We present the fabrication of high-quality graphitic micro-wires in diamond which are conductive in nature using pulsed Bessel beams. The electrodes are created in the bulk of 500 μm thick monocrystalline CVD and HPHT diamond samples perpendicular to the sample surface without sample translation or beam scanning. The role of various beam parameters such as pulse energy and pulse duration, different crystallographic orientations of the sample and two different writing modes of the laser namely burst mode and single-pulse mode in the conductivity of such electrodes are investigated. While the morphology of the electrodes is analysed using optical microscopy, the conductivity is measured experimentally using current-voltage characterisation. Furthermore, micro-Raman spectroscopy is implemented to investigate the graphitic content of electrodes fabricated. We have observed that higher pulse duration favours better conductivity while pulse energy has an optimum value for the same. As for the crystallographic orientations, we have found that it is possible to eradicate the potential barrier in the current-voltage curves completely even for graphitic wires fabricated at low pulse energy and in the fs pulse duration regime in a (110) oriented sample in contrast to the (100) oriented-crystal case where the barrier is generally observed. Finally, in case of wires fabricated with laser bursts with femtosecond sub-pulses, the higher number of sub-pulses, lower time delay between them and longer total burst duration favours better conductivity. Through various optimisation techniques, we report resistivity values as low as 0.01 Ω cm for the Bessel beam written electrodes in diamond.
In the last two decades quartz has become a relevant material for sensing technology since it has been used for realization of various devices, such as Quartz Crystal Microbalance (QCM) or Quartz-Tuning-Fork (QTF). Micromachining of quartz can be realized through various techniques, such as diamond cutting, lithography, wet and dry etching, ion beam etching and Ultra-Short-Pulsed-Laser (USPL) processing. At the state-of-the-art USPL has been efficiently applied to quartz micromachining, e.g., for drilling and stealth dicing. In this study, the influence of the incubation effect and the repetition rate on USPL ablation threshold of quartz was systematically investigated. The multi-pulse ablation threshold of quartz was evaluated using 200 fs laser pulses at a wavelength of 1030 nm, at three different repetition rates, i.e., 0.06, 6, 60 and 200 kHz. Results show a strong decrease in the multi-pulse ablation threshold with the number of pulses N, as a consequence of the effect of incubation during the fs-laser ablation. Conversely, the influence of the repetition rate on incubation is negligible in the investigated frequency range. A saturation of the threshold fluence value occurs at number of pulses N > 100 and this trend is well fitted by an exponential incubation model. Using such a model, the single-pulse ablation threshold value and the incubation coefficient for quartz have been estimated. This investigation represents a first step towards the micro- and nano-texturing of quartz crystal for tailoring its mechanical, electrical, and optical properties.
Development of new lab-on-a-chip (LoC) devices requires an optimization phase in which it could be necessary to continuously modify the architecture and geometry. However, this is only possible if easy, controllable fabrication methods and low-cost materials are available. For this reason, rapid prototyping approaches for the fabrication of polymeric LoC are on the rise, as they allow high degrees of precision and flexibility. Here, we describe the fabrication platform of polymeric microfluidic devices, from the design (CAD) to the proof-ofconcept application as LoC for biological applications. The fabrication procedure is mainly based on fs-laser micromachining techniques. The ability of femtosecond (fs)-laser pulses to produce localized modification of the materials, thereby avoiding either debris, recast layers or unsought thermal affected zones, without restriction of the substrate materials, makes this technology particularly suitable for microfluidic device fabrication. In our work, fs-laser has been also possibly combined with other techniques, without the need for the expensive masks and facilities required by the lithographic process. The LoC devices have been realized in polymethyl methacrylate (PMMA), a low cost and biocompatible material. The fs-based smart fabrication platform has been exploited in the fabrication of disposable LoC devices for particles manipulation. In particular, a serpentine microchannel able to distinguish cancer from non-cancer cells without labeling and a fully inertial sorting 3D device have been fabricated and tested.
We report on an experimental and theoretical investigation on the laser ablation of silicon with THz bursts of fs pulses. Craters were generated by varying the burst features, i.e., the number of pulses and the intra-burst repetition rate, and compared to those obtained in Normal Pulse Mode (NPM). A general reduction of the thermal load was observed using bursts, though with a lower ablation rate. In fact, shallower craters were obtained when increasing the number of pulses and reducing the intra-burst repetition rates at fixed processing time and burst energy. However, for bursts at 2 THz, some combinations of process parameters allowed a higher specific ablation rate compared to NPM. Simulations based on the numerical solution of the density-dependent two temperature model showed that bursts with more pulses or with lower intra-burst repetition rates lead to a lower final temperature, thus supporting the experimental findings. This is ascribed to changes of the reflectivity dependent on the number of pulses. Accordingly, different amounts of energy are transferred from the laser pulse to the sample, which also leads to changes in specific ablation rates. The origin of such a behavior was found to be the non-linear absorption processes, especially the two-photon absorption.
Utilization of parts made by combining dissimilar materials, such as different polymers, metals, or semiconductor to polymers, are nowadays highly demanded for the fabrication of electronic, electromechanical, medical micro-devices, and analytical systems (e.g., lab-on-chip). Techniques for joining such hybrid micro-devices, generally based on gluing or thermal processes, remain a challenging task presenting some drawbacks, such as deterioration and contamination of the substrates. Ultrashort laser welding is a non-contact and flexible technique to precisely weld similar and dissimilar materials. In this case, the only constrain is that the upper substrate is transparent to the laser wavelength. This technique has been demonstrated both for welding polymers and polymers to metallic substrates, but never for joining polymers to silicon. In this work, we report on direct femtosecond laser welding of Poly(methyl methacrylate) (PMMA) and silicon. The laser welding was performed in ambient air by focusing ultrashort laser pulses at high repetition rate at the interface between the two, being PMMA transparent to the laser wavelength. A mechanical homogenous pressure was applied on the sandwiched substrates during all the laser process. The Si-PMMA weld strength was evaluated as a function of the laser and processing parameters, e.g., repetition rate, scan speed, and the overlap between adjacent scan lines.
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.
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 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. The bursts were generated by splitting the pristine 650-fs laser pulses using an array of birefringent crystals which provided time separations between sub-pulses in the range from 1.5 ps to 24 ps. We measured the threshold fluence in Burst Mode, finding that it strongly depends on the bursts features. The comparison with Normal Pulse Mode revealed that the existing models introduced to explain the incubation effect during irradiation with trains of undivided pulses has to be adapted to describe incubation during Burst Mode processing. In fact, those models assume that the threshold fluence has a unique value for each number of impinging pulses in NPM, while in case of BM we observed different values of threshold fluence for fixed amount of sub-pulses but different pulse splitting. Therefore, the incubation factor coefficient depends on the burst features. It was found that incubation effect is higher in BM than NPM and that it increases with the number of sub-pulses and for shorter time delays within the burst. Two-Temperature-Model simulations in case of single pulses and bursts of up to 4 sub-pulses were performed to understand the experimental results.
We report on an experimental investigation of ultrafast laser ablation of silicon with bursts of pulses. The pristine 1030nm-wavelength 200-fs pulses were split into bursts of up to 16 sub-pulses with time separation ranging from 0.5ps to 4080ps. The total ablation threshold fluence was measured depending on the burst features, finding that it strongly increases with the number of sub-pulses for longer sub-pulse delays, while a slowly increasing trend is observed for shorter separation time. The ablation depth per burst follows two different trends according to the time separation between the sub-pulses, as well as the total threshold fluence. For delays shorter than 4ps it decreases with the number of pulses, while for time separations longer than 510ps, deeper craters were achieved by increasing the number of subpulses in the burst, probably due to a change of the effective penetration depth.
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.
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