Multi parallel ultrashort pulse laser processing
Arnold Gillner, M. Jüngst, P. Gretzki

Ultra-short pulse lasers present a new class within high-performance laser beam sources for industrial applications [Du et al 2012]. Due to the outstanding features of the radiation emitted from these sources, which are addressing important physical principles of light-matter interaction, traditional processes of deposition of light energy into the material can be circumvented. With pulse durations in the picosecond and femtosecond range, the absorbed energy is concentrated in the material to a few nanometers, so that thermal damage to the materials can be avoided.

These properties have generated numerous processes in in precision machining at solar cells, batteries, injection molding tools and electronic components [Hartmann et al 2008]. Due to the current developments for power scaling of ultrafast lasers in the kilowatt range, also potential applications for macro processing are obtained, which opens large markets in other than the micro processing field [Russbüldt et al 2010]. Thus, with high-power ultrafast lasers, fiber reinforced composites can be processed without thermal influence and large surfaces can be provided with friction-minimizing microstructures. However, using high power ultrashort pulsed lasers with high repetition rates in the MHz region can cause thermal issues like overheating, melt production and low ablation quality as long certain parameter sets and fluence ranges have been considered. High ablation quality only can be achieved, when the processing fluence is closed to the ablation threshold, which requires new processing strategies and innovative system components. Beside ultra high speed scanning using polygon scanners the use of multiple laser beams provide the best and most versatile high power ablation solution. With switchable single beams out of a special light modulator or a diffractive optical beam splitter high ablation rates can be achieved while maintaining the high processing quality of ultra short pulse laser ablation. With this approach a next step up to an all optical manufacturing system can be provide

_____

Investigation of Femtosecond Laser Texturing in Cemented Carbide Cutting Tools
Patrícia Alves Barbosa, Marcelo Bertolete, Ricardo Elgul Samad, Nilson Dias Vieira Júnior, Izabel Fernanda Machado, Álisson Rocha Machado, Rui Vilar, Wagner de Rossi

The aim of this work is to characterize uncoated cemented carbide tools textured by femtosecond laser and to
investigate the influence of texturing on the cutting tool in machining. Thus, parallel micro-grooves were machined on
the rake face of uncoated cemented carbide inserts at 100, 200, 300 and 400 μm from the cutting edge using a
Ti:Sapphire laser set for 15 μJ, 30 fs pulses emission centered at 775 nm and at 4 kHz of repetition rate. The cutting
forces were monitored along the semi-orthogonal turning of austenitic stainless steel (V304UF) for textured tools and
compared with non-textured ones (reference). The cutting conditions were kept constant. The results showed
reproducible grooves with mean width and depth of 35±3 and 32±6 μm, respectively; a reduction of 27.6% in the
machining force for textured tool at 200 μm from the cutting edge and lower surface roughness for textured tools at 200
and 300 μm when compared with reference ones.

Keywords: Texturing; Femtosecond laser; Cemented carbide tools; Machining force; Surface roughness

_____

Shorter than short: How does the pulse duration influence the process efficiency and the quality of conductive materials?
Simone Russ, Uwe Keller, Lara Bauer, Tilo Meyer, Raphael Gebs, Birgit Faisst, Jörg Roller, Benjamin Führa

Material micro processing with ultrashort laser pulses has been established during the last few years. There are a lot of processes that ultrashort pulsed (USP) lasers can be used for. Drilling of injection nozzles and cutting of Nitinol stents are already established in production lines. Other examples for the use of those kinds of lasers are scribing of silicon as well as processing of hard and brittle transparent materials. The range of possible applications is huge and nevertheless the micromachining market is still in its infancy. For many years since the invention of USP based material micro processing, pulse durations in the range of ten picoseconds were considered as the optimum choice for micro machining applications regarding process quality (Dausinger, Hügel, & Konov, 2003). In scientific laboratories a well known technique to decrease the pulse duration of high power lasers from the picosecond into the femtosecond regime was based on chirped pulse amplification (CPA) since many years (Strickland & Mourou, 1985). However, this technique for a long time was too expensive to transfer it to industrial reliable high power lasers. Today there are cost efficient and reliable high power CPA based femtosecond laser sources available for the industry. This ability seems to offer new micromachining opportunities for better quality and often also for even higher efficiency. We have examined the influence on the efficiency and the quality for an ablation process on conductive materials. Therefore the pulse duration was reduced from 6 ps to 900 fs and even down to 400 fs. By comparing those three pulse durations it will be much easier to decide which laser should be used for a certain process and material. And on the other hand it offers a good chance to discover new applications for lasers with
ultrashort pulses.

Keywords: micro processing; picoceconds; femtoseconds; ultrashort pulses; ablation process

_____

Laser micro structuring using adaptive mirror for extra-cavity beam-shaping of high-power ultra-short laser pulses
Marco Smarra, Johannes Neyer, Klaus Dickmann, Jean Pierre Bergmann

In laser micro processing there is a demand of shaped beams to improve the ablation results. Beam shaping is used e.g. for
generating isolation grooves in solar cells. In addition the variation of the beam profile allows modifications of the surface
properties of the processed material. These modifications can result in surface functionalization e.g. hydrophobic behavior.
Phase changing optical elements are used to influence the wave-front and vary the beam shape. For high-power laser
applications these elements have to be compliant with large pulse energies or high average power. A deformable mirror is
one of the most suitable tools for the application with high power lasers, because of the absence of intensity losses due to
diffraction. In this study a unimorph deformable mirror was used for beam-shaping during a picosecond laser ablation on
metals and dielectrics. A piezoelectric disc behind the deformable mirror is separated into 35 segments, which can be
individually driven by a voltage from -100 V up to 250 V. This feature allows complex deformation of this mirror which
results in an individual variation of the spot geometry. The generation of different focus geometries, e.g. elliptical or line
geometries, were analyzed. For this purpose on one hand the intensity profile and the beam propagation and on the other
hand the influence to the surface modifications were studied. Another field of application for deformable mirrors is the
variation of the focus position. By using additional optical components a controllable focus shift of 5 mm with a step size
down to 25 μm was realized. This feature was required to keep a continuous spot size on the ablation surface. The influence
of this defined defocusing during the ablation process was analyzed and compared to a focus shift realized by a motorized
translation axis. It could be shown that beam-shaping based on a deformable mirror is a precise method for intensity
variations of the laser spot and focus shifting without loss of intensity.

Keywords: micro processing; ablation; surface functionalization

_____

Cutting diamond tools using the Laser MicroJet® technology on a 5 axis machine
Annika Richmann, Sébastien Kurzen, Benjamin Carron, Bernold Richerzhagen

The Laser MicroJet® technology uses a water coupled ns-laser to cut various materials e.g. diamond, semiconductors,
ceramics and metals. The laser light is guided to the workpiece by the water jet and the material is cut by the laser
radiation. This technology can thus cut the same materials as a dry laser. However, the Laser MicroJet® technology
exhibits several advantages over dry laser cutting such as a parallel sided kerf, less heat damage due to the additional
cooling of the water, no adjustment of the laser focus and higher kerf depth to kerf width aspect ratios. The cutting of
ultra-hard diamond tool materials with this technology is now showing very promising results in terms of low surface
roughness and high cutting speed. PCD, PcBN, and single crystal diamond tool materials, including those backed with
cemented tungsten carbide, can all be cut and shaped with the Laser MicroJet®. Latest results show a surface roughness
Ra for the PCD/PcBN region of the cut surfaces of < 0.3 μm and < 0.5 μm on the cemented carbide. The effective cutting
speed on 2 mm thick, carbide backed PCD/PcBN is 5 mm/min. The cutting edge radius (i.e. sharpness) achieved is r < 10
μm for PcBN samples and r < 5 μm for PCD or CVD samples. For some applications, this level of edge sharpness will be
more than adequate and finish grinding will not be necessary. This paper presents the first results cutting ultra-hard
tooling materials on the 5-axis platform using the already well established Laser MicroJet® technology.

Keywords: diamond tool cutting, Laser MicroJet, precision cutting, PCD, PCBN

_____

Influence of coaxial cw laser heating on the ablation of silicon with ultra-fast lasers
Christian Fornaroli, Arnold Gillner

These days most common way to produce electrical components like LEDs, solar cells or transistors is a batch process. Therefore a lot of
identical components are processed parallel on one big wafer and eventually each chip has to be singulated. Currently two dicing
technologies have established themselves, which can be devided in mechanical blade sawing and laser based processes with nanosecond
lasers. In contrast to these technologies, laser dicing with picosecond lasers offers fundamental advantages like smaller kerf width and
marginal heat effected zones. However the process efficiency and the attainable aspect ratio are limited and thus some deep cutting or
drilling processes are not feasible. In this paper the influence of coaxial laser heating on the cutting process of Si wafers with ps lasers is
investigated. It turns out that already with cw average powers in the range of 20 W a significant decrease of the ablation th reshold can
be obtained. Furthermore the aspect ratio can be increased by approx. 20 %.

Keywords: Cutting; Dicing; Singulation; Si Wafer, picosecond Laser, UKP

_____

Effects of ultrashort laser ablation in copper and stainless steel
Wagner de Rossi, Denilson de Camargo Mirim, Nilson Dias Vieira Júnior, Ricardo Elgul Samad

Precision machining with ultrashort laser pulses strongly depends on the relationship between process parameters and
physical and metallurgical properties of the metal machined. This relationship, however, varies in the course of the
process depending on the number of pulses that hit the same spot on the material being processed, and this n eed to be
known and compensated for precision machining. This variation arises due to defects in the crystal lattice produced by
the laser pulses. These defects are cumulative and their influence is described by the incubation effects. Thus, the
relationship between the threshold fluency Fth and the number N of overlapping pulses is a fundamental condition to
obtain controlled and precise ablation with ultrashort laser pulses.
By means of a method developed by the authors, the D-Scan (Diagonal Scan), this study acquired a great amount of
experimental data that allowed obtaining many Fth × N curves. These curves were measured for pulsewidths of 25, 80
and 120 fs for both copper and stainless steel. The results show differences with respect to the temporal wid ths and
even more significant variations between the two metals. Copper, presented a 25 fs single pulse ablation threshold of
0.80 J/cm2, much greater than for stainless steel, which is 0.30 J/cm2. The incubation factor for copper, 0.78 is smaller
than that for stainless steel, which is 0.85. After N=1,000 pulses the two metals presented Fth of around 0,07 J/cm2.
The D-Scan technique also allowed observing many different morphologies of the ablated region. The process
parameters (fluency, N and temporal width) related to these morphologies could be easily determined in a clear and fast
way. Therefore, the process conditions to obtain a specific surface structure as LIPSS, nanocones, fusion and fusion with
phase explosion was determined to these two materials and the method can be used to any other kind of material.

Keywords: Micro processing; ablation; surface processing; femtosecond laser.

_____

Direct Laser Beam Interference Patterning for Fabrication of Plasmonic Hole Arrays
Simonas Indrisiunas, Bogdan Voisiat, Gediminas Raciukaitis

Periodic arrays of apertures in thin metallic films act as metamaterials with interesting properties such as extraordinary
transmission due to surface plasmon resonances and can be applied as biosensors or filters of electromagnetic radiation.
We have tested the Direct Laser Beam Interference Patterning (DLIP) technique as a tool for the fabrication of the
periodical hole’ arrays in thin metal films using picosecond laser pulses. A novel scanning technique allowed the
production of uniform patterns in the tens of square millimetres sized areas. Such device areas are sufficient for most
applications. Properties of the hole’ array depend on the shape of the hole and the lattice structure. The shape of the
hole was controlled by adjusting irradiation fluence and by introducing sample translation between laser exposures. The
lattice structure was adjusted using a certain number of interfering beams. Reflectance measurements of the DLIP
patterned samples show reflectance dips corresponding to the extraordinary transmission. The reflectance modulation
depth strongly depended on fabrication parameters. Longer pulse duration may be favourable to avoid sub-period
irregularities in the ablated structures.

Keywords: Direct Laser Beam Interference Patterning; plasmonic hole arrays

_____

Temporal evolution of hole-geometry and influences of energy deposition during ultra-short pulses helical drilling
Chao He, Claudia Hartmann, Christian Fornaroli, Frank Zibner, Arnold Gillner

This paper presents a static investigation on temporal evolution of hole geometry and influence of laser energy
deposition on hole quality in helical drilling process. By using a rotating dove prism a circular oscillation of the laser sp ots
is performed and holes are drilled at intervals in 1 mm thick stainless steel (1.4301 ) by ultra-short laser pulses of 7 ps at
515 nm. The formation of hole and the behavior of energy deposition differ from other drilling strategies due to the
helical revolution. The temporal evolution of the hole shape is analyzed by means of SEM technique s from which three
drilling phases can be distinguished. The first phase is characterized by a highest drilling rate and the formation of a
sharp-edged circular groove with a pin inside the workpiece. In the following phase, the molten and vaporized materi al
is ejected out from the hole and a funnel-like borehole with a slim tip deepens to the backside of workpiece, growth of
hole depth slows down in this period. The exit is broadened to the final shape in the final phase. Laser scanning
microscope (LSM) measurements of structure details on the hole wall demonstrate that the quality of the helical-drilled
hole is determined by a correlation between the pulse energy applied and the overlapping rate of laser pulses, which is
described mainly by helical path and the rotation speed of laser beam.

Key words: Helical drilling; ultra-short pulses; temporal evolution; energy deposition

_____

Laser Micro-drilling of Multi-layered Artificial Skin
Yasuhiro Okamoto, Kiichi Asako, Akira OKADA, Shogo Minagi, Naoto Maeda, Qiuyue Pan, Keiji Jin, Goro Nishigawa

A multi-layered artificial skin is mainly consisted of three layers of silicone elastomer, print paper, and resin plate.
Sealing of facial defects with the multi-layered artificial skin causes perspired water from the surface of missing parts,
which results in insanitary conditions to the skin surface. Therefore, the laser micro -drilling using nanosecond and
picosecond pulsed laser was investigated to provide the breathability for multi-layered artificial skin without human
eye’s recognition of drilled holes, which can satisfy both the breathability and aesthetic problems. The visibility of drilled
hole was affected by the pitch distance of drilled holes, and its visibility decreased with increasing the pitch distance. A
small color difference measured using a colorimeter led to the low visibility, and laser micro-drilling with small color
difference is effective to obtain the breathable multi-layered artificial skin with low visibility of drilled holes, which can
improve the breathability for the multi-layered artificial skin.

Keywords: micro processing; drilling; multi-layered artificial skin

_____

Energy transfer mechanisms during laser pulsed processing of metals
Daniel Johannes Förster, Volkher Onuseit, Rudolf Weber, Thomas Graf

The basic behaviour of material removal rates in volume laser ablation processes when using ultra-short
laser pulses can be described by simple models based on the Beer-Lambert law with an exponential decay of
the energy density into the material of an externally applied laser field according to preliminary findings of
J.H. Lambert, 1760 and A. Beer, 1852. Neuenschwander et al., 2014 showed that the use of different pulse
lengths results in different “effective” penetration depths of energy into the material. At certain parameters,
these are in the range of the optical penetration depth. However, for a wide range of pulse durations the
effective penetration depth is much larger than the optical penetration depth. The reasons for this
behaviour are several energy transfer mechanisms like photon-electron, electron-electron and electron-
phonon interactions.

For short- and ultrashort-pulsed laser processing, wide-range two temperature model calculations for
several pulse durations were carried out for aluminium. The simulations enable to distinguish between
different energy transfer mechanisms. The influence of optical absorption and energy transport mechanisms
for different pulse durations and applied fluences are discussed in the present paper.

Keywords: Energy transfer; Volume ablation; Two Temperature Model; Fundamentals of pulsed processing

_____

Highly reproducible laser micro drilling of titanium-based HLFC sections
Hamza Messaoudi, Salar Mehrafsun, Geza Schrauf, Frank Vollertsen

The reduction of fuel-consumption is an environmentally relevant priority of the aviation industry. One of the key
technologies in this field is the hybrid laminar flow control (HLFC), which is based on perforated sections on the leading
edge of aircrafts. Up to now the economic efficiency as well as the quality of several micro drilling processes, such as
laser drilling, cannot meet the requirements of a high-speed and large-area generation of micro holes with highly
reproducible diameter. In this paper, an approach for a highly reproducible laser micro drilling of 0.8 mm thick titanium
is presented. The influences of drilling parameters on process stability and bore diameter are discussed. With a
commercially viable system technology based on a short pulsed fiber laser and a galvano-scanner, a stable production of
micro holes of 35 μm to 90 μm in diameter with a through-going bore density of about 95 % was realized at drilling rates
up to 175 holes/s. In terms to improve the inner form and to enlarge the hole diameter, a chemical pickling was carried
out. Depending on the required dimension, this results in bores of 50 μm to 130 μm in diameter with deviations less
than 10 %.

Keywords: Micro processing; ablation; drilling; system technology

_____

Calculating the optimal combination of pulse-to-pulse distance and fluence for scribing and patterning with ultrashort pulsed lasers
Matthias Domke, Giovanni Piredda

The application of ultrashort pulsed lasers for surface structuring or line scribing enables precise control of the ablation
depth. A general challenge for high quality laser ablation is to find the optimal combination of pulse-to-pulse distance and
fluence in order to minimize the surface roughness.
In this study, a model was developed to calculate the surface roughness as a function of the pulse-to-pulse distance and
fluence. In the model, the surface is irradiated by a laser pulse with a Gaussian fluence profile; ablation starts if the fluence
is above a certain threshold fluence, while the depth is proportional to the logarithm of the fluence. This model is used to
calculate the surface profile generated by multiple adjacent craters. The surface roughness was determined as a function of
the pulse-to-pulse distance and the peak fluence.
The calculations indicate that the surface roughness reaches a first minimum when the pulses begin to overlap. If the distance
between the pulses is reduced further, several minima and maxima can be found. The pulse-to-pulse distances, where the
minima of the surface roughness are located, increase with the square root of the logarithm of the fluence.

Keywords: ultrashort pulse; ultrafast; laser; ablation; scribing; patterning; surface roughness; modelling

_____

Laser ablation of SiCp/Al composite
Huanzhen Zhang, Ting Huang, Rongshi Xiao

Particulate reinforced metal matrix composites (PRMMCs) have been known for several decades. The
combination of their outstanding properties such as strength and ductility makes them superior to
many metal alloys. SiCp/Al is one of the most promising PRMMCs for aerospace and automotive
applications. However, SiCp/Al is very difficult to machine with conventional methods due to the hard
SiC particles. Specifically, machining of SiCp/Al with conventional diamond tools always suffers surface
damages and rapid tool wear, which compromises machining precision and increases costs. This
paper involves a fundamental investigation on the precision machining of SiCp/Al composite using
nanosecond laser. The microstructure evolution and product during ablation was demonstrated by
scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. Based on the distinct
morphology features, the ablated surface can be generally divided into three types, i.e. Al is ablated
while SiC particles remain, Al reacts with SiC to form a homogeneous surface layer, and both Al and
SiC particles are ablated. The homogeneous surface layer consisted of mullite (3Al2O3·2SiO2) and
sillimanite (Al2O3·SiO2), which resulted from the decomposition of SiC and the melting of Al. The
effects of mullite and sillimanite on the final properties of the material and the ablation mechanism
are discussed.

_____

Computational Study on the Effect of the Pulse Length on Laser Ablation Processes
Stefan Tatra, Rodrigo Gómez Vázquez, Andreas Otto

Laser assisted ablation technology is widely used for micro-fabrication of metal and semiconductor components in the
electronic industry. The outcome of those processes strongly depends on several variables such as the used material and
different laser parameters. For a better understanding of these processes a study on the effect of the pulse length ba sed on
multi-physical simulations was performed. The use of a complex model which extends original CFD-capabilities of
OpenFOAM simulation software with multiple physics allows us to obtain detailed description of the phenomena taking
place during the process. Special attention is paid to transient process characteristics including melt ejections and metal
vapor dynamics as well as ablation depth and their dependency on the pulse length scales applied. The limitation of the
model, especially at ultra short pulses, is analyzed and possible improvements are discussed. Apart from the
computational investigation comparison of simulated results against available experimental data is finally provided.

Keywords: multi-physical simulations; laser ablation; volume of fluid;

_____

Scaling of ablation rates. Ablation efficiency and quality aspects of “Burstmode”-micromachining of metals.
Marc Sailer, Jonas Kleiner, Myriam Kaiser, Simone Russ

Considering the energy efficiency and the ablation quality, ultra-short double-pulse laser micromachining of stainless
steel is examined for ps and fs pulses. Pulse delays from 0.3 ns up to 220 ns are realized with the help of an external
delay line or a linear MOPA laser system. Burst-mode ablation is found to be less efficient for all investigated double-
pulse delays. Keeping the single-pulse fluence and the overall incident energy constant, a drastic decrease of the
ablation rate is observed when the double-pulse delay is reduced. Despite the lower ablation efficiency, ultra-short
double-pulse laser ablation offers the possibility to machine strongly reflective and smooth surfaces and thus can
improve the ablation quality.

Keywords: burst-mode ablation; ablation efficiency; metals; surface quality; double pulses; ultra-short pulses

_____

Analysis of shape geometry of Ti6Al4V parts fabricated by nanosecond laser ablation
Sabina Luisa Campanelli, Nicola Contuzzi, Fulvio Lavecchia, Gianluca Percoco

The process of laser milling, or laser ablation as it is also known, was developed over the last decade. In conventional
milling techniques material is physically removed by a milling tool. In laser milling the material is removed by a laser
beam through the layer by layer ablation mechanism. Generally, in laser ablation the quality of the processing result is
reduced by melt accretions and thermal damage of the workpiece and therefore increases with shorter pulse duration.
However, ablation efficiency decreases as well. Thus, laser ablation in the nanosecond range is still offering a good
compromise between process quality and efficiency. The aim of this paper was to study the shape geometry and
dimensions of Ti6Al4V parts fabricated by laser milling using a nanosecond Nd:YAG laser source. The impact of the laser
processing parameters onto the machining outcomes was studied in order to find out optimized processing conditions.
Particularly the influence of average power, repetition rate and scan speed was investigated.
The geometry of micro-parts was revealed using a 3D digitizing system Optimet Mini Conoscan 4000, which combine s a
non-contact, single-point measuring sensor based on conoscopic holography technology. The use of this measurement
technology allowed obtaining complete information of the shape geometry and dimensions of built parts.

Keywords: Laser milling, process parameters, 3D Measurement, Conoscopic Holography Main text

_____

Crater shape dependence on pulse duration in crystalline silicon generated using an IR Gaussian laser beam: from femtosecond to microsecond regime
Stefano Buratin, Carol Kong

Semiconductor micromachining is one of the most common industrial applications where laser has been used in the last
few years. Laser ablation is widely applied in the microelectronic field and in the photovoltaic industry. These two
applications typically require a non-contact method for welding, cutting and scribing and the laser is one of the most
suitable solutions for them. The prediction of the crater shape is a relevant issue due to the surface requirements of
these processes. Semiconductors are also very interesting to study in the infrared due to their electromagnetic
penetration that is strongly dependent on the wavelength, because of the material band gap. At the same time the
physical proprieties of semiconductors, in particular the thermal ones, are likely to affect the final crater shape. The
dependence of final crater shape on the laser pulse duration is relevant in laser material processing because different
phenomena occur at different time regimes; this is why a systematic study of the crater shape, depending on the pulse
duration, has been carried out. Four different temporal regimes have been studied. In femtosecond regime, a 330 fs
laser has been used, with central wavelength at 1032 nm with energies below 10 μJ per pulse. In the picosecond regime,
a 10 ps laser has been used, with central wavelength at 1064 nm with energies below 200 μJ per pulse. In nanosecond
regime, the range between 9 ns and 220 ns has been used, with central wavelength at 1064 nm wit h energies below 1.1
mJ per pulse. In microsecond regime, the range between 2μs ns and 20 μs has been used, with central wavelength at
1064 nm with energies below 1.6mJ regime. Different fitting functions have been suggested for the different crater
shapes depending on the phenomena involved in the ablation process, considering the different pulse durations.

Keywords:pulse duration; energy beam; ablation; surface shape.

_____

Nanoparticles fabricated by laser ablation in liquids for biomedical and energy applications
Stephan Barcikowski

After several decades of intense research in the field of nanostructures, nanoparticles are widely implemented for
functionalization on surfaces, into volumes and as nanohybrids, with application in active nanoparticle-polymer-
composites and as nanoparticle-bioconjugates. But nowadays only a limited variety of materials can be integrated into
advanced functional products due to limitations of gas phase and chemical synthesis methods such as particle sintering or
impurity. As alternative synthesis route, laser ablation and nanoparticle generation in liquids has proven its scalability and
capability to generate totally ligand-free colloidal nanoparticle building blocks.
This contribution highlights how the unique properties of laser-generated nanoparticles can be harvested in prospective
real-world applications rapidly via “nanointegration” into the fields of biomedicine and catalysis. Furthermore, it addresses
how laser parameters, chemical environment and reactor design may be tuned in order to obtain monodisperse
nanoparticles and to enhance the productivity of the laser process.

Keywords: laser ablation; ligand-free nanoparticles; nano-integration

_____

Production of silver nanoparticles in liquid by CW and pulsed lasers
Mohamed Boutinguiza, Rafael Comesaña, Fernando Lusquiños, Antonio Riveiro, Jesús del Val, Juan Pou

The new properties of nanoparticles have accelerated the growth of production of nanostructured materials and their
use in many different applications. In particular, silver nanoparticles have attracted much attention as a subject of
investigation due to their well-known properties, such as high electrical and thermal conductivity, antibacterial and
antifungal effects, etc. They are used in many different areas, medicine, industrial applications, and scientific
investigation, etc. The size as well as the shape is very important for certain applications.
Almost all publications reporting synthesis of nanoparticles by laser ablation of solids in liquids use pulsed lasers,
especially nanosecond and femtosecond lasers. Nevertheless nanoparticles can be also obt ained in liquid media using
long pulse lasers and continuous wave (CW) lasers. In the present work we present the results of ablating Ag target in
water using a pulsed, as well as, a continuous wave laser.

The obtained particles consist of pure Ag nanoparticles showing rounded shape and uniform size distribution. Crystalline
phases, morphology and optical properties of the obtained colloidal nanoparticles were characterized by means of X -ray
diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM)
and UV/VIS absorption spectroscopy.

Keywords: laser ablation; silver nanoparticles; pulsed laser; cw laser.

_____

Bioactive glass nanofibers produced by Laser Spinning for biomedical applications
Joaquin Penide Duran, Felix Quintero Martinez, Antonio Riveiro Rodriguez, Jesus del Val Garcia, Rafael Comesaña Piñeiro, Fernando Lusquiños Rodríguez, Juan Pou Saracho

The Laser Spinning technique was employed to produce long, dense and flexible glass nanofibers with different bioactive
compositions. Laser Spinning is a novel technique that allows the rapid and scalable production of high quantity of glass
nanofibers under ambient conditions. The bioactive glass nanofibers have potential utility as reinforcement in
composites, fillers in bone defects or scaffolds for tissue engineering. The morphology and structure of these nanofibers
was analyzed using SEM, XRF, NMR, TEM and ATR-FTIR. The bioactivity of the glass nanofibers is demonstrated with in-
vitro tests. They also present antimicrobial properties against different bacteria. The flexibility of this material, easy of
manipulation and sterilization as well as antibacterial properties, made the laser-spinning glass nanofibers an excellent
alternative to present granulated material for bone defect restoration and for tissue engineering.

Keywords: Laser Spininng; Nanofibers; Glass.

_____

Femtosecond laser ablation in liquids of iron-based nanoparticles
Alexander Kanitz, Jan Hoppius, M'Barek Chakif, Evgeny Gurevich, Andreas Ostendorf

Nanoparticles are intensively investigated in several fields of research due to their wide range of properties and
potential applications. Especially magnetic nanoparticles are of great interest in the field of magnetic fluids, catalysis,
biotechnology/biomedicine, magnetic resonance imaging, data storage, and environmental remediation.
Pulsed laser ablation in liquids (PLAL) is a simple method for the rapid production of colloidal nanoparticles. The
technique requires a suitable laser source, the desired target material and a liquid. One further advantage of the PLAL
produced particles is the ligand-free surface which makes them chemically more active than chemical synthesized ones
[1,2]. Here, we present a study on the generation of magnetic iron-based nanoparticles generated by high power femtosecond
PLAL with peak powers up to 1010 watts. We discuss the influence of the used liquid and applied laser parameters on the
generated nanoparticle properties. These properties, size distribution, shape and composition are investigated by
several methods, e.g. TEM, EDX, dynamic light scattering, and Raman spectroscopy.

Keywords: femtosecond laser, laser ablation, magnetic nanoparticles

_____

Formation of a periodically distributed inverted pyramid structure on silicon using direct laser interference ablation and surface etching processes
Airidas Žukauskas, Bogdan Voisiat, Martynas Gavutis, Gediminas Raciukaitis

In this paper, we present a new method of structuring a crystalline silicon surface using laser ablation and wet
anisotropic etching processes that produce periodic inverted pyramid surface structure. The direct laser interference
patterning technique was used as a first process instead of the single focused laser beam. That resulted in the idea-
periodicity structure produced on a large area with just a single laser pulse. The morphology investigation of the
formed structure depending on the laser processing and etching parameters was performed using SEM and AFM
microscopy.

KeyWords: laser Ablation, silicon, inverted pyramid, periodic structure, etching

_____

Direct laser patterning as alternative method for production of THz components and plasmonic structures
Bogdan Voisiat, Aidas Petryla, Gediminas Raciukaitis, Irmantas Kašalynas, Linas Minkevicius

In this work we discuss the alternative laser patterning method for fabrication of the frequency selective components,
plasmonic structures and compact optics on various metals. Research was focused on demonstration of the suitability of
laser DLP process for THz components fabrication. Experiments with different DLP process parameters was performed to
find optimal ones for fast fabrication procedure of various THz components. The fabricated structures morphology analysis
in terms of shape accuracy and processing quality was estimated. Finally the performance of fabricated components was
tested.

KeyWords: Laser Ablation; Micro-Cutting; THz components; Thin film; Foil;

_____

Evaluation of picosecond laser induced shunt resistance in CIGS thin-film solar cells
Edgaras Markauskas, Paulius Gecys, Gediminas Raciukaitis

CIGS thin films became even more attractive to PV industry after rapid efficiency increase over the last years for small
scale devices. However, the transition from the small scale to full-sized modules introduces efficiency losses due to
increased photocurrent and related ohmic losses. On the other hand, the high efficiency can be maintained if the large
module area is divided into many smaller segments interconnected in series by a three-step patterning process. We
utilized a picosecond laser working at the 532 nm wavelength to perform P3 type scribing process to separate the
adjacent cells. Initially, we applied the technique of nested circular scribes proposed by K. Zimmer to evaluate the laser-
induced shunt resistance after the P3 process. Further, we investigated the model including the top-contact resistance
effects to the I-V measurement. Finally, we applied a simpler laser-induced shunt evaluation model by performing the P3
scribes between screen-printed silver grids of the CIGS mini cells. In both cases, parallel resistance values of the cells
were extracted by analyzing I-V characteristics. Both evaluation techniques allowed to improve the laser parameter
optimization process and the measurement accuracy of shunt formation during the laser scribing.

Keywords: CIGS, laser scribing, P3, parallel conductivity,

_____

Picosecond laser modification of thin-film CIGS solar cell absorber layer for P2 micro-welding process
Paulius Gecys, Andius Žemaitis, Gediminas Raciukaitis

The high efficiency of a large thin-film solar cell can be maintained if cells are divided into smaller segments interconnected in series
in order to reduce photocurrent and resistance losses. Laser patterning is a promising tool for monolithic interconnect formation,
although cell deposition processes have to be interrupted for the laser scribing to b e applied. These issues are especially important
when going to the mass role-to-role production. P2 micro-welding process can be made after the front-contact layer deposition
replacing the standard P2 scribing process which interrupts absorber and window layers deposition. After being affected by laser
radiation CIGS compound melts and recrystallizes becoming a metallic compound which makes the interconnection between two
adjacent cells. The metallic compound can be made due to the formation of Cu-rich areas and diffusion of TCO to the CIGS layer in
the laser affected zone. In this work, we used picosecond laser to form molten lines in CIGS layer. Energy dispersion spectroscopy
analysis together with Raman spectroscopy was applied to investigate the composition of chemical elements and crystal structural
changes along various laser scribes made in CIGS absorber layer. Electrical characterization of the CIGS mini-cells after laser
modification revealed changes in the cell parallel resistance indicating changes of the CIGS conductivity in the laser modified areas.
Our investigations showed the potential of the picosecond laser in local modification process of the CIGS material which could be
applied to the P2 welding process.

Keywords: micro-welding, CIGS, solar cell, picosecond laser, EDS, Raman spectra

_____

Material modification of reinforcing glass fibers through pulsed laser radiation
Niels Schilling, Benjamin Krupop, Udo Klotzbach, Scott White, Raj Patel

In this paper, laser processing of glass fiber reinforced thermoplastics is investigated with different laser
sources. The aim of the study is to determine process windows in which uniform selective ablation of
polymer matrix and homogenous ablation of matrix and fiber occurs. Laser sources with different
wavelengths (10600 nm, 1064 nm and 355 nm) and pulse durations in μs and ns regime are compared on
their ablation behavior of natural and black colored glass fiber reinforced polypropylene. Further the effect
of laser processing on the fiber strength is investigated for different parameter settings where selective
ablation of polymer matrix was achieved.

Keywords: glass fiber reinforced thermoplastics, selctive laser material removal, fiber strength, joining interface, polymer filler

_____

Laser Processing of Lithium Iron Phosphate Battery Electrodes
Adrian Hugh Alexander Lutey, Alessandro Fortunato, Maurizio Fiorini, Simone Carmignato, Alessandro Ascari

Lithium iron phosphate (LFP) battery electrodes are exposed to laser radiation at 100 mm/s and 1000 mm/s while
systematically varying pulse duration (4-200 ns), repetition rate (20-1000 kHz) and average power (1-150 W). An optical
profiler operating in confocal mode is utilised to establish material removal efficiency in all cases, while scanning
electron and Raman microscopes are employed to determine macroscopic, microstructural and chemical changes near
the cut edges. The laser pulse fluence ranges leading to lowest minimum cutting power at 100 mm/s are found to be 35–
40 J/cm2 (100 kHz) and 100–110 J/cm2 (20 kHz) for the cathode and anode, respectively. The same laser parameters are
found to result in the smallest clearance width of the upper coating layer and the smallest defects. By increasing the
exposure velocity to 1 m/s and scaling the average laser power and repetition rate proportionally, clearance width and
defect size are found to reduce moderately, while chemical and microstructural degradation of the active layers is all but
eliminated. This study confirms the process as a viable alternative to mechanical blanking.

Keywords: Laser Ablation; Lithium Ion; LiFePO4; Graphite

_____

Fabrication of microchannels by picosecond laser pulses spatially shaped with cylindrical lens
Ehsan Zahedi, Daniel Förster, Volkher Onuseit, Rudolf Weber, Thomas Graf

Microchannels have applications in heat sinks, liquid dosing, flow rate measurements and structuring of surfaces for tribological
applications. Laser ablation with picosecond pulses promises small melt formation and precise machining of the metals. Howeve r, the
pulse heat accumulation can reduce the advantages of picosecond machining. Bea m shaping strategies are able to improve the
machining quality through manipulation of the heat accumulation. In the current work, microchannels were produced on the surf ace of
milled stainless steel samples. A combination of a cylindrical lens with a plan o-convex lens was used creating a line focus on the sample
surface up to the length of 500 μm. The experiments were performed with a 1 μm-wavelength picosecond laser with repetition rates of
50 kHz and 100 kHz. The maximum pulse energy of 188 μJ yields fluence about twenty times above ablation threshold. The geometrical
properties of the microchannels obtained were compared with the result of conventional processing with a circular s pot with respect to
the width and depth, sharpness of profile. Beamshaping with a cylindrical lens allows improving the quality of microchannels, narrower
microchannels and reduction of debris.

Keywords: Ablation, Drilling and Micro-cutting, Microchannel.

_____

Experimental study on laser marking of alumina
Joaquín Penide, Félix Quintero, Felipe Arias-González, Antonio Fernández, Jesús del Val, Rafael Comesaña, Antonio Riveiro, Fernando Lusquiños, Juan Pou

Alumina is one of the most employed ceramics in industry because of its good properties. Components made of alumina
are essentially identified by a code or a symbol printed directly on them. However, there is still a lack of a reliable and
efficient method to mark alumina. Particularly, laser typically produces marks with poor contrast.

In this work, an extensive experimental study on laser marking of alumina was carried out with the aim of finding out the
optimal parameters to produce high contrast marks. Four different lasers working at three different wavelengths
(infrared, visible and ultraviolet) were employed for the treatments. Colorimetric analyses were carried out in order to
have an objective and quantitative comparison among marks and resulting contrast. The optimum operating conditions
of the laser marking process were determined.

Keywords: laser marking, alumina.

_____

On line evaluation of femtosecond laser ablation efficiency on copper structures
Jan Stefan Hoppius, Alexander Kanitz, Benjamin Schöps, Evgeny Gurevich, Andreas Ostendorf

Femtosecond laser ablation is a flexible method to generate precise structures in the micrometer range. Yet, the
efficiency for the removal of bulk material is lower than ablation with micro- or nanosecond pulses. To evaluate the
efficiency of ultrashort pulse material ablation copper, test stripes were prepared and partly ablated with a titanium
sapphire laser. The electrical resistance of material is inversely proportional to its wire cross-section. Therefore, the
change of resistance is an indirect measure for the removed material. This instantaneous feedback enables on line
optimization of laser processing parameters. The results for various repetition rates, spot overlaps, ablation strategies
and pulse energies with pulse durations of 35 femtoseconds are compared with surface profiles measured by white-light
interferometry.

Keywords: femtosecond laser ablation; copper; electrical resistance; in situ evaluation

_____

Reducing the roughness of the kerf for brass sheet cutting with the Laser MicroJet® by a systematic parameter study
Yixin Bai, Annika Richmann, Jamie Paik, Bernold Richerzhagen

The Laser MicroJet® technology uses a 532 nm ns-laser coupled into a water jet for precision cutting. The water jet is
used to guide the laser and also cools the work piece so that a heat affected zone in the material is reduced or even
eliminated and the kerf is parallel. The Laser MicroJet® is widely used for processing of various materials such as metals,
hard materials and semiconductors.
This paper shows how the roughness on the kerf of 0.2 mm brass sheet can be reduced to < 0.2 μm by choosing laser
parameters and processing strategy. A systematic study for cutting of brass sheet with a thickness of 0.2 mm is carried
out in which laser parameters including frequency, pulse width and laser power are varied. Using three different laser
systems in this study the pulse width ranges from 7-350 ns. Additionally different processing parameters are tested such
as feeding speed, nozzle size and number of passes.
This work provides the users of Laser MicroJet® a deeper understanding of the relation between kerf roughness, laser
parameter and process strategy. We observed that multiple passes lead to higher roughness so that cutting with one
pass is preferred. Above a threshold peak intensity, which enables to cut through with one pass, the roughness
decreases for higher peak intensities with the same fluence. However, for too high peak intensities and energies per unit
length, the cooling effect of the water jet is not sufficient so that a heat affected zone evolves and the roughness
increases. Same as known from dry laser cutting, with low pulse widths, low roughness down to 0.2 μm on the kerf can
be achieved.

Keywords: water coupled laser, thin metal, precision cutting, cold cutting;

_____

3D laser micro-machining for targets manufacturing
Remy Bourdenet

High power experiments require complex targets that include various geometries and materials. Laser micro-machining
processes offer reliable and accurate solutions, able to fulfill demanding specifications. This paper gives examples of
recent developments in 3D laser micro-machining of targets components, involving either UV nanosecond pulses
(excimer laser) or ultrashort pulses (Ti:Sa femtosecond laser).

Keywords: femtosecond laser ; excimer laser ; pulsed UV laser ; micro-machining ; targets

_____

Mechanical properties of ultrafast-laser cut polylactic acid films
Giovanni Piredda, Rein Andreas, Johann Zehetner, Victor Matylitsky

The use of ultrashort laser pulses to cut polymers for medical applications has important advantages. Heat transmission
to the region surrounding the cuts is limited, so that the cuts are precise and the effects on the regions around the cuts
are small; in this way, the need for post-processing is reduced and ultrashort-pulse laser cutting becomes interesting for
industrial applications. In general, both cutting speed and heat effects increase with the energy of the pulses and the
repetition rate of the laser; it is important to identify process parameters with which polymer samples can be cut quickly
and without compromising the chemical and mechanical properties of the polymer. In this work we present measurements
of mechanical properties (elastic modulus, ultimate tensile strength and elongation at break) of cut samples of films of
poly(lactic acid) – a biodegradable polymer with many different medical applications – as a function of laser repetition
rate and examine how mechanical properties are correlated with the width of the heat-affected zones that can be
observed with an optical microscope.

Keywords: Micro-Cutting; Ultrafast laser sources; Processing of transparent materials; Polymers

_____

Effect of the pulse duration on the surface roughness and the heat affected zone in laser micro polishing processes
Mikel Gomez-Aranzadi, Antonio Dias, Miguel Martinez-Calderon, Ainara Rodriguez, Santiago Miguel Olaizola

Micro polishing of metal surfaces by means of pulsed laser has attracted increased interest in the last
decade, especially due to the precision it offers and the absence of waste byproducts in relation to
traditional mechanical abrasive methods. The process for micro laser polishing requires melting a thin
surface layer in order to reduce the roughness by means of the surface tension of the melt pool. This in turn
produces a heat affected zone beneath the melted layer. In applications where the structural properties of
the material must remain unchanged, the depth of the melted layer plus the heat affected zone have to be
tightly controlled. One of the most important parameters that conditions both the maximum affected spatial
period of the surface roughness (which increases the efficiency of the process) and the maximum thermally
affected depth is the pulse duration. In this work the authors have studied the effects in these magnitudes
as a function of the pulse duration between 20 and 200 ns. This has been done with a variable pulse laser of
a wavelength of 1064 nm over an iron and nickel alloy, scanning the surface with a fixed frequency of 5 kHz.
In the case of the evaluation of the surface roughness, a frequency-domain spectral analysis of the images
has been performed to study the effect of the pulse duration on the frequency spectrum of the surface
roughness. Considering the half width at half maximum (HWHM) of the frequency-domain spectral content
of the images, the results show a gradual decrease of the minimum affected frequency of the surface
roughness down to 0.21 μm-1 (which corresponds to a spatial period of 4.76 micrometers). This means that
the process can erase surface features with a maximum width of 4.76 micrometers. In the case of the depth of the heat affected zone (plus the melted layer), a range of 1.5 to 4.5 μm was measured for pulse widths
between 20 and 200 nanoseconds.

Keywords: laser micro polishing; nanosecond laser; FFT analysis

_____

Scanned Mask Imaging: The economical approach to high resolution micro-machining using UV solid state lasers
Zoe Knill, David Milne

Two methods currently exist for micro-structuring materials by laser: mask projection and direct write. This paper outlines
a novel and alternative method called Scanned Mask Imaging (SMI), the resulting quality of SMI is comparable to that of
excimer laser mask projection systems, but delivered at a fraction of the cost and burden of ownership. SMI has the
potential to unlock the development of novel micro-machining techniques via programmable illumination patterns and
dual plane imaging optics. Advantages of this new Scanned Mask Imaging method compared with conventional excimer
laser based mask projection systems are discovered and summarized in this paper.

Keywords: Micro-machining; Scanned Mask Imaging; Mask projection; UV; Solid state lasers; Ablation; Imaging