From ultrafast laser-matter interaction to permanent modification: x-ray and optical insights into femtosecond-laser induced crystallization

Femtosecond lasers can drive amorphous-to-crystalline transitions, yet the underlying mechanism remains debated. We investigate this process in tellurium-oxide glass using operando X-ray diffraction combined with ultrafast transient optical reflectivity. Our results reveal a cascade of events: plasma generation, melt pool formation during the laser pulse train, and post-irradiation phase separation leading to crystalline tellurium. The key parameter governing crystallization is the laser repetition rate, indicating a fundamentally thermal process. Modifications occur near the surface, accompanied by morphological changes and oxygen release. This study clarifies the dynamics of laser-induced crystallization and demonstrates the value of X-ray methods for optimizing femtosecond laser microfabrication.