Among recent advanced manufacturing techniques introduced over the last decades, non-ablative femtosecond laser processing has gained a lot of attention thanks to its applicability to a variety of substrates and its unique ability to locally process transparent materials in their volumes. The laser-induced taxonomy of structural modifications is rich and, despite the extreme brevity of the laser-matter interaction, includes nano-crystallization events as recently reported in various amorphous substrates. Yet, the mechanism leading to these nano-crystallization phenomena driven by locally extreme exposure conditions, similar to warm-dense state of matter (WDM), remains elusive.

We present in situ nano-crystallization dynamics using X-ray microdiffraction, reporting such experiments for the first time to our knowledge. Specifically, we investigate the case of a femtosecond laser-induced nano-crystallization process in an amorphous multilayer stack of Al2O3/Nb2O5 layers using operando X-ray micro-diffraction at the microXAS beamline of the Swiss Light Source (SLS). We identify the crystalline phases and the timescales of the transition using varying laser exposure conditions.