Ultrafast Mechanisms of Femtosecond Laser-Induced Structural Reconfiguration in Fluoride Glasses

Femtosecond laser direct writing enables precise three-dimensional structuring of transparent materials, yet the underlying modification mechanisms in fluoride glasses—key platforms for mid-infrared photonics—remain elusive. Here, we elucidate the ultrafast-to-microscale mechanisms governing femtosecond laser-induced modification in fluoroindate glass by integrating time-resolved pump–probe shadowgraphy, birefringence imaging, and microscopic elemental analysis. Plasma evolution dynamics reveal peak electron densities approaching 5×10 ²⁰ 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 selective migration of heavy and light ions, 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 originate from cation densification (Pb/In enrichment), whereas negative regions arise from anion expansion (F/Ba migration). This mechanistic insight provides general design principles for controllable femtosecond-laser processing of fluoride glasses and extends to the rational engineering of low-loss, mid-infrared integrated photonic components.