Processing of Transparent Materials (LiM 2017)

A novel 2 µm ultrashort pulsed laser source for selective laser-induced etching of glass
Sotier, Florian; Kipnusu, Wycliffe K.; Hermans, Martin; Gottmann, Jens; Geiger, Stephan; Marowsky, Gerd; Siefermann, Katrin R.

We present a novel source of ultrashort light pulses in the 2 μm wavelength range and demonstrate its application for
selective laser-induced etching of different glasses, such as borosilicate glass and fused silica. A chirped pulse master
oscillator power amplifier (MOPA) system is based on a mode-locked seed oscillator followed by a chain of Thulium
doped fiber amplifiers. The laser system delivers pulse energies of up to 2 μJ at repetition rates between 30 kHz and 2
MHz and pulse durations as short as 540 fs. The output beam of laser light features an M2 better than 1.3.
Focusing ultra-short laser pulses into a glass work piece generates material modifications at the position where the field intensity exceeds the threshold for modification. This laser-induced structure alteration of the glass material is attributed to a highly nonlinear process involving ultrafast multi-photon excitation of electrons and subsequent energy
transfer to the lattice. Here, we demonstrate that modifications inscribed by the 2 μm laser into the different glasses can
be selectively removed in a wet chemical etching process. This manufacturing procedure is termed “selective laser-
induced etching” or SLE. The key is that the etching rate of modified material is higher than the etching rate of
unmodified material. For borosilicate glass we demonstrate etching selectivities of 450 and for fused silica of 500. These
high selectivities will allow manufacturing of tunnels, three-dimensional cavities, and even complex networks for
microfluidic devices in both materials. The ultrashort 2 μm laser pulse thus makes borosilicate glass the most
important glass for chemical laboratory equipment due to its superior durability, chemical and heat resistance
accessible to manufacturing via selective laser-induced etching.

Keywords: ultrashort pulsed fiber laser; 2 micron laser; selective laser induced etching; borosilicate glass; glass processing


Direct femtosecond laser irradiation of polymeric substrates for high resolution ink-jet printing of conductive lines
Rodriguez, Ainara; Martinez-Calderon, Miguel; Fraile, Itziar; Gomez-Aranzadi, Mikel; Olaizola, Santiago Miguel; Castro-Hurtado, Irene

In this work, polyethylene terephthalate (PET) and polyimide (PI) films of 125 m, two polymers commonly used for ink-
jet printing, have been processed with a Ti:Sapphire laser system which generates 130 fs pulses at a central wavelength
of 800 nm, with a 1 kHz repetition rate. A study of the formation of surface micro/nanostructures was performed by
scanning the selected areas at a constant speed for different laser parameters. These structures have notably increased
the surface water and Ag ink wettability. The fabricated polymer surfaces were subsequently used as substrates for ink-
jet printing of Ag conductive lines for improving and controlling the ink jet flow in a highly efficient way, giving rise to Ag
lines with a resolution five times better than the one for non-treated polymers.

Keywords: ultrafast laser; polymer; ink-jet printing.


Strong Connection: Welding of different kinds of glass using femtosecond laser pulses
Russ, S.; Müller, C.; Budnicki, A.; Wieduwilt, J.; Lang, M.; Richter, S.; Hesse, T.

In this paper, we report on latest welding results of different glass substrates. All substrates have been welded by
utilizing a femtosecond (fs) laser of the TruMicro series 2000 applying a pulse duration of 300 fs. The research focuses on
welding of fused silica and alkali- aluminosilicate glass. We report also on results of alkaline earth boro- aluminosilicate
and borosilicate glass bonds.
The femtosecond laser source has been applied on welding two optically contacted samples. To generate a dedicated
melt volume different parameters have been varied during the study. Aside from the number of pulses within a burst
train the base frequency has been determined as one important parameter to control the generation of a suitable melt
After joining the glass samples the quality of the welded seams have been evaluated. Besides the quality, the stability of
the welds was examined.

Keywords: micro processing, femtosecond, ultra-short laser pulses, glass, welding


Cutting thin glasses with ultrafast lasers
Domke, Matthias; Piredda, Giovanni; Bernard, Benjamin; Matylitsky, Victor

In recent years, the non-linear interaction phenomena of ultrafast laser pulses inside transparent media have attracted
many researches and companies to develop several innovative glass-cutting applications. However, cutting of thin
glasses with a thickness of about 50 μm may still be difficult or even impossible. This paper presents a novel cutting
strategy that combines back and front side ablation. The bending test results demonstrate that this strategy also
improves the breaking strength of the cut chips by about 20 % to above 300 MPa compared to a conventional full cut by
front side ablation.

Keywords: ultrafast, laser, patterning, superhydrophobic, superhydrophilic, surfaces


Automated color printing of glass by using a laser-burning process
Plat, Kristin; von Witzendorff, Philipp; Suttmann, Oliver; Overmeyer, Ludger

In recent years, the production of individual, colored glass products has been increased. As a result, flexible production
processes and short delivery times for glass finishers should be profitable. Currently, the colors have to be applied on the
glass surface before tempering in the oven. As a result, the color will be burned with the glass. Therefore, the process is
reserved for large companies with an oven for the production of toughened safety glass. In this project, a new process
will be developed. The applied color is baked locally on the glass surface by laser radiation. Float glass was used for the
investigations. Laser firing experiments were carried out with different color layers with thicknesses of about 30 μm. A
coloring could be achieved by using a laser wavelength of 10.6 μm. The process limits were determined and the
following phenomena were observed: no coloring is possible with low material stress; with adapted laser parameters, a
coloring is visible; with an increase in the laser power, stresses in the glass were introduced. Therefore, the risk of crack
initiation will be increased. The colors created with the oven fired process and with the laser process have identical
appearances. Using the laser process, different colors could be burned locally on the glass surface and the production of
large-format and colored images, for example in architectural glass, are possible.

Keywords: glass; printing techniques; surface treatment


Patterning of graphene from ps to fs pulses
Piredda, Giovanni; Domke, Matthias; Coca Lopez, Nicolas; Hartschuh, Achim; Matylitsky, Victor; Bernard, Benjamin

Single-layer graphene is a one-atom thick carbon crystal with unique material properties: extremely high mechanical
strength and elasticity, very high electrical and thermal conductivity, uniform absorption over all of the optical and
infrared spectrum.
These properties make it highly attractive for many applications, among which flexible electronics. The properties of
graphene depend on the number of layers so that patterning with a controlled number of layers affords flexibility in the
fabrication of graphene devices; selective ablation leading to the control of the number of graphene layers down to a
monolayer has recently been demonstrated using picosecond laser pulses.
In our work we study selective patterning of graphene as a function of pulse length from 340 fs to 14 ps both on glass
and on PET polymer substrates. We characterize the patterned graphene using optical transmission and Raman

Keywords: ultrashort pulses; graphene; ablation; laser


In-volume structuring of silicon using ultrashort laser pulses
Kämmer, Helena; Matthäus, Gabor; Nolte, Stefan; Chanal, Margaux; Utéza, Olivier; Grojo, David

We investigate in-volume structuring of crystalline silicon with ultrashort laser pulses. The processing threshold is
analyzed in dependence of pulse length (0.8 10 ps), pulse energy, pulse number and repetition rate (30 200 kHz).
Moreover, the generated morphology of the irradiated region as well as the dynamic evolution during laser irradiation
was studied. This work has the potential to pave the way for future silicon processing methods like stealth dicing or even
the generation of buried integrated optical elements.

Keywords: ultrashort pulses; in-bulk processing; silicon;


Telecentric CO2 laser ring-cutting system with adjustable diameter
Kraus, Matthias; Förster, Erik; Bohnert, Patrick; Kilper, Roland; Müller, Ute; Buchmann, Martin; Brunner, Robert

This contribution presents an optical setup for the generation of a telecentric ring focus with an adjustable diameter for
cutting applications. The system is designed for the use of a CO2 laser at 10.6 μm working wavelength and is adapted to
an observation microscope for examining and selecting the sample at first. Subsequently, it cuts a small circular region
with a single shot. The main application is the cutting of thin polymer films such as forensic adhesive tapes for trace
selection. The diameter of the ring can be continuously tuned between 500 μm and 2 mm where a kerf of less than 20
μm is realized. To compensate potential height differences of the samples, telecentricity in the target plane for all
adjustable ring diameters is demanded. Therefor the aperture stop of the focussing lens has to be filled with a variable
angular spectrum. Variable ring diameter and the compliance of the telecentricity condition have been achieved by a
modular optical setup which includes three axicon elements as key components. An axial shift of one of these axicons
results in a varying angular spectrum in the aperture stop and therefore, the required specifications in the target plane
are fulfilled. The optical design principle which creates the telecentric and size-variable ring focus will be discussed at
first. Secondly, the adaption of the ring-cutting system to the observation microscope will be presented.
Observation focus and ring cutting profile are aligned in the target plane and are coupled dynamically to allow a combined movement e.g. for focus variation. Subsequently, the properties of the realized system will be demonstrated by reference to cutting experiments on different polymer tapes. Finally, further cutting examples, for instance from the field of biotechnology, will be shown.

Keywords: micro-cutting of polymer films; axicon; annular laser profile; telecentricity;


Additive manufacturing by UV laser direct writing of UV-curable PDMS
Obata, Kotaro; Nakajima, Yasutaka; Hohnholz, Arndt; Koch, Jürgen; Terakawa, Mitsuhiro; Suttmann, Oliver; Overmeyer, Ludger

The combination of photo-curable polymers and laser direct writing technique established one of the well-known
technologies in the field of additive manufacturing. Recently, 2D/3D structuring of an elastic polymer material is
required as an advanced technique for various different research fields. In this study, we have demonstrated UV laser
photo-polymerization of elastic 2D/3D structures using UV-curable Polydimethylsiloxane (PDMS). In this technique, UV
curable PDMS was locally polymerized to fabricate 1D and 2D single layer structures, as well as 3D multi-layer structures
with resolution in micrometer-scale and structure dimension in millimeter-scale. In addition, a hybrid technique of
aerosol jet printing followed by UV laser direct writing was developed to realize 2D patterning of thinner UV curable
PDMS layer without solvents. The polymerized PDMS had good transparency and elasticity as same as common thermal
curing-type PDMS. The hybrid technique of aerosol jet printing followed by UV laser direct writing achieved the thinnest
layer thickness of UV curable PDMS as small as 1.6 μm on the free-form surface.

Keywords: UV-curable PDMS, UV laser direct writing, Aerosol jet printing, Additive manufacturing


Laser-based process for polymeric coatings on temperature-sensitive metallic components
Sändker, Hendrik; Stollenwerk, Jochen; Loosen, Peter

Multiple applications, especially in the automotive sector and in mechanical engineering, are predominantly affected by
friction and wear stress and, therefore, represent a substantial challenge for the components being used. Oftentimes,
the endurance and the efficiency of these components can be enhanced by means of application-specific coatings. For
multiple applications, the functional requirements can be met sufficiently by polymeric coatings. Particularly, coatings
based on high performance thermoplastic polymers like Polyetheretherketone (PEEK) have an outstanding potential due
to their excellent properties in terms of temperature resistance and wear protection.

Conventionally, PEEK is deposited in powder form and, subsequently, workpiece and powder are heated above the
melting temperature of PEEK (approx. 340°C) by oven. Accordingly, this process is not suitable for temperature-sensitive
base materials which show undesirable thermally induced effects (e.g. decrease of hardness) for temperatures below
340°C. A promising approach to enlarge the range of processable materials consists in the investigation of a laser-based
process. Due to melting the PEEK with temporally and spatially controllable laser radiation, the thermal load of the
workpiece is reduced. This process comprises four consecutive steps: a laser-based pre-treatment of the components,
the preparation of a hydrous dispersion based on PEEK powder, the deposition of the dispersion by e.g. spray coating,
and the laser-based melting of the PEEK powder.
Regarding the pre-treatment using pulsed laser radiation, current investigations primarily focus on the dependence
between the induced roughness and the adherence of the coating. For the laser melting process, the main goal is
identifying a process window for dense and adherent coatings without thermally induced effects on the base material.
By means of this new coating process, dense and adherent coatings with a thickness of 15 45 μm can be applied on
steel substrates. The adherence is significantly increased by the laser-based pre-treatment of the metallic substrates.

Keywords: Surface Functionalization; Fundamentals and Process Simulation


Induction of low-stress and crack-free laser micro holes in sapphire: optimization of the process parameters under different ambient conditions
Gantner, Christina; Liebold, Karsten; Kaszemeikat, Tristan; Schädel, Daniela; Knappe, Verena

Objective: Generation of high-precision micro holes in thin substrates made of sapphire with the aim of
avoiding residual stress while maintaining high edge strength and processing quality.
Materials and methods: Micro holes of 1000 μm and 500 μm in diameter were drilled in 1mm and 0.5mm thick sapphire samples using an UV nanosecond laser (355 nm, 7 ns). The tests were carried out both under
normal pressure and under reduced ambient pressure of 10mbar generated by a specially designed vacuum
chamber. Chipping was assessed and measured by means of optical microscopy. Furthermore a polarization
microscope was used to quantify tensions, which were induced throughout the drilling process.
Results: Holes drilled in sapphire with pulse durations of about 10ns show significantly smaller chipping compared to drills in other transparent materials (e.g. borosilicate glass). Due to the faster removal of
erosion products from the drill channel, higher ablation rates were reached under reduced pressure. Moreover, a 20% reduction of the laser power caused a considerable decrease of stress in the material.
Conclusion: From the literature and own preliminary investigations it is known that sapphire can be hardly processed from the reverse direction since very high mechanical tensions occur immediately which can destroy the entire sample. This effect was also confirmed in the present experiments where front-side laser irradiation proved to be more efficient than rear-side irradiation. Overall, higher laser powers and slower
feed rates are necessary in order to achieve an ablation in sapphire.

Keywords: micro processing; drilling; processing of transparent materials; sapphire


Spatio-temporal beam shaping for glass cutting with femtosecond lasers
Mishchik, K.; Lopez, J.; Duchateau, G.; Chassagne, B.; Kling, R.; Audouard, E.; Mottay, Eric

We develop a dedicated method for glass cutting by laser cleaving. The presented method allow to control the crack
orientation and thus optimize the cutting velocity with high quality (pending patent) via a beam engineering module
added to the laser. We cut a 500μm thick sample of glass with a single pass. At repetition rate of 100kHz and 80μJ per
pulse, cutting is possible at a speed of 600mm/s. Our original method allow also curve cutting. Resulting average
roughness of sidewall is less than 1μm. Cutting of other material like sapphire, quartz, fused silica, tempered glass was
equally demonstrated.

Keywords: femtosecond laser processing, glass cutting, beam shaping


Inducing scattering centers in medical optical fibers by pulses in the range of ps.
Wall, Alexander; Hoffmann, Hans-Jürgen; Knappe, Verena

Opti ca l fibers are used in dental medicine to treat gingival s ulcus with light of 450nm wavelength. About 3mm long
di ffusers a t the end of the fiber i ncrease the irradiation angle by s cattering the l ight inside the core. I n order to maximize
the s tability of the fiber, we i nvestigated a novel a pproach to i nduce scattering ce nters inside the fiber. Limits and
pos sibilities of the front-end induction inside fused silica fibers were s tudied using laser pulses i n the ra nge of 10 ps .
Thre s hold intensities for the generation of s cattering ce nters we re determined as well as their growth as a function of
i nte nsity, number of pulses and time interval between pulses. The threshold was compared with predictions re sulting
from a ra te -equation. Beside the i ntended scattering ce nters other phenomena occurred s uch a s periodical ripples a nd
ma te rial alteration i nside the fiber that s eem to i nterfere with the cre ation of the s cattering ce nters.

Keywords: Silica fiber; picosecond pulse; medical optical fibers; fiber processing; scattering center; diffuser; Microstructure


Nanosecond laser ablation of different crystallographic planes of sapphire
Saeidi, Fatemeh; Mouhamad Ali, Freidy; Wasmer, Kilian

In this work, nanosecond laser ablation of different crystallographic planes (c-, a- and m-plane) of sapphire produced by
Verneuil, Kyropoulos, and EFG methods have been studied. The ablation thresholds as well as the crack orientations
were analyzed by single laser shots at different laser fluence ranging from 4 to 200 J/cm2. The results showed that the
production methods and the orientation planes of sapphire have slight effect on both the ablation threshold and the
ablated depth of sapphire. However, the crack morphology strongly depends on the planes orientations.

Keywords: Sapphire; ns-laser ablation; crystallographic plane


Damage mechanisms of ultrashort pulsed laser processing of glass in dependency of the applied pulse duration
Kalupka, Christian; Holtum, Tim Henrik; Reininghaus, Martin

We report on the transmission of ultrashort pulsed laser radiation for glass processing with single laser pulses. The
applied pulse duration is varied from 80 fs to 9 ps and the intensity from 5*1011 to 5*1014 W/cm², respectively. Above a
certain intensity threshold, the transmission of the laser radiation continuously decreases. This indicates an increased
absorption of the laser pulse due to the generation of a denser free-electron plasma. Additionally, we observe visible
damage of the glass by means of optical microscopy imaging. The intensity threshold decreases by more than one order
of magnitude from 22*1012 W/cm² for 80 fs to 2*1012 W/cm² for 9 ps. This can be explained by different dominant
ionization mechanisms, which depend on the applied pulse duration and intensity. By comparing our experimental
results with simulations performed by other groups, we find that for a pulse durations of 80 fs, the damage of material is
ascribed to the generation of a free-electron plasma by multiphoton ionization. For longer pulse durations 1 ps and 9 ps,
the generation of the free-electron plasma and the damage of material is induced primarily by impact ionization.

Keywords: Ultrashort laser pulses, processing transparent materials, absorption, ionization mechanisms


Ablative processing of fine features in brittle materials with ultrashort laser pulses
Lott, Geoffrey; Lafoy, Guillaume; Falletto, Nicolas; Devilder, Pierre-Jean; Kling, Rainer

Laser processing of transparent brittle materials has proven to be an attractive alternative to conventional mechanical
methods due to increases in quality and yield, while also allowing the processing of difficult feature geometries. Here we
present a summary of recent experiments aimed at optimizing laser ablation drilling of relatively high aspect ratio (>1:1)
features in brittle materials, focusing on sapphire. We present results for laser drilling of holes in sapphire wafers with
both 1030nm and 515nm sub-picosecond laser sources. We determine the optimum process window as a function of
wavelength, pulse energy, repetition rate, overlap, and beam waist / z-axis translation speed. In ambient air, we find
that ~400μm diameter holes in 430μm thick sapphire wafers (aspect ratio depth:diameter of ~1:1) can be drilled with
average taper angles of ~4° in <3 seconds, and ~2° in ~12 seconds. We determine broad ranges of process parameters
that yield high-quality holes for both 1030nm and 515nm, while also identifying important process limitations. We
identify the recast of sticky particulates generated during the ablation process as the main detriment towards the drilling
of zero-taper holes in sapphire. We present results for reducing recast with liquid-assisted drilling, which we observe to
efficiently reduce redeposition of ablated material along the hole sidewall, allowing for the drilling of holes with <2°
taper in ~5 seconds. Lastly, we explore the ability to minimize drilled hole diameters in sapphire by applying the totality
of the general knowledge learned throughout these studies, demonstrating the ability to drill holes with aspect ratios of
up to ~10:1.

Keywords: laser ablation; through holes; femtosecond laser; transparent media; aluminum oxide


Single pass cutting of glass substrates >4mm with ultra-short laser pulses
Bergner, Klaus; Thomas, Jens Ulrich; Kumkar, Malte; Seyfarth, Brian; Schatz, Jonas; Lentes, Frank-Thomas; Gross, Herbert; Feuer, Anne; Weber, Rudolf; Tünnermann, Andreas; Nolte, Stefan

The precise separation of glass is of great importance for various applications, e.g. for the fabrication of displays. The use
of ultra-short laser pulses enables localized energy deposition inside the glass sample. The resulting stress fields can be
used to define breaking lines enabling glass cutting with high surface quality, no chipping and no need for post-
processing. In this presentation, we report on in-situ measurements of the interaction between the laser pulses and the
glass. This allows a fundamental understanding of the interaction process, to identify critical processing parameters and
to tailor the scribing process. Using adapted laser beam shaping we were able to induce homogeneous modifications
and to process glass thicknesses of >4mm in a single pass.

Keywords: Laser marials processing; Glass processing; In-volume modification; Beam shaping; Time-resolved microscopy


Novel optical concept for large area rapid thermal processing
Tillkorn, Christoph; Canova, Lorenzo; Dorer, Stefan; Lang, Michael; Huonker, Martin

With the availability of high-power infrared laser sources, laser-based rapid thermal processing of large area substrates
becomes more and more attractive. Aside from the laser source, one of the core components is the beam shaping optics
for generation of a line-shaped intensity profile with extreme aspect ratio. We report on a novel optical concept for
laser-based large area rapid thermal processing which relies on the established thin disk laser platform providing an
output power of 12 kW per unit. One of the unique features of this modular concept is the capability to precisely
combine optical units to realize a processing width of larger than 3 m. To fulfill the high demands on homogeneity and
depth of focus we make use of advanced beam-shaping and measurement techniques. Together with the proven
robustness of fiber-coupled laser sources this modular approach has demonstrated the ability of 24/7 operation with
industrial standards in terms of process quality, treatment speed and efficiency at our strategic partner. For the first
time in history rapid thermal annealing was applied to thin films deposited on jumbo-size architectural glass without
affecting the glass substrate. Due to the highly improved crystallization, the resistivity of e.g. thin Ag-based stacks could
be decreased up to 30%. This led to a breakthrough in the development of high energy efficiency functional coatings on
This technology opens a large field to new cost-effective glass products with improved energetic balance, simultaneous
high transparency and improved electronic transport properties and many others. Perspectively applications for sheet-
metal refinement can also be considered.

Keywords: Rapid thermal processing; laser annealing; functional coatings; disk laser; high power optics; robustness; scalability, stitching.