To monitor the density of photo-generated charge carriers on a semiconductor surface, we demonstrate a detectorless imaging system based on the analysis of the optical feedback in terahertz quantum cascade lasers. Photo-excited free electron carriers are created in high resistivity n-type silicon wafers via low power (?40 mW/cm2) continuous wave pump laser in the near infrared spectral range. A spatial light modulator allows to directly reconfigure and control the photo-patterned intensity and the associated free-carrier density distribution. The experimental results are in good agreement with the numerical simulations.

We study the incubation effect during laser ablation of stainless steel with ultrashort pulses to boost the material removal efficiency at high repetition rates. The multi-shot ablation threshold fluence has been estimated for two pulse durations, 650-fs and 10-ps, in a range of repetition rates from 50kHz to 1 MHz. Our results show that the threshold fluence decreases with the number of laser pulses N due to damage accumulation mechanisms, as expected. Moreover, approaching the MHz regime, the onset of heat accumulation enhances the incubation effect, which is in turn lower for shorter pulses at repetition rates below 600 kHz. A saturation of the threshold fluence value is shown to occur for a significantly high number of pulses, and well fitted by a modified incubation model. (C) 2014 Optical Society of America

We have investigated the friction properties of lubricated laser micro-textured surfaces. The micro-texture consists of a square lattice of micro-holes whose diameter, depth and spacing are controlled during the laser texturing process. All surfaces have the same texture area density, but different diameters and depths of the micro-holes. We measure the coefficient of friction on a range of sliding velocities from the mixed lubrication regime to the hydrodynamic regime. We find that the depth and the diameter of the micro-holes have a huge influence in determining the amount of friction reduction at the interface. Interestingly experiments also show that optimal micro-hole depth values, minimizing the friction in the hydrodynamic regime, are remarkably effective also in the mixed lubrication regime. (C) 2014 Elsevier Ltd. All rights reserved.

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. © 2014 SPIE.

We present measurements of friction coefficient of lubricated laser surface textured (LST) microstructures with two different geometries. The former is made of a square lattice of microholes; the latter is constituted by a series of microgrooves. We analyze sliding velocities spanning more than two orders of magnitude to cover the entire range from the boundary to the hydrodynamic regime. In all cases, the interfacial pressure is limited to values (relevant to particular manufacturing processes) which allow to neglect macroscopic elastic deformations, piezo-viscosity and oil compressibility effects. The measured Stribeck curves data are compared with those obtained for the flat control surface and show that the regular array of microholes allows to reduce friction over the entire range of lubrication regimes with a decrease of about 50 % in the hydrodynamic regime. On the contrary, the parallel microgrooves lead to an increase of friction compared to the flat control surface with a maximum increase of about 80-100 % in the mixed lubrication regime. These remarkably opposite friction results are then explained with the aid of numerical simulations. Our findings confirm that LST may have cutting edge applications in engineering, not only in classical applications (e.g.; to reduce piston-ring friction losses in internal combustion engines) but also, in particular, in technological processes, such as hydroforming, superplastic forming, where the mapping of the frictional properties of the mold has a crucial role in determining the final properties of the mechanical component. © 2012 Springer Science+Business Media New York.

We report on an experimental study of the incubation effect during laser ablation of stainless steel with fs- and ps-pulses at high repetition rates. Ablation thresholds for multiple pulses N have been estimated. As expected, the ablation threshold decreases with N due to damage accumulation. The related incubation coefficient has been determined at different repetition rates, from 50-kHz to 1-MHz and two pulse durations: 650-fs and 10-ps. Results show that the incubation effect is lower for fs-pulses below 600 kHz. At higher repetition rates incubation is more pronounced regardless of the pulse duration, probably due to heat accumulation. © 2013 The Authors.

Ultrafast laser ablation in liquids is an easy, fast and versatile method to generate nanoparticles. Metal nanoparticles have been demonstrated to possess excellent antimicrobial properties thanks to their very high surface area to volume ratios which provide better contact with microorganisms [1]. For this reason, they are attracting growing interest as a base to develop novel nanocomposites preventing biocontamination in several application fields. In particular, copper nanoparticles (CuNPs) can be used, under controlled ionic release conditions [2], to inhibit bacteria proliferation in food packaging [3] as well as in other applications in medicine, agriculture or pharmaceuticals. In order to prevent human toxicity, CuNPs need to be carefully embedded into polymer matrices acting as immobilizing component and potentially bringing additional properties to the final nanocomposite [4]. Among the possible dispersing polymers, chitosan (CS) is a well-known antimicrobial material, widely exploited for its biodegradability and nontoxicity [5]. © 2013 IEEE.

The instantaneous measurement of both ablation front displacement and removal rate during ultrafast laser microdrilling is demonstrated by on line sensing technique based on optical feedback interferometry in both unipolar and bipolar semiconductor laser. The dependence of laser ablation dynamics on pulse duration, energy density and working pressure has been investigated, thus allowing a significant advancement of the basic understanding of the ultrafast laser-material interactions. Moreover, the detection system results high-sensitive, compact, and easily integrable in most industrial workstations, enabling the development of real-time control to improve ablation efficiency and quality of laser micro-machining processes. © 2013 The Authors.

We experimentally investigate and theoretically interpret the effect of varying the microstructure geometry introduced by laser surface texturing (LST), on the frictional properties of interacting components. The ability to control the coefficient of friction under lubricated conditions is demonstrated. Particularly, the LST optimization of a regular pattern of microholes on steel allows to reduce friction over the entire range of sliding velocities with respect to the untextured case. Moreover, we measure the Stribeck curves on a range of sliding velocity covering the entire lubrication range, i.e. from the boundary to the hydrodynamic regime under the so called iso-viscous rigid condition. Our measurements show a friction reduction up to 50% in the hydrodynamic regime. © 2013 The Authors.

The impact of quantum cascade lasers (QCLs) intrinsically high sensitivity to external optical feedback intended for sensing applications such as in-line ablation rate measurements is experimentally demonstrated. We developed a QCL-based sensor to assess the voltage modulation at the laser terminals induced by fast displacement of the ablation front during the process. This work shows that the detection range of our diagnostic system is only limited by the emission wavelength of the QCL probe source and the capability to measure ablation rates as high as 160 nm/pulse was reported. This sensing technique can be employed with the whole class of quantum cascade lasers, whose emission spans from mid-IR to THz spectral region, thus enabling the extension of its applications to ultra-fast laser ablation processes.

In this paper we describe a novel spectroscopic closed loop control system capable of stabilizing the penetration depth during laser welding processes by controlling the laser power. Our novel approach is to analyze the optical emission from the laser generated plasma plume above the keyhole, to calculate its electron temperature as a process-monitoring signal. Laser power has been controlled by using a quantitative relationship between the penetration depth and the plasma electron temperature. The sensor is able to correlate in real time the difference between the measured electron temperature and its reference value for the requested penetration depth. Accordingly the closed loop system adjusts the power, thus maintaining the penetration depth.

We studied the laser ablation dynamics of steel in the thermal regime both experimentally and theoretically. The real-time monitoring of the process shows that the ablation rate depends on laser energy density and ambient pressure during the exposure time. We demonstrated that the ablation efficiency can be enhanced when the pressure is reduced with respect to the atmospheric pressure for a given laser fluence, reaching an upper limit despite of high-vacuum conditions. An analytical model based on the Hertz-Knudsen law reproduces all the experimental results.

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.

We report on the instantaneous detection of the ablation rate as a function of depth during ultrafast microdrilling of metal targets. The displacement of the ablation front has been measured with a sub-wavelength resolution using an all-optical sensor based on the laser diode self-mixing interferometry. The time dependence of the laser ablation process within the depth of aluminum and stainless steel targets has been investigated to study the evolution of the material removal rate in high aspect-ratio micromachined holes.

We demonstrate that a non-invasive sensing technique based on optical feedback interferometry is capable to instantaneously measure the ablation front displacement and the removal rate during ultrafast laser percussion drilling of metallic plates. The sawtooth-like modulation of the interferometric signal out of the detecting sensor has been analyzed to reveal the time dependence of the removal depth with sub-micrometric resolution. Various dynamic factors related to the influence of laser pulse duration and peak energy have been assessed by in-situ spatial- and time-dependent characterization all through the ablation process. The importance of realtime measurement of the ablation rate is crucial to improve the basic understanding of ultrafast lasermaterial interactions. Moreover, the detection system results high-sensitive, compact, and easily integrable in most industrial workstations, enabling the development of on-line control to improve the ablation efficiency and the quality of laser micromachining processes.

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.

High-energy ultra-short pulse laser ablation is a fast-growing technology in precision laser micromachining of transparent as well as opaque materials. Accurate in-situ measurements of physical parameters such as the penetration depth and the removal rate are crucial to fully characterize the ultrafast laser-material interactions [1-5]. Nonetheless, the laser drilling is still lacking of a real-time technique able to monitor and control the spatial- and time-dependent evolution of the hole-depth in metallic plates.