Laura Gemini

Metal-like self-organization of periodic nanostructures on silicon and silicon carbide under femtosecond laser pulses

Periodic structures were generated on Si and SiC surfaces by irradiation with femtosecond laser pulses. Self-organized structures with spatial periodicity of approximately 600 nm appear on silicon and silicon carbide in the laser fluence range just above the ablation threshold and upon irradiation with a large number of pulses. As in the case of metals, the dependence of the spatial periodicity on laser fluence can be explained by the parametric decay of laser light into surface plasma waves. The results show that the proposed model might be universally applicable to any solid state material.

High throughput laser texturing of super-hydrophobic surfaces on steel

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.

Towards Laser-Textured Antibacterial Surfaces

Escherichia coli and Staphylococcus aureus bacterial retention on mirror-polished and ultrashort pulse laser-textured surfaces is quantified with a new approach based on ISO standards for measurement of antibacterial performance. It is shown that both wettability and surface morphology influence antibacterial behavior, with neither superhydrophobicity nor low surface roughness alone sufficient for reducing initial retention of either tested cell type. Surface structures comprising spikes, laser-induced periodic surface structures (LIPSS) and nano-pillars are produced with 1030 nm wavelength 350 fs laser pulses of energy 19.1 μJ, 1.01 μJ and 1.46 μJ, respectively. SEM analysis, optical profilometry, shear force microscopy and wettability analysis reveal surface structures with peak separations of 20–40 μm, 0.5–0.9 μm and 0.8–1.3 μm, average areal surface roughness of 8.6 μm, 90 nm and 60 nm and static water contact angles of 160°, 119° and 140°, respectively. E. coli retention is highest for mirror-polished specimens and spikes whose characteristic dimensions are much larger than the cell size. S. aureus retention is instead found to be inhibited under the same conditions due to low surface roughness for mirror-polished samples (Sa: 30 nm) and low wettability for spikes. LIPSS and nano-pillars are found to reduce E. coli retention by 99.8% and 99.2%, respectively, and S. aureus retention by 84.7% and 79.9% in terms of viable colony forming units after two hours of immersion in bacterial broth due to both low wettability and fine surface features that limit the number of available attachment points. The ability to tailor both wettability and surface morphology via ultrashort pulsed laser processing confirms this approach as an important tool for producing the next generation of antibacterial surfaces.

Formation of upconversion nanoparticles of 18%Yb:1%Er:NAYF4 by ultra-short pulse laser ablation in water

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:NAYF4, 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.

Surface functionalization of metal surfaces by large-area USP laser texturing

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.

Laser fluence dependence of periodic structures on metals produced by femtosecond double pulse laser

The formation mechanism of LIPSS has been investigated for titanium irradiated by double pulse. We found that variation of the surface plasma density characterized by first pulse led to a variation of the grating interspaces.

Formation of laser induced periodic surface structures (LIPSS) on Ti upon double fs pulse exposure

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-2 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-2 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.

Advanced LIDT testing station in the frame of the HiLASE Project

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 [1] 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.