Self-Arranged Periodic Nanovoids by Ultrafast Laser-Induced Near-Field Enhancement

The know-how of periodic nanostructuring at tens of nanometers scale is crucial for surface engineering, and the understanding of the physical mechanism underlying it remains of fundamental importance. The fact that ultrafast laser irradiation enables formation of nanostructures with dimensions far below the diffraction limit questions the triggering events. Optically, local near-field enhancement is supposed to be responsible for initial redistribution of the electromagnetic field, while the role of periodic thermomechanical dynamics in this swift and strongly confined regime has not been elucidated. By revealing the periodic nanovoids trapped under the surface as well as nanocavities emerging at the surface, we demonstrate that they are behind the formation of high spatial frequency structures. Driven far beyond equilibrium by an ultrashort laser pulse, the system experiences a phase transition and a cavitation process as a source of punctual nanorelief. The kinetics are probed and evaluated by an original strategy combining double-pulse irradiation and a hydrodynamic modeling approach. The surface self-arrangement mechanism addressed in this work opens the route to further breakthrough in geometric reduction of nanopattern dimensions.