Femtosecond laser (Pharos, Light Conversion) with the possibility to produce bi-burst pulses with a repetition rate of 64.5 MHz or/and 4.88 GHz was used to structure copper and steel samples. Ablation efficiency and quality were studied by varying number of pulses per burst in conventional burst regimes and in the bi-burst mode. The comparison of burst, bi-burst and single-pulse ablation efficiencies was done for beam-size-optimised regimes, showing the real advantages and disadvantages of milling and drilling processing approaches.
Surfaces inspired by nature and their replication find great interest in science, technology, and medicine due to their unique functional properties. This research aimed to develop an efficient laser milling technology using single-pulse- and burst-modes of irradiation to replicate bio-inspired structures over large areas at high speed. The ability to form the trapezoidal-riblets inspired by shark skin at high production speeds while maintaining the lowest possible surface roughness was demonstrated.
Ultrashort laser pulses are already widely used in material processing due to high flexibility and precision. However, competition in industrial applications demands growth of processing efficiency every year. In our previous works, the laser ablation efficiency versus various conventional processing parameters like laser fluence, beam scanning speed, pulse repetition rate and hatch distance was studied and beam-size-optimization was introduced. Also burst mode processing was applied, since it attracted a lot of attention by scientific and technological communities. The purpose of our current work was to investigate the real advantage of the burst regime by comparing the beam-size-optimized single-pulse regime with the beam-size-optimized multiple-pulse burst regime, which was never previously demonstrated. The optimization was done by increasing the spot size to find the maximum ablation efficiency for various pulse numbers per burst and various pulse durations.
The modification of transparent materials with femtosecond lasers has a lot of interest in data processing, waveguides and diffractive optical elements (DOE) development fields. In our research, we are focused on transparent materials processing with a deeply focused Gaussian beam. As well known, the internal modifications induced in fused silica with high numerical aperture objectives are affected by spherical aberration. In most cases, this phenomenon is unwanted and needs to be compensated to get the width to height ratio close to 1:1 for high-quality waveguides writing etc. However, when the focusing is quite deep (> 1 mm), due to the energy dissipation the modification can be formed only in the central part of the laser beam. Consequently, the radial size of modifications is reduced less than the diffraction limit. This property can be successfully used to record high-density volume DOE with the diffraction efficiency > 90%.
In this work, we develop the method to record DOE with multi-level binary refractive index modification distribution in bulk fused silica. In the beginning, the transverse modification length induced by single pulse elongation to the wider area is investigated to find the conditions where only type-I modification is induced. The two-level binary phase by a single modification depth > 50 µm can be achieved. In the next step, the desired multi-level binary phase distribution is simulated according to the required intensity distribution. Then the slicing of the multi-level binary phase to the two-level binary phase images is performed. This method involves recording the phase elements slice by slice with the resolution limited to the minimal induced phase change in one separate layer.
Laser writing for selective plating of electro-conductive lines for electronics has several significant advantages, compared to conventional printed circuit board technology. Firstly, this method is faster and cheaper at the prototyping stage. Secondly, material consumption is reduced, because it works selectively. However, the biggest merit of this method is potentiality to produce moulded interconnect device, enabling to create electronics on complex 3D surfaces, thus saving space, materials and cost of production. There are two basic techniques of laser writing for selective plating on plastics: the laser-induced selective activation (LISA) and laser direct structuring (LDS). In the LISA method, pure plastics without any dopant (filler) can be used. In the LDS method, special fillers are mixed in the polymer matrix. These fillers are activated during laser writing process, and, in the next processing step, the laser modified area can be selectively plated with metals.
In this work, both methods of the laser writing for the selective plating of polymers were investigated and compared. For LDS approach, new material: polypropylene with carbon-based additives was tested using picosecond and nanosecond laser pulses. Different laser processing parameters (laser pulse energy, scanning speed, the number of scans, pulse durations, wavelength and overlapping of scanned lines) were applied in order to find out the optimal regime of activation. Areal selectivity tests showed a high plating resolution. The narrowest width of a copper-plated line was less than 23 μm. Finally, our material was applied to the prototype of the electronic circuit board on a 2D surface.
Metal films on transparent substrates are widely applied for mask production in lithography, and lasers are frequently
used for their patterning. Quality of the patterning is limited by fundamental phenomena taking place close to edges of the laser ablated area. We experimentally and numerically investigated transformations in metal films during their
irradiation with the nanosecond laser beam with fluence above the ablation threshold. Ridges of the resolidified metal with non-uniform thickness were always formed on edges of the cleaned area. Instabilities during the ablation process forced the molten metal in the ridges to break up into droplets with the periodicity predicted by the Plateau–Rayleigh instability. The droplets on ridges were starting points for formation of self-organized lines of metal film by irradiation with partially overlapping laser pulses. The initial droplets and later the self-organized parallel lines of chromium metal were heat sinks that cooled down the metal in their close proximity. Temperature modulation along the laser irradiation spot was high enough to initiate the Marangoni effect which resulted in movement of the molten metal from hot to colder areas.
During the recent few years picosecond lasers have been proved as a reliable tool for microfabrication of diverse
materials. We present results of our research on structuring of thin films and surfaces using the direct laser writing and
the laser beam interference ablation techniques. The processes of micro-pattering were developed for metallic, dielectric
films as well as complex multi-layer structures of thin-film solar cells as a way to manufacture frequency-selective
surfaces, fine optical components and integrated series interconnects for photovoltaics. Technologies of nano-structuring
of surfaces of advanced technical materials such as tungsten carbide were developed using picosecond lasers as well.
Experimental work was supported by modeling and simulation of energy coupling and dissipation inside the layers.
Selectiveness of the ablation process is defined by optical and mechanical properties of the materials, and selection of the
laser wavelength facilitated control of the structuring process. Implementation of the technologies required fine
adjustment of spatial distribution of laser irradiation, therefore both techniques are benefiting from shaping the laser
beam with diffractive optical elements. Utilization of the whole laser energy included beam splitting and multi-beam
processing.
Solid oxide fuel cells (SOFC) are widely studied because of their potential usage in power source applications. At
present huge attention is paid to micro solid oxide fuel cells (μ-SOFC) based on thin film technologies with power
capacity in the range of several watts. Porous nickel is an important part in many types of solid oxide fuel cells. This
work presents experimental results of laser micro-channel formation in the 200 nm thick nickel and platinum films for
the fuel cell membranes. The four-beam interference ablation was applied for fast and parallel formation of microchannel
over a large area in thin metal film on a silicon substrate for μ-SOFC. Using this technique, regularly arranged
circular holes with a period of 4.2 μm were formed in the 200 nm thick nickel and platinum films. The diameter of the
holes ranged from 1.7 to 2.7 μm. The area where holes were ablated by a single laser exposure was approximately
250x250 μm. A silicon substrate was chemically etched from backside to release the patterned nickel film.
Effects related to the use of high repetition rate lasers in ablation of metals (aluminum, copper, stainless steel) and silicon
were investigated. The multi-pulse irradiation with the laser beam significantly lowered the ablation threshold and led to
a relative increase in the ablation rate at the higher repetition rate. The reason of alteration could be accumulation of
structural defects on the metal surface formed by irradiation with a laser of the sub-threshold fluence. The mean
volumetric ablation rate in laser milling experiments was a non-linear function of the pulse energy. Plasma shielding was
the main limiting factor in processing efficiency of metals with the high power picosecond lasers. Increasing the
repetition rate keeping the pulse energy below the plasma formation threshold is a way to increase the efficiency of
material removal with nanosecond lasers. Thermal management of the specimen could be a problem at high repetition
rates because of the laser energy wasted in the bulk. The reduction in the ablation threshold by irradiation with a series of
laser pulses might be useful in application of the high- repetition-rate lasers with the low pulse energy.
Indium-tin oxide (ITO) is the main material for making transparent electrodes in electronic devices and flat panel
displays. Laser-direct-write technology has been widely used for patterning ITO. The well defined edges and good
electrical isolation at a short separation are required for the modern OLED and RFID devices of high packing density.
High repetition rate lasers with a short, picosecond pulse width offer new possibilities for high efficiency structuring of
transparent conductors on glass and other substrates. The results of patterning the ITO film on glass with picosecond
lasers at various wavelengths are presented. Laser radiation initiated ablation of the material, forming trenches in ITO.
Profile of the trenches was analyzed with a phase contrast optical microscope, a stylus type profiler, SEM and AFM.
Clean removal of the ITO layer was achieved with the 266 nm radiation when laser fluence was above the threshold at
0.20 J/cm2, while for the 355 nm radiation the threshold was higher, above 0.46 J/cm2. The glass substrate was damaged
in the area where the fluence was higher than 1.55 J/cm2. The 532 nm radiation allowed getting well defined trenches,
but a lot of residues in the form of dust were generated on the surface. UV radiation at the 266 nm provided the widest
working window for ITO ablation without damage of the substrate. Use of UV laser radiation with fluences close to the
ablation threshold made it possible to minimize surface contamination and the recast ridge formation during the process.
UV picosecond laser pulses have been shown to be able to etch clean holes in polymers. This process has numerous applications in processing polymers. We present the results obtained by the investigation of the laser ablation process with picosecond pulses in 19 different organic materials. Micro-drilling and cutting of polymers were performed by the 355 and 266 nm radiation. The ablation rate versus laser fluence was investigated. Polymers showed the logarithmic ablation rate versus the laser fluence proving a well known Beer's model of laser ablation. Two ablation thresholds were observed in most of the polymers. One of them was below 1 J/cm2 with a low material removal rate. Another threshold was spread in the range of 3 - 30 J/cm2 and a rapid increase in the ablation rate was found above it. The processing of polymers was tested in fabrication of cavities relevant to different micro-devices. Combined cavities for the ink-jet printer head and micro-fluidic set were formed by laser etching. UV laser radiation with the picosecond pulse duration can be useful in manufacturing micro-systems for diverse technical applications.
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