Laser micro- and nano-material processing – Part 2

This special issue of Advanced Optical Technologies (AOT) is dedicated to the field of laser-based microand nanostructuring methods. Due to the unique characteristics of pulsed laser systems, among them ultrafast sources with picosecond and femtosecond pulse durations, today we are experiencing an explosion of new technological developments that will open new perspectives for industrial applications in the near future. This becomes possible due to a continuous reduction in the cost of laser sources as well as the outstanding improvement of the power stability, increased pulse repetition frequencies, as well as the simplicity of the new laser devices. However, although these developments are necessary for boosting the availability of lasers in modern industrial manufacturing, they alone will not define the industrialization of laser-based applications. In this context, additional efforts are still necessary for understanding how specific surface functionalities on different materials can be created or even improved by developing specific textured surfaces as well as how to produce these topographies at high throughput by the full utilization of the laser performance. Our list of contributors for this issue reflects a leading-edge mix of experts in these areas, from all around the world. The special issue is published in two parts. Part 2 of the special issue ‘Laser Microand NanoMaterial Processing’ contains four original research articles that are briefly summarized here: Fosodeder et al. demonstrate in in-vitro experiments that a ring of hierarchical micro-nanostructures (selforganized micro-spikes covered with laser-induced periodic surface structures) processed on a titanium alloy cylinder by femtosecond laser irradiation and subsequent anodic oxidation can act as an efficient barrier preventing the overgrowth with fibroblast cells. These results pave way for applications in miniaturized cardiac pacemakers that can be implanted directly into the heart. Ocaña et al. process hierarchical micro-nanostructures (complex two-dimensional periodic multi-gratings) on plane titanium alloy samples by combining the techniques of nanosecond direct laser writing (DLW) with picosecond direct laser interference patterning (DLIP). The processed surfaces feature a strongly hydrophobic surface wettability and may exhibit an improved electrochemical corrosion resistance. Genieys et al. study the ablation of four different metals (Al, Cu, Ni, W) irradiated by single titanium sapphire laser pulses with durations ranging between 15 fs and 100 fs. For these metals, a constant ablation threshold is reported and the energy specific ablation efficiency is quantified on the basis of an analysis of the ablation depths. The authors demonstrate that for metals there is no real interest in using few-optical-cycle pulse durations for ablation-based application processes. Bauerhenne and Garcia analyze the phenomenon of non-thermal melting by performing systematic ab-initio molecular dynamics (MD) simulations of femtosecond laser excited silicon using electron temperature-dependent density functional theory (DFT). The simulations reveal that the indirect electronic band gap decreases as a universal function of the atomic mean-square displacement almost independently of the electronic temperature (laser fluence) and that the dependence is linear for a wide range of mean-square displacements. We would like to thank all authors for their contributions to this special issue, reporting on new insights in this fascinating topic that significantly increase the capabilities in manufacturing technology. We would also like to acknowledge AOT for coordinating and guiding this special issue as well as all reviewers for their fruitful comments, which permitted improving the quality of the presented articles. We hope you will enjoy reading the articles in this special issue as much as we have enjoyed putting them together. *Corresponding authors: Andrés Fabián Lasagni, Technische Universität Dresden, Institute for Manufacturing Technology, 01062 Dresden, Germany, e-mail: andres_fabian.lasagni@tu-dresden.de; and Jörn Bonse, Bundesanstalt für Materialforschung und -prüfung (BAM), Unter den Eichen 87, 12205 Berlin, Germany, e-mail: joern.bonse@bam.de. https://orcid.org/0000-00034984-3896