Volume 9, Issue 3 - Laser Micro- and Nano-Material Processing – Part 2 / Guest Editors: Jörn Bonse, Andrés Fabian Lasagni

The invention of new miniaturized and smart medical implants continues in all medical fields, including miniaturized heart pacemakers. These implants often come with a titanium (Ti) casing, which may have to be removed after several months or years and shall therefore not be completely overgrown by cells or scar tissue after implantation. Scar tissue is mainly formed by fibroblast cells and extracellular matrix proteins like collagen produced by them. Suppression of fibroblast growth at Ti surfaces could be achieved by 800 nm femtosecond laser-ablation creating self-organized sharp spikes with dimensions in the 10 μm-range which are superposed by fine sub-μm parallel ripples. On flat Ti control samples, the best results regarding suppression of cell growth were obtained on spike-structures which were additionally electrochemically anodized under acidic conditions. When Ti cylinders with a diameter of 8 mm (similar as the pacemakers) were placed upright in a culture of murine fibroblasts, a multi-layer cell growth up to a height of at least 1.5 mm occurred within 19–22 days. We have demonstrated that a laser-structured and anodized ring around the Ti cylinder surface is an effective way to create a barrier that murine fibroblasts were not able to overgrow within this time.

The use of pulsed laser irradiation techniques has proven to be a clearly effective procedure for the achievement of surface properties modification via micro-/nano-structuration, different conceptual approaches having been the subject of research and extensively reported in the literature. Completing the broad spectrum of applications developed mostly involving the generation of structured surfaces (particularly of metallic materials) with specific contact, friction and wear functionalities, the application of laser sources to the surface structuration of metal surfaces for the modification of their wetability and corrosion resistance properties is considered. The particular problems found for the generation of the appropriate surface microstructure able to replicate the hydrophobic behaviour of some live structures present in nature, their long term stability and their amenability to macroscopic scale are discussed along with innovative methods to generate the required hierarchical micro-/nano-structures by a combination of the DLW and DLIP techniques.

We provide measurements of the ablation of four post-transition and transition metals [aluminum (Al), copper (Cu), nickel (Ni) and tungsten (W)] irradiated by single 800 nm laser pulses, in ultrashort regime from 100 femtosecond (fs) pulse duration down to 15 fs covering a temporal range little explored as yet. For each metal and pulse duration tested, we measured its ablation characteristics (depth and diameter) as a function of incident energy allowing us to determine its laser-induced ablation threshold and ablation rate in a single-shot regime. For all the metals studied, we observed a constant ablation threshold fluence as a function of pulse duration extending this scaling law to pulse duration of few-optical-cycles. We provide evidence of the interest of adjusting the incident fluence to maximize the energy specific ablation depth but also of the absence of any peculiar advantage related to the use of extremely short-pulse duration for ablation purposes. Those informative and detailed ablation data have been obtained in the single-pulse regime and in air ambiance. They can serve as rewarding feedback for further establishing smart strategy for femtosecond laser micromachining and laser damage handling of metallic and metal-based components as well as for enhancing accuracy of modeling of fs laser interaction with metals in ultrashort regime.

We performed systematic ab-initio molecular dynamics (MD) simulations of fs-laser-excited silicon (Si) using the T e -dependent density functional theory (DFT). We considered the case in which the potential energy surface (PES) is strongly modified by the laser excitation, so that nonthermal melting occurs. We analyzed the correlation between the time dependence of electronic properties like the band gap and the laser-induced atomic motion. Surprisingly, we found that the indirect electronic band gap decreases as a universal function of the atomic mean-square displacement (MSD) almost independently of the electronic temperature (laser fluence) and that the dependence is linear for a wide range of MSDs. We also found that a universal dependence is also present when analyzing the band gap as a function of the relative Bragg peak intensities, which can be directly measured in experiments.

Heterojunction solar cells based on ternary blends of two donors (absorbers and one acceptor) were investigated using modeling. The Tauc-Lorentz model and experimental absorption spectra of selected compounds were used in the simulations. The optimization process was carried out in this way to maximize the absorption of the system. Poly(3-hexylthiophene-2,5-diyl) (PEHT) was investigated as a first donor, which was mixed respectively with poly(3-octylthiophene-2,5-diyl) (P3OT), coumarin 153, purpurin, fluorescent brightener 184, N-chloroethylene carbazole, and 1,3,6,8 tetrachloro 9n amylocarbazole. Simulations were also performed for the Tauc-Lorentz model.