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.

Coupling ultrafast light irradiation to surface nanoreliefs leads to periodic patterns, achieving record processing scales down to tens of nanometers.

In the course of laser-induced surface self-organization, an optical feedback mechanism is generally evoked as the main process driving the final rippled topography. To unravel the role of light and transient nanostructures in a multipulse irradiation regime, we use a high-resolved imaging technique, the photoemission electron microscopy. The pulse-topulse evolution of the inhomogeneous laser field distribution on a titanium surface nanostructured by a femtosecond laser is investigated at the nanoscale. Photoelectron imaging allows to separate the contributions of radiative and nonradiative scattered fields and to unveil the correlation between the nanocavity density and the quasi-periodic light absorption. The multipulse experimental observations are supported by electromagnetic calculations showing that the absorption of light on an evolving surface relief drives the selection of the final periodicity. The surface distribution of light absorption is influenced by surface topography that transiently adapts following the collective optical response of nanocavities, namely, the reinforcement of the dipole−dipole coupling as the nanocavity density increases. Mapping the evolution of the photoelectron emission sheds new light on the intricate mechanisms controlling the laser-induced surface self-organization features.