establish a unified framework linking energy deposition and structural reconfiguration. The results clarify that positive index regions (i.e., regions with a higher refractive index than the unmodified glass matrix) originate from cation densification (Pb/In enrichment), whereas negative regions are associated with outward F/Ba redistribution and reduced local polarizability. This mechanistic insight provides a physically grounded framework for controllable femtosecond laser processing of fluoride glasses and for mechanism-guided design of mid-infrared integrated photonic structures.