Relation between porosity‐based and crystal‐based femtosecond laser‐induced nanogratings in oxide glasses

Femtosecond laser irradiation in glass, under controlled conditions, can induce two types of nanogratings (NG): porous nanogratings (pNG) and crystal/glass nanogratings (cNG). While most glasses exhibit only one type, binary aluminosilicate glasses have shown both types of NG. This work investigates the conditions enabling such possibility in this glass system. First, we combine the Johnson‒Mehl‒Avrami‒Kolmogorov equation with laser thermal treatment curves, varying laser parameters (repetition rate and pulse energy). This investigation clarifies crystallization conditions during irradiation and identifies those required to form NG in crystalline regions. Second, we propose an explanation for the unexpected superior thermal stability of cNG compared to pNG, as reported in the literature. Our model predicts a 200°C improvement considering 1% retardance drift over 25 years. Finally, we map the domains of existence and coexistence of pNG and cNG within a typical repetition rate ‐ pulse energy landscape. This approach enables the determination of optimal femtosecond laser crystallization conditions by integrating glass composition with laser parameters (speed, pulse energy, and repetition rate). It offers a practical toolbox for glass design and optical property engineering.