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Graphical abstract Highlights • The inversion of the laser intensity profile by polarization control was demonstrated. • Detailed theoretical and experimental analysis of the formed profiles are performed. • The pillar formation using the inverted interference intensity profile was shown. • The growth of the pillar with additional laser pulses was demonstrated. • The mechanism of the structures formations is discussed in details. Abstract Three-beam laser interference patterning, with the ability to control the polarization of each beam, is used to produce crater and pillar like periodic structures on steel surfaces. The experiments are performed using parallel and radial polarization vector orientations of the interfering beams. In case of parallel vector orientation, common intensity profiles consisting of intensity peaks distributed in triangle lattice geometry are formed. By changing the polarization to the radial set, the intensity distribution of the interference pattern could be inverted while the geometry also has a triangle lattice shape. The equations describing both intensity profiles are derived from the basic interference equation and the modeled interference intensity profiles are compared with measured intensity profiles. Two different types of structures containing periodically distributed craters and pillars are produced using these polarization sets. Using a detailed analysis of the scanning electron microscope images and confocal data, the mechanisms responsible of the formation of the pillar- and crater-like geometries are introduced. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Square micro-structures were fabricated with nanosecond laser pulses in Ti-6Al-4V. • LIPSS with a spatial period of 810 nm were created in top of the prior structures. • Air and polyethylene storage conditions were used to analyze aging and wettability. • Hierarchical structures were hydrophobic with SCA over 160° for polyethylene. • Hydrophobic surface had the lower concentration of polar molecules. Abstract In the present work, a study of laser-based surface structuring of aerospace-grade titanium alloy (Ti-6Al-4V) with subsequent ageing by employing two different storage methods is undertaken. The titanium alloy samples were patterned using UV-ns and IR-fs pulsed lasers in a two-step process to fabricate bio-inspired hierarchical structures. The resulting surface structure consisted in regular periodic square-shaped micro-pillars covered by 810 nm-periodic LIPSS. After the laser processing the samples were kept in two different storage conditions: exposed to ambient air and inside polyethylene bags. The polyethylene bags were found to be beneficial for the surface ageing of laser-fabricated titanium surfaces, increasing the ageing time when compared to ageing by exposure to ambient air. Hierarchical surface topographies exhibited higher water-repellency when compared to non-hierarchical structures. Especially, hierarchical structures reached a hydrophobic state with water contact angle over 160° after 3 weeks storage in polyethylene bags. The micro-structured surfaces were characterized by using confocal microscopy, scanning electron microscopy, static contact angle measurements and X-ray photoelectron spectroscopy. [ABSTRACT FROM AUTHOR]

The adaption and inclusion of electrical technologies with various applications have motivated researchers to develop the most efficient and effective sensors, batteries and capacitors for safe, reliable and prolonged operations. However, miniaturization of sensors and practically associated with batteries and capacitors have driven them to employ nanofibers and nanotubes made of various material for reliable operations. Considering wide acceptance and available research for silicon material, nanofibers and nanoparticles made using silicon can be a great candidate for manufacturing of such sensors and electrical components. To employ nanofibers for those applications, understanding of its resistive characteristics can be crucial. In our study, we have manufactured hybrid silicon nanofibrous structure using novel approach of pico-second laser ablation and performed 4-point resistivity test to comprehend the change in its electrical behavior with variations in different laser parameters like frequency, power and scanning parameter of number of loops. Significant change in resistivity and topological structure was observed for surface ablated with various laser and scanning parameters. [ABSTRACT FROM AUTHOR]

Periodic surface patterns can provide materials with special optical properties, which are usable in decorative or security applications. However, they can be sensitive to contact wear and thus their lifetime and functionality are limited. This study describes the use of direct laser interference patterning for structuring a multilayered polymer film at its interface creating periodic in-volume structures which are resistant to contact wear. The spatial period of the structures are varied in the range of 1.0 µm to 2.0 µm in order to produce decorative elements. The pattern formation at the interface is explained using cross sectional observations and a thermal simulation of the temperature evolution during the laser treatment at the interface. Both, the diffraction efficiency and direct transmission are characterized by light intensity measurements to describe the optical behavior of the produced periodic structures and a decorative application example is presented. [ABSTRACT FROM AUTHOR]

In order to achieve a surface pretreatment of the bearing steel 100Cr6 (1–1.5 wt.% Cr) a laser-based process was used. The obtained modification may result in an optimization of the adhesive properties of the surface with respect to an anticorrosion polymer coating on the basis of PEEK (poly-ether-ether-ketone), which is applied on the steel surface by a laser melting technique. This work deals with the influence of the laser-based pretreatment regarding the surface microstructure and the micro-hardness of the steel, which has been examined by scanning electron microscopy (SEM), light microscopy and automated micro-hardness testing. The most suitable parameter set for the laser-based pretreatment leads to the formation of very hard white etching zones (WEZ) with a thickness of 23 μm, whereas this pretreatment also induces topographical changes. The occurrence of the white etching zones is attributed to near-surface re-austenitization and rapid quenching. Moreover, dark etching zones (DEZ) with a thickness of 32 μm are found at the laser path edges as well as underneath the white etching zones (WEZ). In these areas, the hardness is decreased due to the formation of oxides as a consequence of re-tempering. [ABSTRACT FROM AUTHOR]

Diffraction limits the focusing capabilities of an optical system seriously constraining the use of lasers for nanopatterning. In this work, we present a novel and simple approach to reduce the minimum feature size of a laser-direct write system by ablating a pre-stretched material. In particular, by focusing and scanning a femtosecond laser beam on the surface of a uniaxially pre-stretched elastomeric membrane we are able to obtain microstructures according to a desired pattern. After removing the stress applied to the elastomer, the membrane relaxes to its original size and the ablated patterns shrink while preserving their shape. In this way, the minimum feature size that is typically determined by the optical properties of the focusing system can be now controlled by the strain applied to the elastomer during the ablation process. We demonstrate this approach by ablating lines on a stretchable polymeric membrane at different strain conditions. Experimental results are in good agreement with theoretical predictions. The proposed method opens up new interesting possibilities for the rapid prototyping of micro- and nano-structures suitable for a wide range of applications such as soft-lithography, micro-/nano-fluidics and lab-on-chip. [ABSTRACT FROM AUTHOR]

The interaction of short and ultrashort pulse laser radiation with glass materials is addressed. Particular attention is paid to regimes which are important in industrial applications such as laser cutting, drilling, functionalization of material surfaces, etc. Different factors influencing the ablation efficiency and quality are summarized and their importance is illustrated experimentally. The effects of ambient gas ionization in front of the irradiated target are also analyzed. A possibility to enhance laser coupling with transparent solids by bi-wavelength irradiation is discussed. [ABSTRACT FROM AUTHOR]

Highlights: [•] The paper reports use of laser-based methods for structuring PDMS-moulds. [•] The laser-based methods used include ablation and photo-polymerisation. [•] These methods employ a digital multi-mirror device as a spatial light modulator. [•] PDMS moulds were used to demonstrate micro-contact printing of proteins. [Copyright &y& Elsevier]

Highlights: [•] We studied evolution of porosity inside ablated crater in PMMA as a function of number of laser pulses. [•] For multiple laser pulses, the porous area fraction decreases with increasing pulse energies. [•] But the pore size distribution peaks at smaller pore areas. [•] For line ablation, the porous area fraction increases with scanning speeds. [Copyright &y& Elsevier]

Abstract: The measurement of absorptance is important for the analysis and modelling of laser–material interactions. Unfortunately, most of the absorptance data presently available considers only polished pure metals rather than the commercially available (unpolished, oxidised) alloys, which are actually being processed in manufacturing. This paper presents the results of absorptance measurements carried out at room temperature on as-received engineering grade steels including hot and cold rolled mild steel and stainless steels of various types. The measurements were made using an integrating sphere with an Nd:YLF laser at two wavelengths (1053 and 527nm, which means that the results are also valid for Nd:YAG radiation at 1064 and 532nm). The absorptance results obtained differ considerably from existing data for polished, pure metals and should help improve the accuracy of laser–material interaction models. Some clear trends were identified; for all materials studied, the absorptance was considerably higher than the previously published values for the relevant pure metals with polished surfaces. For all 15 samples the absorptance was higher for the green than for the infrared wavelength. No clear trend correlating the absorptance with the roughness was found for mild steel in the roughness range Sa 0.4–5.6μm. A correlation between absorptance and roughness was noted for stainless steel for Sa values above 1.5μm. [Copyright &y& Elsevier]

Abstract: Photostructurable glass ceramic (PSGC) materials contain a sensitizer that is used to facilitate the optical exposure process. The primary role of the sensitizer is to absorb incident radiation and generate photoelectrons. With thermal treatment, these photoelectrons can then interact with nascent metal ions to induce the formation of metallic clusters and the precipitation of a soluble crystalline phase in the glass matrix. The photo-ionization efficiency of the sensitizer species is strongly dependent on its spectral absorption and oxidation state in the base glass. Stabilizing compounds are typically added to the glass matrix to maintain the photo-active oxidation state and promote efficient exposure. To investigate the effectiveness of the photo-initiator, we have conducted experiments in which sample coupons of a commercial PSGC material (Foturan™, Schott Corp., Germany) were carefully exposed to various photon doses by pulsed UV nanosecond lasers at λ =266nm and 355nm. Foturan is a lithium aluminosilicate glass that contains trace amounts of cerium as the photosensitive agent (0.01–0.04wt.% admixture Ce2O3). The photo-initiator efficiency was investigated by using samples with cerium and without cerium. The irradiation wavelengths were selected because they lie above and below the primary absorption band of the cerium photo-initiator. Optical transmission spectroscopy (OTS) was employed to identify and monitor the population density of the photo-induced trapped electron state as a function of incident laser irradiance. The irradiated samples were thermally processed and then analyzed again with OTS to measure the quenching of the trapped electron state and the concurrent growth of a spectral band associated with the formation of nanometer-scale metallic clusters. The growth of metallic clusters signifies the “fixing” of the exposure and permanent image formation in the glass. The OTS results reveal that for λ =266nm laser irradiation, at least two photoelectron donors (including cerium) contribute to the formation of the trapped electron (defect state) density and fixing of the exposure. For λ =355nm laser irradiation, however, the photo-initiator is critical to the exposure process and nearly all of the trapped electron density is formed via cerium photoelectron donors. [Copyright &y& Elsevier]

Abstract: We report the evolution of sharp conical spike array formed on the silicon surface under the cumulative picosecond (ps) and femtosecond (fs) laser pulse irradiations in SF6 ambient. The experimental results suggest that the physical mechanisms of conical spike evolutions under ps and fs laser irradiations are different. Under the ps laser irradiation, silicon surface is melted before the spike arrays formed, while under the fs laser irradiation, the formation of spike arrays does not pass through the liquid phase. The evolution of microstructure is strongly dependent on the laser duration. [Copyright &y& Elsevier]

Using X-ray photoelectron spectroscopy (XPS), we have investigated the surface of polyimide (kapton), which was subjected to processing by femtosecond UV radiation from an excimer laser (KrF: λ=248 nm, tp∼380 fs) in air. Significant variations were observed in the C 1s and O 1s spectra of the processed surface of kapton, indicating chemical modifications due to loss of volatile groups such as C&z.dbnd6;O and C&z.sbnd;N. Incorporation of oxygen via bonding as C&z.sbnd;O&z.sbnd;C of the aromatic rings was observed during processing. Bulk characterization of the processed kapton by Fourier transform infrared spectroscopy (FTIR) allowed a study of laser-induced modifications in the material at energy densities below the ablation threshold. Scanning electron microscopy (SEM) studies of the surface morphology of the processed zone of kapton indicates mechanically clean and damage-free processing with good edge quality. [Copyright &y& Elsevier]

We have investigated by X-ray photoelectron spectroscopy (XPS) the surface of poly(tetrafluoroethylene) (PTFE) films, which were subjected to processing by femtosecond (fs) UV radiation from an excimer laser (KrF: λ=248 nm, tp∼380 fs) in air. Bulk characterization of processed PTFE films by Fourier transform infrared spectroscopy (FTIR) permit an investigation of the laser induced modifications in the material at energy densities below the ablation threshold. No features in XPS and FTIR spectra indicated the incorporation of hydrogen and/or oxygen, or the formation of a cross-linked network of carbon indicating chemically clean processing in contrast to nanosecond excimer laser processing which chemically degrades the surface. Scanning electron microscopy (SEM) of the micrometer size vertical interconnect (microvia) indicated mechanically and thermally damage free processing of PTFE with good edge quality, in contrast to nanosecond excimer laser processing. [Copyright &y& Elsevier]

• Development of an optical method to quantify the self-cleaning functionality of superhydrophobic aluminium surfaces. • DLIP structures of 7.0 µm spatial period demonstrate a cleaning of up to 99% of the contaminating particles. • High-speed recordings show the interaction between contamination and single water droplets. Self-cleaning ability on technical surfaces can increase the added value of a product. A common path to achieve this property is making the surface superhydrophobic so that water droplets can roll down, picking up dirt particles. In this contribution, the self-cleaning efficiency of Al surfaces structured with direct laser writing (DLW), direct laser interference patterning (DLIP) and a combination of both technologies was quantitatively determined. This was performed by developing a characterization method, where the treated samples are firstly covered with either MnO 2 or polyamide micro-particles, then tilted by 15° and 30° and finally washed applying up to nine water droplets (10 µl) over the contaminated surfaces. Then, an optical analysis by image processing of the remaining contamination particles on the textured surfaces was realized after each droplet rolled over the surface. The DLIP textures showed the best performance, allowing the removal of more than 90% of the particles after just three droplets were released. High-speed videos and scanning electron microscopy characterization allowed a deeper understanding on the cleaning behavior and on the relationship between surface microstructure and particle size and shape. [ABSTRACT FROM AUTHOR]