Ultrafast laser nanostructuring in bulk silica, a “slow” microexplosion

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