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Many surfaces in nature, e.g. lotus leaf, exhibit superhydrophobicity. Some of the most attractive applications of these surfaces are based on their self-cleaning properties and anti-icing capability. Many strategies are used by researchers to replicate these natural phenomena on metallic substrates. Among them, short/ultrashort pulsed laser technologies can functionalize surfaces with micro/nano-textures enabling strong water-repellent properties and low adhesiveness, which represent a promising solution to anti-icing properties. In this work, several patterns of micro-structures were textured by femtosecond laser on metallic materials of aeronautic and aerospace interest. The wettability properties of the surfaces were investigated in terms of water contact angle (CA) under different ambient conditions. The reversibility of the sample superhydrophobicity after exposure to a highly humid environment was studied. Water-dripping tests were carried out at subzero temperature finding that, while the untreated samples were covered with ice, no frozen spot was observed on the superhydrophobic textured surfaces.

In this work, we report on a single-pass method for cutting 250-μm thick Z-cut quartz plates using 200 fs laser pulses at the wavelength of 1030 nm. In particular, we delve into the influence of the process parameters, i.e. laser repetition rate, scan speed and pulse energy, on the generation of a controlled stress-induced fracture which ultimately leads to the final cut. Processing above a certain threshold pulse energy caused significant damage, resulting in poor quality cuts. Whereas, a correct combination of these parameters led to a flat and almost defect-free cut edges, in a single pass.

Inertial microfluidic particles sorting represents a critical task in many areas of biology, biotechnology, and medicine, including the isolation from blood of rare target cell populations, like e.g. circulating tumor cells (CTCs) and circulating fetal cells (CFCs). Usually, cell sorter microfluidic devices are fabricated by PDMS soft lithography, which is the most widespread micromanufacturing platform enabling to cost-effectively produce Lab-on-a-Chip with resolution in the nanometer scale. However, this technology presents some drawbacks: (i) due to PDMS softness, especially for high-pressure flows, the microfluidic behavior may change along the devices, leading to ambiguous results; (ii) soft-lithography allows to pattern structures on just one side of the chip thus limiting the affordable geometries to enhance the throughput of target particles. In this work, we develop a PMMA continuous size-based inertial microfluidic sorter by femtosecond laser microfabrication (FLM). The device design includes contracting and expanding channels (microchambers) provided with siphoning outlets on the backside of the chip. Since FLM technology is in principle applicable to any type of polymer, we chose PMMA, which is a biocompatible and transparent thermoplastic polymer much stiffer than PDMS. FLM allows machining the microfluidic network on both sides of the chip, making it possible the parallelization of the sorting process. In addition, thanks to the FLM flexibility, we easily varied the chambers number and the collecting strategy (at different flow rates) in order to define a device layout maximizing the trapping efficiency and throughput.

In this work, we report on the fabrication of laser induced periodic surface structures (LIPSS) on stainless steel, using bursts of 200 fs sub-pulses at a wavelength of 1030 nm. A cascade of birefringent crystals was used to generate the bursts with tunable number of sub-pulses and intra-burst delays varying between 1.5 ps and 24 ps. Being such a delay shorter than the typical electron-lattice relaxation time in metals, the sub-pulses impinge on the sample surface when the material is still in a transient state after excitation from the first sub-pulse, thus allowing peculiar structures to be generated depending on the burst features. We obtained 1-D and 2-D periodic surface structures and investigated the influence of number of sub-pulses and polarization on their morphology. In particular, when bursts composed by all-aligned linearly polarized sub-pulses were used, 1-D LIPSS were obtained with different periodicity and depths depending on the number of sub-pulses. Bursts with crossed linear polarization or circular polarization sub-pulses produced 2-D LIPSS with morphology varying from triangular structures arranged in hexagonal lattice to pillar-like ordered or disordered structures depending on the bursts features. In most cases these structures exhibit a superhydrophobic behavior, as assessed by static contact angle measurements, which is achieved after a time of exposition to laboratory air. By XPS analysis we investigated the chemical variations occurring on the surfaces over this time.

We report on an experimental study of the incubation effect during irradiation of stainless steel targets with bursts of femtosecond laser pulses at 1030 nm wavelength and 100 kHz repetition rate. ...

Femtosecond-pulsed laser welding of transparent materials on a micrometer scale is a versatile tool for the fabrication and assembly of electronic, electromechanical, and especially biomedical micro-devices. In this paper, we report on microwelding of two transparent layers of polymethyl methacrylate (PMMA) with femtosecond laser pulses at 1030 nm in the MHz regime. We aim at exploiting localized heat accumulation to weld the two layers without any preprocessing of the sample and any intermediate absorbing media, by focusing fs-laser pulses at the interface. The modifications produced by the focused laser beam into the bulk material have been firstly investigated depending on the laser process parameters aiming to produce continuous melting. Results have been evaluated based on heat accumulation models. Finally, fs-laser welding of PMMA samples have been successfully demonstrated and tested by leakage tests for application in direct laser assembly of microfluidic devices.

Surface micro-texturing has been widely theoretically and experimentally demonstrated to be beneficial to friction reduction in sliding contacts under lubricated regimes. Several microscopic mechanisms have been assessed to concur to this macroscopic effect. In particular, the micro-textures act as lubricant reservoirs, as well as traps for debris. Furthermore, they may produce a local reduction of the shear stress coupled with a stable hydrodynamic pressure between the lubricated sliding surfaces. All these mechanisms are strongly dependent both on the micro-texturing geometry and on the operating conditions. Among the various micro-machining techniques, laser ablation with ultrashort pulses is an emerging technology to fabricate surface textures, thanks to the intrinsic property of laser light to be tightly focused and the high flexibility and precision achievable. In addition, when using sub-ps pulses, the thermal damage on the workpiece is negligible and the laser surface textures (LST) are not affected by burrs, cracks or resolidified melted droplets, detrimental to the frictional properties. In this work several LST geometries have been fabricated by fs-laser ablation of steel surfaces, varying the diameter, depth and spacing of micro-dimples squared patterns. We compared their frictional performance with a reference nontextured sample, on a range of sliding velocities from the mixed lubrication to the hydrodynamic regime. The measured Stribeck curves data show that the depth and diameter of the microholes have a huge influence in determining the amount of friction reduction at the interface. Different theoretical interpretations to explain the experimental findings are also provided.

Surface hardening with discrete laser spot treatment is an interesting solution since the adoption of a single pulse allows the treatment of different surface geometries avoiding the effect of back tempering. The aim of this work is to find a suitable process window in which operate to get best results in terms of hardness, diameter and depth of the treated region. A single pulse out of a fiber laser source impinging on a bearing hypereutectoid steel was used using different power values, pulse energy and defocussing distances, in order to get the optimal process parameters. The dimensions of the hardened zone and its hardness were then acquired and related to the laser process parameters, to the prior microstructure of the steel (spheroidized and tempered after oil quenching) and to the roughness on the specimen before the laser treatment. Experimental results highlighted that both the surface condition (in terms of roughness) and the initial steel microstructure have a great influence on the achieved hardness values and on the dimension of the laser hardened layer. The pulse energy and power strongly affected the dimension of the hardened layer, too.

We demonstrated a sensing technique for in-line ablation rate detection using a quantum cascade laser (QCL) under external optical feedback. The design of the QCL-based diagnostic system allowed to monitor the voltage modulation at the laser terminals induced by fast dynamics in the ablation process. Real-time detection of the ablation front velocity as well as in-situ investigations of the surface temperature were provided. Experimental results on fast ablation rates per pulse correlate well with the theoretical prediction. The detection range was demonstrated to be limited only by the QCL-probe emission wavelength, which is scalable up to the THz spectral region.

In-process monitoring and feedback control are fundamental actions for stable and good quality laser welding process. In particular, penetration depth is one of the most critical features to be monitored. In this research, overlap welding of stainless steel is investigated to stably reproduce a fixed penetration depth using both CO 2 and Nd:YAG lasers. Plasma electron temperatures of Fe(I) and Cr(I) are evaluated as in process monitoring using the measurement of intensities of emission lines with fast spectrometers. The sensor system is calibrated using a quantitative relationship between electron temperature and penetration depth in different welding conditions. Finally closed loop control of the weld penetration depth is implemented by acquiring the electron temperature value and by adjusting the laser power to maintain a pre-set penetration depth. A PI controller is successfully used to stabilize the electron temperature around the set point corresponding to the right penetration depth starting from a wrong value of any initial laser power different than the set point. Optical inspection of the weld surface and macroscopic analyses of cross sections verify the results obtained with the proposed closed-loop system based on a spectroscopic controller and confirms the reliability of our system.

Direct real-time measurements of the penetration depth during laser micromachining has been demonstrated by developing a novel ablation sensor based on laser diode feedback interferometry. Percussion drilling experiments have been performed by focusing a 120-ps pulsed fiber laser onto metallic targets with different thermal conductivity. In-situ monitoring of the material removal rate was achieved by coaxially aligning the beam probe with the ablating laser. The displacement of the ablation front was revealed with sub-micrometric resolution by analyzing the sawtooth-like induced modulation of the interferometric signal out of the detector system.

We report the realization of an evanescently coupled laser-written type II array in χ-cut Lithium niobate. Certain processing parameters allow evanescent fields to extend beyond the regions of damage, while still increasing the index sufficiently to guide light. An array consisting of eleven coupled waveguides was fabricated. Coupling was evaluated by observing discrete diffraction patterns of single waveguide excitations at various array sites. Homogeneous coupling was verified within the array, while the outermost guides are slightly detuned due to being formed by just one damage structure.

We present an experimental study of the drilling of metal targets with ultrashort laser pulses with pulse durations from 800 fs to 19 ps at repetition rates up to 1 MHz, average powers up to 70 Watts, using an Ytterbium-doped fiber CPA system. Particle shielding and heat accumulation have been found to influence the drilling efficiency at high repetition rates. Particle shielding causes an increase in the number of pulses for breakthrough. It occurs at a few hundred kHz, depending on the pulse energy and duration. The heat accumulation effect is noticed at higher repetition rates. Although it overbalances the particle shielding thus making the drilling process faster, heat accumulation is responsible for the formation of a large amount of molten material that limits the hole quality. The variations of the pulse duration reveal that heat accumulation starts at higher repetition rates for shorter pulse lengths. This is in agreement with the observed higher ablation efficiency with shorter pulse duration. Thus, the shorter pulses might be advantageous if highest precision and processing speed is required.

Aluminum alloys are interesting in many and many industrial applications, from the classical aircraft industry to rail and road vehicles manufacturing (High Speed Train, Car Structure and Body). Recently much more attention for Aluminum Alloys, 5000 and 6000 Series, has been carried out by Shipbuilding Industry, especially for using in the H.S.L.C. (High Speed Light Craft). Therefore the aim of this experimental work has been to study, develop and test a reproducible CO2 laser welding procedure and technique of four specific alloys, that is AA 5083, AA 5383, AA 5059 (Al-Mg Alloys), and AA 6082 (Al-Mg-Si Alloy). Different techniques, methodologies, covering gases, nozzles, focusing lenses and mirrors, welding speed range, laser power range (1000 and 2500 W) have been carefully experimented. The melted zones properties have been evaluated by cross sections, and some visual inspections by a NIKON LUCIA Imaging System correlating each experimental test, results and evaluations to the adopted process parameters and to the thermo-physical properties of the tested alloys.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In this work we present an innovative optical sensor for on- line and non-intrusive welding process monitoring. It is based on the spectroscopic analysis of the optical VIS emission of the welding plasma plume generated in the laser- metal interaction zone. Plasma electron temperature has been measured for different chemical species composing the plume. Temperature signal evolution has been recorded and analyzed during several CO2-laser welding processes, under variable operating conditions. We have developed a suitable software able to real time detect a wide range of weld defects like crater formation, lack of fusion, excessive penetration, seam oxidation. The same spectroscopic approach has been applied for electric arc welding process monitoring. We assembled our optical sensor in a torch for manual Gas Tungsten Arc Welding procedures and tested the prototype in a manufacturing industry production line. Even in this case we found a clear correlation between the signal behavior and the welded joint quality.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.