Ultra-short pulse laser texturing is a well-known one-step technique used to transform the surface properties of different materials in order to functionalize them for specific applications. According to the laser and process parameters, several features can be achieved, as surface coloring, blackening and super-hydrophobicity. In this work, an upscaling approach is considered for generation of surface structures and thermal effects, connected to the use of high-average power lasers are considered in relation to the influence of the laser pulse duration and repetition rate on the final surface morphology. Mirror-polished 316L steel samples were textured by an UPS laser source with pulse duration of about 450fs and running at 1030nm, at two different repetition rates, 250kHz and 1000kHz. Results show that two main sources of thermal effects are identified: (i) heat accumulation due to the use of high repetition rates and (ii) thermal diffusion effects linked to the intrinsic nature of the material. When employing high repetition rates, a lower cumulative energy is necessary to highlight the influence of the pulse duration on the surface morphology. Finally, the influence of pulse duration and wavelength on the wetting properties of the material surface are also investigated.
Super-hydrophobic surfaces are nowadays of primary interest in several application fields, as for de-icing devices in the automotive and aerospace industries. In this context, laser surface texturing has widely demonstrated to be an easy one-step method to produce super-hydrophobic surfaces on several materials. In this work, a high average power (up to 40W), high repetition-rate (up to 1MHz), femtosecond infrared laser was employed to produce super-hydrophobic surfaces on 316L steel. The set of process and laser parameters for which the super-hydrophobic behavior is optimized, was obtained by varying the laser energy and repetition rate. The morphology of the textured surfaces was firstly analyzed by SEM and confocal microscope analyses. The contact angle was measured over time in order to investigate the effect of air environment on the hydrophobic properties and define the period of time necessary for the super-hydrophobic properties to stabilize. An investigation on the effect of after-processing cleaning solvents on the CA evolution was carried to assess the influence of the after-processing sample handling on the CA evaluation. Results show that the highest values of contact angle, that is the best hydrophobic behavior, are obtained at high repetition rate and low energy, this way opening up a promising scenario in terms of upscaling for reducing the overall process takt-time.
Pulsed laser ablation in liquid (PLAL) is nowadays gaining popularity as innovative, reliable and efficient technique to produce high-purity nanoparticles (NPs) of many inorganic and organic materials. In this context, attention has been recently focused on luminescent up-conversion NPs (UCNPs) which, being characterized by sharp emission bands in ultraviolet (UV)-to-near-infrared (NIR) range upon NIR irradiation, are in fact of great interest in many biological and biomedical applications. Moreover, with respect to organic dyes NPs and quantum dots, UCNPs show less toxicity, increased chemical stability, long-lifetime decays and lack of photo-bleaching. Our research focuses on generation of UCNPs of rare earth lanthanide-doped crystalline material, namely 18%Yb:1%Er:NAYF<sub>4</sub>, by PLAL in water. It is well known that optical properties of NPs strongly depend on their features, as for instance size and shape, which in turn may be controlled by laser ablation parameters. Therefore, two different laser sources are used for the ablation processes in order to find the set of laser parameter, i.e. pulse duration, laser fluence and repetition rate, for which the luminescence of UPNPs is optimized: (i) Amplitude Satsuma HP3 system: 330 fs pulse duration, 1030 wavelength and (ii) Eolite Hegoa system: 50 ps pulse duration, 1030 nm wavelength. UCNPs are finally characterized by spectrophotometer analyses to define emission range and intensity under NIR light and by transmission electron microscopy (TEM) to determine their size and shape.
Recently a parametric decay model was proposed in order to foresee LIPSS interspaces, and experimental results are in reasonable agreement. To confirm the possibility assumed by the model of pre-formed plasma generation, Ti surface was irradiated by a femtosecond (fs) laser beam composed by double fs pulses, with a fixed delay of 160 fs. The fluence of the first pulse (FPP), responsible for surface plasma formation, was varied in the range 10-50 mJ cm<sup>-2</sup> and always kept below the LIPSS formation threshold fluence (FLIPSS) of Ti for 50-single-shots exposure. The fluence of the delayed pulse (FLP), responsible for LIPSS formation, was varied in the range 60-150 mJ cm<sup>-2</sup> and always kept above FLIPSS. Regardless the specific fluence FLP of the delayed pulse, the interspace of the grating structures increases with the increase of FPP, that is the increase of the surface plasma density. This tendency suggests that a variation of the surface plasma density, due to a variation of FPP, actually leads to a modification of the grating features, highlighting the driving role of the first pulse in LIPSS formation. Moreover, we observed that the LIPSS periodicities after double pulse exposures are in quite good agreement with data on LIPSS periodicities after single 160 fs pulse irradiations on Ti surface and with the curve predicted by the parametric decay model. This experimental result suggests that the preformed plasma might be produced in the rising edge of the temporal profile of the laser pulse.
An overview of Czech national R&D project HiLASE (High average power pulsed LASEr) is presented. The HiLASE project aims at development of pulsed DPSSL for hi-tech industrial applications. HiLASE will be a user oriented facility with several laser systems with output parameters ranging from a few picosecond pulses with energy of 5 mJ to 0.5 J and repetition rate of 1-100 kHz (based on thin disk technology) to systems with 100 J output energy in nanosecond pulses with repetition rate of 10 Hz (based on multi-slab technology).
Nowadays, more powerful and challenging laser systems are built to meet the need of evolving technology. In this
context, the aim of the HiLASE project  is to develop a multi-joule picosecond laser system working in kHz repetition
rate regime. The outputs of the project will provide not only unique source for both scientific and industrial applications,
but also great challenge for supporting technologies. The key parameter of all optical components in laser and beam
delivery structure is the laser induced damage threshold, which limits intensities manageable by the system. The
following paper presents results of LIDT test of mirrors intended to use in laser system built within the HiLASE project
as well as advanced LIDT test station design, which will use HiLASE laser as source.
We have developed a novel method for efficient structuring of the surface of materials by applying femtosecond near
infrared laser pulses simultaneously with a weak extreme ultraviolet beam, which leads to a very strong radiation-matter
interaction and brings a dramatic increase of the surface processing speed. We present our recent experimental results on
surface nanostructuring of thin films of amorphous carbon and polymethyl methacrylate deposited on bulk substrates and
discuss the underlying physical mechanisms. In the case of amorphous carbon, large areas of laser-induced periodic
surface structures with a spatial period of 550 nm were created, having their origin in laser-induced convective currents.
Our method provides a powerful tool for fast modification of tribological properties of the irradiated sample.