Dynamic Ultrafast Laser-Induced Structural Changes and Extreme Nanostructuring in Hard Dielectric Materials

Abstract

Understanding matter transformation under light enables ultrafast 3D laser structuring to attain precise control down to the nanoscale. This is challenging for hard crystals given their mechanical resistance; nonetheless, it is key in structurally designing matter, pendling between crystalline and amorphous phases. The particular time evolution of structural and morphological changes can pinpoint either dynamics related to shock compaction or to thermal relaxation with phase transition. We report quantified the time-resolved dynamics of laser modifications induced by nondiffractive ultrafast laser beams in bulk sapphire using qualitative and quantitative phase-contrast microscopy to link optical changes to thermodynamic and structural evolutions. The final morphological changes of irradiated structures are revealed by high-resolution electron microscopy. Observations confirm that Bessel pulse irradiation transforms the pristine crystalline structure into a homogeneous amorphous phase in tens of ns, via the passage through a liquid phase nucleated at the early stages of the process. This ns-lived liquid phase is subject to cavitation at higher energy concentrations on the cooling phase (100 ns), facilitating nanoscale void fabrication with high aspect ratios. The outcomes strongly support bulk modification without shock assistance, governed instead by thermal relaxation. This determines a robust path for extreme laser structuring down to the nanoscale.