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

Abstract

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.

Light absorption by surface nanoholes and nanobumps

“…The energy absorption maps on the surfaces are particularly required to understand laser interac-tion with matter, because the patterns of maximum absorbed energy define precisely the thermoaffected zones, where the material modification such as ablation is likely to take place after the energy deposition [12][13][14][15][16][17][18][19]. This way, the laseraffected surface can be efficiently treated pulse by pulse.…”

“…This way, the laseraffected surface can be efficiently treated pulse by pulse. The modifications strongly influence the surface roughness and the energy deposition below the surface [18,20], therefore, it appears crucial to predict in which way the absorbed energy profiles change. Self-organization of polarizationdependent periodic surface nanostructures is among the impressive phenomena induced by multipulse ultrashort laser irradiation [17,18,21,22].…”

“…In this paper, we investigate numerically the energy absorption below dielectric and metallic surfaces with a single or several holes or ridges and analyze the influence of light polarization, optical properties of material, and arbitrary shape of nanoobjects on the resulting scattered radiative and nonradiative fields. For this, we solve a full system of Maxwell's equations by Finite-Difference Time-Domain approach [33,34], which was successfully used for scattering, transmission and absorption data analysis in case of subwavelength individual, periodic and randomly distributed holes and bumps on the surface [17,18,21,22,28,35,36]. We focus on two different materials, widely used in industry: unexcited fused silica glass (dielectric) and stainless steel (metal).…”

Ultrasensitive SERS-Based Plasmonic Sensor with Analyte Enrichment System Produced by Direct Laser Writing

“…To produce the SERS-active structures on the central site, we chose a rather simple approach based on the spallation (or "swelling") of a noble (or semi-noble) metal target when subjected to fs laser pulses. The mechanism of this process involves laser-induced melting of a surface layer of the target followed by lift-off of this layer owing to inertial stress confinement [30][31][32][33][34] or even subsurface boiling [35]. As a result, the ejected surface layer leaves a crater of a few microns in diameter and with multiple self-organized nanospikes.…”

Ultrasensitive SERS-Based Plasmonic Sensor with Analyte Enrichment System Produced by Direct Laser Writing

“…To produce the SERS-active structures on the central site, we chose a rather simple approach based on the spallation (or "swelling") of a noble (or semi-noble) metal target when subjected to fs laser pulses. The mechanism of this process involves laser-induced melting of a surface layer of the target followed by lift-off of this layer owing to inertial stress confinement [29][30][31][32][33] or even subsurface boiling [34]. As a result, the ejected surface layer leaves a crater of a few microns in diameter and with multiple self-organized nanospikes.…”