Ultrafast laser nanostructuring in bulk silica, a slow microexplosion

Ultrafast laser microexplosions in bulk material create extreme conditions at mesoscopic scales and are essential to the synthesis of extraordinary matter structural phases and to light structuring beyond the diffraction limit. Observing the transformation cycle can elucidate their evolution. We discuss multiscale relaxation dynamics in the formation of nanoscale structures in laser-irradiated fused silica. Tightly focused ultrafast nondiffractive Bessel beams are used to generate microexplosions that lead to uniform voids. These trigger thermodynamic nonequilibrium conditions in one-dimensional geometries with record excitation confinement down to 100 nm and electronic pressures in the gigapascal range. Time-resolved phase-contrast microscopy on nanosecond to microsecond scales indicates that void formation is a slow process developing from low-viscosity phases after persistent plasma fluid stages signaled via nanosecond-long luminescence. The void evolution is not necessarily driven by rarefaction following initial pressure relaxation, but involves molecular kinetics and stress mechanisms that interfere with the evolution of the liquid phase and induce cavitation. Heat transport is also visualized. Higher energy leads to hydrodynamic instabilities and void fragmentation. The dynamic view helps us understand material transformation under confinement. (C) 2017 Optical Society of America