Making Light Matter

Lasers permeate our daily lives in many ways and specifically materials processing with lasers is key for numerous applications in science, technology, medicine, and industry. During the last years, different trends have been observed in the field of laser materials processing, i.e., 1) the pursuit of reaching extremely small scales and ultimate resolution in laser nanostructuring,1 and 2) an up-scaling of the processing rates for meeting industrial demands.2 Both trends are driven by the rapid advancements in ultrafast laser technologies in terms of throughput, complexity, and interoperability—currently manifesting in a Moore-law-like scaling of the average output powers with time—and, at the same time, benefiting from the newest developments in data-driven processes. Most applications of laser processing do manifest between these two extremes and are critically impacted by latest developments in material sciences. Europe plays a major role in each of these fields, as it successfully combines an excellent scientific environment with advanced technological capabilities and, thus, provided the ground for developing a successful industry in laser technologies. Long lasting scientific conferences, such as the Spring Meeting of the European Materials Research Society (E-MRS)3 represent an important platform to exchange the most recent developments among those fields. This Special Issue represents the proceedings of the Symposium L "Making light matter: lasers in material sciences and photonics" held from May 29th to June 2nd 2023 in the Congress negative chirp: "blue" spectral components are arriving first). Dominic et al. (article number 2300703) investigated sub-wavelength-sized high spatial frequency LIPSS (HSFL) formed upon fs-laser irradiation (60 fs pulse duration, 800 nm center wavelength, 1 kHz pulse repetition rate) of tungsten in ultrahigh vacuum conditions. An analysis of the surface topography along with two-temperature model coupled molecular dynamics simulations revealed that the HSFL formation on tungsten originates from laser-induced thermal stresses that are involving both, surface tension and tensile forces. Moreover, the experimental observation of sub-surface cavitation effects points towards a hydrodynamics-based origin for these nanostructures. Müller et al. (article number 2300719) explored in a multi-method surface analytical approach the influence of the laser pulse repetition rate and the number of passes (over-scans) on the surface topography and chemistry during the ultrashort pulsed laser processing (925 fs pulse duration, 1030 nm center wavelength) of sub-100 nm sized HSFL on Ti-6Al-4V titanium alloy in ambient air environment. The authors demonstrated that it is possible to increase the processing speed of HSFL to pulse repetition rates up to 400 kHz in an industrially suitable manner, while simultaneously avoiding limiting heat accumulation effects. Hard X-ray photoelectron spectroscopy (HAXPES) and depth-profiling time-of-flight secondary ion mass spectrometry (ToF-SIMS) revealed that the several tens of nanometer shallow HSFL nanostructures on Ti-6Al-4V are accompanied by a graded near-surface oxide layer here that extents a few tens of nanometers into depth and consists mainly of TiO2 with a small admixture of Ti2O3. Madapana et al. (article number 2300610) also used Ti-6Al-4V titanium alloy for scan-processing of areas filled with parallel micrometric ablation lines that are covered with nanometric surface features (including LSFL). The ultrafast laser processed surfaces (100 fs–3 ps pulse duration, 800 nm center wavelength, 10 kHz pulse repetition rate) were subsequently characterized microscopically and surface analytically and corrosion test were performed by electrochemical impedance spectroscopy (EIS). The authors reported the presence of strain in the grains of the irradiated material for depths up 150 μm, aligned in the direction of some specific crystallographic planes. Additionally, a more than 100 nm thick laser-induced oxide layer consisting of TiO2 and Ti2O3 was found at the surface. Moreover, the authors concluded from the EIS measurements that the laser processing resulted in a thick protective passive layer (being responsible for a higher corrosion resistance), covered by a thin outer porous layer. Voss et al. (article number 2300920) combined electrochemical pre- and post-anodization steps with the ultrashort pulsed laser processing (925 fs pulse duration, 1030 nm center wavelength) of hierarchical micro-spikes covered with nanoscale laser-induced periodic surface structures (LIPSS) on Ti-6Al-4V in ambient air environment Scanning electron and white light interference microscopy, X-ray diffraction analyses, micro-Raman spectroscopy and hard X-ray photoelectron spectroscopy are employed to evaluate morphological, topographical, structural, and chemical surface alterations for processing conditions previously explored for applications in medical implants. Lorenz et al. (article number 2300482) combined the ultrashort pulsed laser surface texturing (10 ps pulse duration, 1030 nm wavelength, 100 kHz pulse repetition rate) of stainless steel (AISI 304) with additional chemical modification through a self-assembled monolayer of a fluorinated, phosphonic acid-modified alkane, resulting in the formation of a hierarchical micro-nanostructured surfaces (including LSFL) with highly hydrophobic properties. Additional exposition to mechanical wear led to the abrasive removal of parts of the topmost surface layers, causing a reduction of the water contact angle. This surface wettability change is influenced by the design pattern of the hierarchical surface, the laser processing parameters, and the strength of the abrasive wear. The authors developed a model that is capable to explain several of the observed trends. Nedyalkov et al. (article number 2300478) studied the influence of the ambient pressure on the electrical resistance of surface tracks processed by ps-laser pulses (10 ps pulse duration, 355 or 1064 nm wavelength, 1 kHz pulse repetition rate) at the surface of AlN ceramic in different vacuum conditions (10−4 torr up to atmospheric pressure). It was demonstrated that under certain conditions the ps-laser processing can turn the irradiated material electrically conductive. A decrease of the resistivity by up to a few orders of magnitude, from about tens of kΩ to a few Ω was observed, depending on the processing conditions. Laser processing in the presence of nitrogen gas flow led to a reduction of the tracks' resistance. The systematically explored effects were attributed to the formation of an Al-enriched layer and its oxidation either during (at high pressures) or after (at low pressures) laser-induced material decomposition and surface cooling. Ogor et al. (article number 2300486) studied the speed-up and parallelization of the additive manufacturing approach based on multi-photon polymerization. The authors developed a phenomenological digital model of the photochemical process for parallelized multiphoton-based fabrication of three-dimensional structures by imaging a 1920 × 1080 pixels spatial light modulator onto an ultrasensitive polymeric triplet–triplet annihilation resist. In that approach, optical propagation and chemical diffusion effects are involved. For single laser exposures, the simulation provided reasonable predictions of 2D and 2.5D microscale structures and helped to improve 3D structures' resolution upon two laser exposure treatments. Heinke et al. (article number 2300485) combined the process of laser cleaning with atmospheric pressure plasma jet (APPJ) processing of different multi-component optical materials (N-BK7, Zerodur®). The laser processing step enables the removal of nonvolatile reaction products formed during the APPJ etching from metal oxides contained in the optical glasses. The processed samples were examined by scanning electron microscopy. Sets of optimized laser processing parameters (laser fluence, pulse number ranges) were identified that allowed the removal of the APPJ residual layer without any observable damage of the substrate material. Lorenz et al. (article number 2300481) studied the fabrication of plasmonic microcubes by laser ablation of gold nanoparticle (Au-NP) loaded acrylates. The samples were prepared by UV light curing of an Au-filled acrylate film and subsequent thermal treatment, leading to the precipitation and growth of 1 to 10 nm sized Au-NPs as characterized by transmission electron microscopy (TEM) and optical emission spectroscopy. Additional laser processing (sub-ps pulse duration, 343 nm wavelength, 100 kHz pulse repetition rate) of the Au-NP acrylate films in a cross-hatched line processing scheme using a moderately focused laser beam (Gaussian beam radius ≈16 μm) then led to the formation of gold micro-cuboids with sizes down to 15 μm. This concept allows an easy and large-scale fabrication of microstructures with independently adjustable plasmonic properties. A working safety aspect of growing relevance was tackled by Kraft et al. (article number 2300725). The ever-growing pulse repetition rates of high energy ultrashort laser pulses can drive the secondary emission of hard X-rays during the laser machining of technically materials over the (country specific) regulatory safety limits. The article reports on a set of experiments using differently laser-pretreated stainless steel (AISI 304) sheets processed with high repetition rate ultrashort laser pulses (600 fs pulse duration, 1030 nm wavelength, 35 μJ laser pulse energy, 0.5 to 2.0 MHz pulse repetition rate) focused by a 167 mm focal length f-theta objective. The measurements showed an influence of the surface roughness and the number of laser scans on the laser-induced X-ray skin dose rate. Additionally, the dependency of X-ray emission on the scan direction relatively to the laser beam polarization state was explored. The authors concluded that in almost every of their situations the laser-induced secondary X-ray emission exceeded the German legal limitation. In such circumstances protection measures must be taken to shield against this hazardous ionizing radiation, while operating the laser system. In a guest contribution from the Symposium E "Carbon- and/or nitrogen-containing thin films and nanomaterials", Naser et al. (article number 2300750) explored the fast large-area characterization of silicone films by Coherent Raman Scattering (CRS) imaging, providing a chemically specific contrast. The authors compared their results to that of conventional (spontaneous) Raman (SR) spectroscopy, stimulated Raman scattering (SRS), and coherent anti-stokes Raman scattering (CARS). Jörn Bonse received a Diploma degree in Physics from the University of Hannover in 1996, and a Doctoral degree from the Technical University of Berlin in 2001. He held research positions at the Laser Zentrum Hannover, the Max-Born-Institute in Berlin, the Spanish National Research Council in Madrid, and was appointed as a Senior Laser Application Specialist at Newport Spectra-Physics. Currently, he is a Senior Scientist at the German Federal Institute for Materials Research and Testing (BAM). His research interests include fundamentals and applications of laser-matter interaction, laser-induced periodic surface structures, surface functionalization, time-resolved optical techniques, and laser processes in photovoltaics. Irina Alexandra Paun graduated at the Faculty of Physics, University of Bucharest (2001). She received the PhD Diploma at Politehnica University from Bucharest in 2009, where she became Full Professor in Physics in 2023. In 2022, she obtained the Habilitation in Physics and became PhD Supervisor at Doctoral School of Applied Sciences. Since 2011 she also activates at National Institute for Lasers, Plasma and Radiation Physics (INFLPR), where she became Scientific Researcher First Degree in 2024. Between 2016-2017, 2021-2022 she was Head of the Laser Materials Processing Laboratory at INFLPR. Her research interests are laser 3D and 4D printing of biocompatible materials. Johannes Heitz is expert in the field of laser-matter interaction at surfaces including photo-induced nanopatterning and modification of polymer surfaces, and deposition of thin polymer films by laser-ablation. He studied Physics in Freiburg (Germany) until 1989. Since then he is employed at the Institute of Applied Physics at the Johannes Kepler University Linz (Dissertation 1993, Habilitation 1999). Since 1999 he is Associate Professor at the university in Linz. From January 1995 he stayed for one year in Tsukuba, Japan as research fellow. Razvan Stoian graduated in 1996 from the Bucharest University and received his PhD degree in 2000 from the Free University, Berlin. He was with the Ultrafast Laser Material-Processing Group at the Max-Born Institute, Berlin between 1997 and 2004, and joined the Centre Nationale de la Recherche Scientifique France in 2004 where he is now Research Director. He is leading the Laser-Matter Interaction group at the Laboratoire Hubert Curien in St. Étienne. He is equally scientific supervisor of Manutech-USD, a technological platform for advanced laser processing. His research interests include laser–matter interactions, ultrafast phenomena, and laser material processing.