Femtosecond laser interaction with ULE® glass

Ultra-Low Expansion (ULE ®) glass, a silica-based glass with TiO 2 content below 10%, possesses the lowest coefficient of thermal expansion among engineered materials and is integral to high-precision optical systems. Despite its technological importance, its interaction with ultrafast laser irradiation remains insufficiently understood. Here, we investigate structural and chemical modifications induced by sub-ablation femtosecond laser pulses in ULE ® glass and assess how Ti incorporation alters energy absorption and relaxation pathways compared with pure silica. Using Raman spectroscopy, optical microscopy, and birefringence analysis, we show that TiO 2 modifies the nonlinear absorption threshold and promotes defect-assisted energy localization, leading to distinct regimes of photodarkening, localized densification, and nanocrystallization. The presence of Ti further drives photo-induced phase segregation and Ti-rich nanocluster formation, suppressing the welldefined nanograting periodicity typically observed in fused silica. These results reveal how Ti-induced network heterogeneity and Ti-O bonding chemistry govern ultrafast energy deposition and structural evolution, offering new guidelines for controlled laser processing of ultra-stable low-expansion glass systems.