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Paper des Monats
- ja (3)
Laser cleaning of soiled paper is a challenging task due to the fact that a contamination has to be removed and a fragile organic material has to be preserved. The ejection of particles forms a significant channel for the removal of unwanted surface contaminations and can be exploited for an in-situ monitoring of the cleaning procedure. 532-nm-nanosecond single and multi pulse laser cleaning of artificially soiled Whatman© paper was performed. Particles were registered with a dust monitor. These in-situ experiments were combined with ex-situ investigations of cleaning and substrate damage thresholds by means of light and scanning electron microscopic techniques. The cleaning efficiency measured by a multi-spectral imaging system was compared to the in-situ particle monitoring. Additionally, possible color changes of the paper substrate were evaluated.
Mikromaterialbearbeitung mit ultrakurzen Laserimpulsen: Vorzüge, Probleme, Sicherheitsaspekte
(2005)
A first investigation on the ablation of composite materials like carbon and silicon-carbide reinforced alumo- and borosilicate glasses by 300 fs laser pulses (620 nm) in comparison to experiments with 17 ns pulse excimer laser (308 nm) treatment is presented. In all composites, femtosecond laser scans with a fluence of less than 2 J cm-2 produce well defined cuts with smooth side walls in contrast to the nanosecond laser result where extremely incongruent ablation is observed. Visible lasers should not be applicable because SiC and the glasses are practically transparent. Visible subpicosecond pulses of high intensity in the TW cm-2 range allow multi-photon absorption accompanied by incubation phenomena. The morphology of the groove edges reveal the contrasting ablation thresholds and rates of the fibre and glass materials. The ablation thresholds of the transparent components, i.e. the glass matrices and SiC, are about one order of magnitude greater than that of carbon which is ~ 0.15 J cm-2. Incubation effects are important for the absorption mechanisms in the transparent materials. When a critical number of pulses has not been reached at the SiC-glass composites, only the glass is preferentially ablated, and the fibres remain intact. This is in contrast to the C-glass composites where the ablation behaviour is opposite.
High-power lasers in industrial and R & D applications raise the general problem of reliability and degradation of optical components. A systematic study of nonlinear interaction of various transparent dielectric materials as e.g. glasses, fused silica, and polymers, with laser-pulses in the intensity range of up to 1013 W cm-2 is presented. On the other hand, femtosecond-pulse laser processing in the visible spectral range (300 fs; 620 nm, ~ 2 eV) allows precise microstructuring of transparent dielectrics without disruption of the remnant material. Damage and ablation threshold fluences occur above 1.2 J cm-2 at both silicate glasses and fused silica. Two different photon absorption mechanisms have been observed. The first occurs during the initial laser pulses in the incubation range. There, multiphoton absorption results in moderate energy volume densities. These are sufficient to generate morphological changes and optically active defect sites (colour centres) which provide a much higher absorptivity relevant for the second mechanism. It results in gasification without participation of melt.
Analyse häufig verwendeter europäischer Papiere. Im Fokus: Fasermaterial, Füllstoffe und Leimung
(2007)
Restauratoren können verschiedene Papierarten mit dem bloßen Auge aufgrund von Merkmalen in der Textur und der Oberflächenbeschaffenheit, seinem Aussehen im Durchlicht sowie an Griff und Klang grob unterscheiden. Dabei ist sich der Restaurator stets der Tatsache bewusst, dass es sich lediglich um eine erste grobe Einschätzung handeln kann. Erst bei mikroskopischen Analysen und physikalischen Experimenten treten Charakteristika zu Tage, die eine weitere Annäherung an die Zusammensetzung des Papiers und damit seiner Provenienz erlauben. Gegenstand dieser Arbeit war es, ein europäisches Hadernpapier, ein holzhaltiges Zeichenpapier und ein Whatman®-Filterpapier mit zwei mikroskopischen Methoden und einer Verbrennungstechnik zu untersuchen. Das Fasermaterial, die Füllstoffe, die Leimung und eventuelle Verunreinigungen wurden detailliert evaluiert. Neben konventioneller Lichtmikroskopie kam die weniger bekannte Umweltrasterelektronenmikroskopie (ESEM = Environmental Scanning Electron Microscopy) zum Einsatz, die neben der bildlichen Darstellung der Probenoberflächen zusätzlich eine Elementanalyse derselben gestattet. Letzteres erfordert allerdings das Vorhandensein eines zusätzlichen Röntgenspektrometers. Darüber hinaus wurden die verschiedenen Papiere trocken verascht. Es wurde eine oxidative Zerstörung der organischen Stoffe gewährleistet bis nur noch nichtbrennbare mineralische Asche zu verzeichnen war. Die Komplementarität der eingesetzten Methoden wird demonstriert und ein Einblick in die Zusammensetzung und Struktur der Papiere ermöglicht. Die Analysemethoden sind durch ihren partiell zerstörenden Charakter bzw. Limitierungen hinsichtlich der Objektgröße nur bedingt für die Untersuchung am Original geeignet, sehr wohl aber dafür, das Probenmaterial für Versuchsreihen genau zu definieren und zu standardisieren.
In recent years, femtosecond lasers matured from sensitive research setups to turnkey systems for an increasing number of applications. Now that running an ultra-short pulse laser system needs no longer a specialized scientist, safety issues become more crucial. Ultrashort laser pulses have extremely high peak powers and even scattered radiation may possess severe risks to the unprotected eye. Accordingly,
special protection schemes are necessary for these unique laser systems.
High-power optical multimode fibers are essential components for materials processing and surgery and can limit the performance of expensive systems due to breakdown at the end faces. The aim of this paper is the determination of laser-induced damage thresholds (LIDT) of fibers (FiberTech) and preforms (Heraeus Suprasil F300). Preforms served as models. They were heated up to maximum temperatures of 1100, 1300 and 1500°C and cooled down to room temperature at rates of 10 K min-1 (oven) and ~105 K min-1 (quenched in air) to freeze in various structural states simulating different conditions similar to a drawing process during the production of fibers. Single- and multi-pulse LIDT measurements were done in accordance with the relevant ISO standards. Nd:YAG laser pulses with durations of 15 ns (1064 nm wavelength) and 8.5 ns (532 nm) at a repetition rate of 10 Hz were used. For the preforms, LIDT values (1-on-1) ranged from 220 to 350 J/cm² (1064 nm) and from 80 to 110 J/cm² (532 nm), respectively. A multi-pulse impact changed the thresholds to lower values. The LIDT (1064 nm wavelength) of the preforms can be regarded as a lower limit for those of the fibers.
Optical multimode fibers are applied in materials processing (e.g. automotive industry), defense, aviation technology, medicine and biotechnology. One challenging task concerning the production of multimode fibers is the enhancement of laser-induced damage thresholds. A higher damage threshold enables a higher transmitted average power at a given fiber diameter or the same power inside a thinner fiber to obtain smaller focus spots.
In principle, different material parameters affect the damage threshold. Besides the quality of the preform bulk material itself, the drawing process during the production of the fiber and the preparation of the fiber end surfaces influence the resistance. Therefore, the change of the laser-induced damage threshold of preform materials was investigated in dependence on a varying thermal treatment and preparation procedure.
Single and multi-pulse laser-induced damage thresholds of preforms (F300, Heraeus) were measured using a Q-switched Nd:YAG laser at 1064 nm wavelength emitting pulses with a duration of 15 ns, a pulse energy of 12 mJ and a repetition rate of 10 Hz. The temporal and spatial shape of the laser pulses were controlled accurately.
Laser-induced damage thresholds in a range from 150 J cm-2 to 350 J cm-2 were determined depending on the number of pulses applied to the same spot, the thermal history and the polishing quality of the samples, respectively.
Characterization of laser-generated microparticles by means of a dust monitor and SEM imaging
(2006)
Nanosecond laser (1064 nm wavelength) cleaning of artificially soiled paper as a model sample simulating a real-world artwork was performed. During the cleaning process, the ejection of particles was monitored in situ by means of a dust monitor (8 size classes, ranging from 0.3 µm to >2 µm) and ex situ using a mini-cascade impactor (MKI, 5 stages). The cleaning result was analyzed by scanning electron microscopy (SEM) considering possible laser-induced damages to the substrate. Size distributions of emitted particles were measured depending on the processing parameters: laser fluence, F, and pulse number per spot, N. High numbers of large (>2 µm) particles were collected by the mini-cascade impactor indicating a gas dynamical liftoff process. Obviously, these particles were not affected by the laser-matter interaction. The different methods (SEM, MKI, and dust monitor) are compared with respect to their usefulness for a proper interpretation of the cleaning results.
High-power optical multimode fibers are essential components for materials processing
and surgery and can limit the reliability of expensive systems due to breakdown at the end faces.
The breakdown threshold of fibers is determined by intrinsic materials properties and parameters of
the technology applied. The aim of this paper is the identification of technological parameters that
are crucial for the fiber quality.
Fibers were drawn from preforms of Heraeus SWU with core material F300 and a low amount of
OH-. Both, the cladding (fluorine doped SiO2) to core diameter ratio (CCDR) and the drawing speed
were varied. CCDR values between 1.05 and 1.4 were used. Afterwards, the laser-induced damage
thresholds (LIDT) of the fibers were determined. For comparison, also samples from preforms,
which underwent different thermal treatments above the transition temperature, were tested with
respect to their damage resistivity. Single and multi pulse LIDT measurements were done in
accordance with the relevant ISO standards. Nd:YAG laser pulses with durations of 15 ns (1064 nm
wavelength) and 8.5 ns (532 nm) at a repetition rate of 10 Hz were utilized. For the fibers, LIDT
values (1-on-1, 1064 nm and 532 nm) increased with growing CCDR and with decreasing drawing
velocities.
Cleaning of paper is a challenging task due to the fact that a contamination should be removed and a fragile organic original material has to be preserved. Pulsed laser cleaning of artificially soiled Whatman© filter paper samples serving as models for historical paper was performed. Different cleaning strategies employing 8-ns laser pulses at 532 nm wavelength were applied to clean paper avoiding undesired effects like discoloration (yellowing) and mechanical deterioration of the substrate. Multi shot experiments with low-energy pulses were compared with single pulse investigations utilizing high pulse energies achieving a constant energy load incident on the samples in both cases. The cleaning efficiency and possible yellowing effects were evaluated by means of a multi spectral imaging system. An extensive microscopic analysis of the cleaned parts of the samples provided insight into the remaining soiling on the surface and in the bulk of the paper material after laser treatment. As a reference, a hard and a soft eraser were used to clean the samples.
Subpicosecond Emission from Mercury and Silver into Electrolyte Solution: An Experimental Study
(1997)
The influence of different laser pulse lengths on the removal of a polymer layer from metal substrates was investigated. As model systems, doped poly(methylmetacrylate) (PMMA) on titanium and tungsten substrates were selected.
The ablation threshold and irradiation spot morphology of titanium and tungsten were compared for femtosecond (fs) and nanosecond (ns) laser irradiation and different pulse numbers. Nanosecond laser treatment resulted in a non-homogeneous surface morphology for both titanium and tungsten substrates. Femtosecond irradiation of tungsten revealed a homogeneous ablation spot with little changes in the surface morphology. For titanium, the formation of columnar structures within the irradiation spot was observed.
Two different dopant concentrations were used for PMMA to achieve an equal linear absorption coefficient for the femto- and nanosecond laser wavelengths of 790 and 1064 nm. The best results were achieved for the removal of doped PMMA by femtosecond laser irradiation, where only a minimal modification of the metal surface was detected. In the case of nanosecond laser exposure, a pronounced change of the structure was observed, suggesting that damage-free cleaning of the selected metal may only be possible using femtosecond laser pulses. Different experimental parameters, such as laser fluence, pulse repetition rate and sample speed were also investigated to optimize the cleaning quality of doped PMMA from tungsten substrates with femtosecond laser pulses.
The formation of near-wavelength laser-induced periodic surface structures (LIPSS) on silicon upon irradiation with sequences of Ti:sapphire femtosecond laser pulse pairs (pulse duration 150 fs, central wavelength 800 nm) is studied theoretically. For this purpose, the nonlinear generation of conduction band electrons in silicon and their relaxation is numerically calculated using a two-temperature model approach including intrapulse changes of optical properties, transport, diffusion and recombination effects. Following the idea that surface plasmon polaritons (SPP) can be excited when the material turns from semiconducting to metallic state, the 'SPP active area' is calculated as function of fluence and double-pulse delay up to several picoseconds and compared to the experimentally observed rippled surface areas. Evidence is presented that multi-photon absorption explains the large increase of the rippled area for temporally overlapping pulses. For longer double-pulse delays, relevant relaxation processes are identified. The results demonstrate that femtosecond LIPSS on silicon are caused by the excitation of SPP and can be controlled by temporal pulse shaping.
Cleaning of artifically soled paper using nanosecond, picosecond and femtosecond laser pulses
(2009)
A new approach is presented to quantify the so-called "heat affected zone" (HAZ) during
femtosecond laser pulse processing. Ablation of titanium nitride (TiN) thin films (~3 μm
thickness) by multiple femtosecond laser pulses (τ=130 fs, λ=800 nm) in air environment was
studied by means of two different surface analytical methods both being sensitive to chemical
alterations at the surface. Scanning Auger electron microscopy was applied for a visualization of the
spatial distribution of specific elements (Ti, O) within the laser-modified areas. The chemical state
of the irradiated surface was revealed by complementary x-ray photoelectron spectroscopy. Both
methods were used for a depth-profiling chemical analysis (tracking the elements Ti, N, O, and C)
using an Ar-ion beam for surface sputtering. In a narrow laser fluence range slightly below the
ablation threshold of TiN significant superficial oxidation can be observed leading to the formation
of substoichiometric TiO2-x. At fluences above the ablation threshold, an increased titanium
concentration is observed within the entire ablation craters. Following upon sputter removal the
elemental distribution into the depth of the nonablated material, the results allow an estimation of
the heat-affected zone for femtosecond laser ablation in air environment. According to our analyses,
the HAZ extends up to a few hundreds of nanometers into the nonablated material.
Hochleistungsfasern am Limit - Laserinduzierte Zerstörung als Einsatzgrenze bei Lichtwellenleitern
(2012)
Aufgrund der steigenden Anforderungen an Fasern für Hochleistungsanwendungen hat die Business Unit Fiber Optics der Leoni Gruppe gemeinsam mit der BAM Bundesanstalt für Materialforschung und -prüfung die Herausforderung angenommen, erstmalig die Zerstörschwellen eigener large core Spezialfasern zu untersuchen.
The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of fused silica with multiple irradiation sequences consisting of laser pulse pairs (50 fs single-pulse duration) of two different wavelengths (400 and 800 nm) is studied experimentally. Parallel polarized double-pulse sequences with a variable delay Δt between -10 and +10 ps and between the individual fs-laser pulses were used to investigate the LIPSS periods versus Δt. These two-color experiments reveal the importance of the ultrafast energy deposition to the silica surface by the first laser pulse for LIPSS formation. The second laser pulse subsequently reinforces the previously seeded spatial LIPSS frequencies.
Ultrashort pulse laser materials processing can be accompanied by the production of X-rays. Small doses per laser pulse can accumulate to significant dose rates at high laser pulse repetition rates which may exceed the permitted X-ray limits for human exposure. Consequently, a proper radiation shielding must be considered in laser machining. A brief overview of the current state of the art in the field of undesired generation of X-ray radiation during ultrashort pulse laser material processing in air is presented.
Laser-induced periodic surface structures (LIPSS) are a universal phenomenon and can be generated on almost any material by irradiation with linearly polarized radiation. This chapter reviews the current state in the field of LIPSS, which are formed in a “self-ordered” way and are often accompanying materials processing applications. LIPSS can be produced in a single-stage process and enable surface nanostructuring and, in turn, adaption of optical, mechanical, and chemical surface properties. Typically, they feature a structural size ranging from several micrometers down to less than 100 nm and show a clear correlation with the polarization direction of the light used for their generation. Various types of LIPSS are classified, relevant control parameters are identified, and their material-specific formation mechanisms are analyzed for different types of inorganic solids, i.e., metals, semiconductors, and dielectrics. Finally, technological applications featuring surface functionalization in the fields of optics, fluidics, medicine, and tribology are discussed.
Ultrashort laser pulses have found their way into industrial material processing. They take advantage of the fact that moderate laser fluences can produce high quality material removal without significant thermal influence. The growing availability of powerful, highly repeating laser sources and an advanced laser beam control have favored these developments. However, the laser-matter interaction can be accompanied by the production of X-rays. Small doses per laser pulse can accumulate to significant dose rates at high laser pulse repetition rates which may exceed the permitted X-ray limits for human exposure. Consequently, a proper radiation shielding must be considered in laser material processing. The paper summarizes the current state of the art in the field of undesired generation of X-ray radiation during ultrashort pulse laser material processing in air.
Ultrakurzpuls-Lasertechnik ist heute der Schlüssel für eine Vielzahl von Anwendungen in der industriellen Lasermaterialbearbeitung. Beim Einsatz hoher Laser-Intensitäten kann allerdings unerwünschte Röntgenstrahlung erzeugt werden. Die Röntgendosis und Energieverteilung ist von Laser-, Material- und Prozessparametern abhängig. Es können Röntgendosen über den erlaubten Grenzwerten für die Exposition des Menschen erreicht werden.
For pulse laser materials processing often optical step index and gradient index multimode fibers with core diameters ranging from 100 to 600 μm are used. The design of a high power fiber transmission system must take into account limitations resulting from both surface and volume damage effects. Especially, breakdown at the fiber end faces and self-focusing in the fiber volume critically influence the fiber performance. At least operation charts are desirable to select the appropriate fiber type for given laser parameters.
In industry-relevant studies the influence of fiber core diameter and end face preparation on laser-induced (surface) damage thresholds (LIDT) was investigated for frequently used all-silica fiber types (manufacturer LEONI). Experiments on preform material (initial fiber material) and compact specimens (models of the cladding and coating material) accompanied the tests performed in accordance with the relevant LIDT standards ISO 21254-1 and ISO 21254 2 for 1-on-1 and S-on-1 irradiation conditions, respectively. The relation beam diameter vs. LIDT was investigated for fused silica fibers. Additionally, laser-induced (bulk) damage thresholds of fused silica preform material F300 (manufacturer Heraeus) in dependence on external mechanical stress simulating fiber bending were measured. All experiments were performed with 10-ns laser pulses at 1064 and 532 nm wavelength with a Gaussian beam profile.
The current state in the field of laser-induced periodic surface structures (LIPSS, ripples) is reviewed. Their formation mechanisms are analyzed in ultrafast experiments (time-resolved diffraction and polarization controlled double-pulse experiments) and technological applications are demonstrated.
For pulse laser materials processing often optical step index and gradient index multimode fibers with core diameters ranging from 100 to 600 μm are used. The design of a high power fiber transmission system must take into account limitations resulting from both surface and volume damage effects. Especially, breakdown at the fiber end faces and self-focusing in the fiber volume critically influence the fiber performance. At least operation charts are desirable to select the appropriate fiber type for given laser parameters.
In industry-relevant studies the influence of fiber core diameter and end face preparation on laser-induced (surface) damage thresholds (LIDT) was investigated for frequently used all-silica fiber types (manufacturer LEONI). Experiments on preform material (initial fiber material) and compact specimens (models of the cladding and coating material) accompanied the tests performed in accordance with the relevant LIDT standards ISO 11254-1 and ISO 11254-2 for 1-on-1 and S-on-1 illumination conditions, respectively. The relation beam diameter vs. LIDT was investigated for fused silica fibers and preforms. Additionally, the laser-induced (bulk) damage threshold of fused silica preform material F300 (manufacturer Heraeus) in dependence on external mechanical stress simulating fiber bending was measured. All experiments were performed with 10-ns laser pulses at 1064 and 532 nm wavelength with a Gaussian beam profile.
Die Verleihung des Deutschen Zukunftspreises an ein Team zum Thema "Ultrakurzpulslaser für die industrielle Massenfertigung - Produzieren mit Lichtblitzen" im Jahr 2013 belegt die industrielle Relevanz des Ultrakurzpulslasereinsatzes in der Materialbearbeitung. Zunehmend werden weitere Anwendungen z.B. in der Mess- und Medizintechnik erschlossen. Femtosekunden-Laserpulse weisen prinzipbedingt ein breites Spektrum auf und besitzen bereits bei moderaten Pulsenergien extreme Pulsspitzenleistungen, womit über „konventionelle“ Laserquellen hinausgehende Herausforderungen an den Laserschutz entstehen. Die in entsprechenden Laserschutzbrillen verwendeten Laserschutzfilter müssen eine große spektrale Absorptionsbandbreite aufweisen und dürfen nicht aufschalten, d.h. keine induzierte Transmission zeigen. Im Vortrag soll dargestellt werden, dass Laserschutzfilter auf Glasbasis, die über ein breites Absorptionsspektrum verfügen, einen ausreichenden Schutz gegenüber fs-Laserstrahlung bieten, wohingegen Laserschutzfilter auf Polymerbasis eine induzierte spektrale Transmission bei steigender Energiedichte aufweisen können, was zu einem drastischen Verlust der Schutzfunktion führt.
A new technique to fix bioceramic powder on a titanium alloy by using femtosecond laser pulses is presented. It is shown that gentle fixation of a bioactive dielectric material on a metallic model implant is successful. This is potentially a new tool for the improvement of bone prostheses. An advantage of the ultrashort pulses is the very low heat influx into the whole sample. There is only a very thin interaction zone during the fixing, which is the metal surface in contact with the ceramic layer. Neither the fixed ceramic particles nor the major part of the metal suffer any modification. The stability of the model implant (ceramic on metal) is investigated by rotating bending fatigue tests. No indication of a reduction of the mechanical stability compared to untreated metallic reference samples was found.
The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of silicon by multiple (N = 100) linearly polarized Ti:sapphire femtosecond laser pulses (duration τ = 30 fs, center wavelength λ0 ~ 790 nm) is studied experimentally in air and water environment. The LIPSS surface morphologies are characterized by scanning electron microscopy and their spatial periods are quantified by two-dimensional Fourier analyses. It is demonstrated that the irradiation environment significantly influences the periodicity of the LIPSS. In air, so-called low-spatial frequency LIPSS (LSFL) were found with periods somewhat smaller than the laser wavelength (ΛLSFL ~ 0.7 × λ0) and an orientation perpendicular to the laser polarization. In contrast, for laser processing in water a reduced ablation threshold and LIPSS with approximately five times smaller periods ΛLIPSS ~ 0.15 × λ0 were observed in the same direction as in air. The results are discussed within the frame of recent LIPSS theories and complemented by a thin film based surface plasmon polariton model, which successfully describes the tremendously reduced LIPSS periods in water.
The formation of laser-induced periodic surface structures (LIPSS, ripples) upon irradiation of silicon with multiple irradiation sequences consisting of femtosecond laser pulse pairs (pulse duration 150 fs, central wavelength 800 nm) is studied numerically using a rate equation system along with a two-temperature model accounting for one- and two-photon absorption and subsequent carrier diffusion and Auger recombination processes. The temporal delay between the individual equal-energy fs-laser pulses was varied between 0 and ~4 ps for quantification of the transient carrier densities in the conduction band of the laser-excited silicon. The results of the numerical analysis reveal the importance of carrier generation and relaxation processes in fs-LIPSS formation on silicon and quantitatively explain the two time constants of the delay-dependent decrease of the low spatial frequency LIPSS (LSFL) area observed experimentally. The role of carrier generation, diffusion and recombination is quantified individually.
For many applications, optical multimode fibers are used for the transmission of powerful laser radiation. High light throughput and damage resistance are desirable. Laser-induced breakdown at the end faces of fibers can limit their performance. Therefore, the determination of laser-induced damage thresholds (LIDT) at the surface of fibers is essential.
Nanosecond (1064 nm and 532 nm wavelength) single-shot LIDT were measured according to the relevant standard on SiO2 glass preforms (Suprasil F300) as basic materials of the corresponding fibers. For 10 kinds of fused silica fibers (FiberTech) with core diameters between 180 µm and 600 µm, an illumination approach utilizing a stepwise increase of the laser fluence on a single spot was used. For both wavelengths, the LIDT values (0% damage probability) obtained by means of the two methods were compared. The influence of surface preparation (polishing) on damage resistance was investigated. For equal surface finishing, a correlation between drawing speed of the fibers and their surface LIDT values was found. In addition to the surface measurements, bulk LIDT were determined for the preform material.
The influence of paper type and state of degradation on laser cleaning of artifically soiled paper
(2009)
This contribution compares traditional cleaning and laser methods. Partial laser cleaning with a nanosecond pulse laser (wavelength of 532 nm) has proved very promising for future application in paper conservation-restoration. Traditional cleaning methods are not always sufficient or successful in surface cleaning of objects of art. Comparative studies of traditional paper cleaning methods and laser cleaning were made on several historic picture postcards printed with the chromolithography technique.
Laser cleaning for works of art on paper might be a supplemental, noncontact method to overcome some of the limitations of traditional dry cleaning techniques. Three different types of paper (pure-cellulose filter paper, rag paper, and wood-pulp paper) were mechanically soiled with pulverized charcoal in a standardized procedure to make model samples. These samples were characterized microscopically and by means of lightness measurements using a multi-spectral imaging system. A prototype laser workstation with Laser Class I conditions for the operator was used for the cleaning experiments. For 10-ns laser pulses at a wavelength of 532 nm, a set of laser parameters was established for a successful cleaning of the samples avoiding damage to the paper substrate. Single- and multi-pulse illumination conditions were tested. An extensive microscopic analysis after laser treatment of the cleaned parts of the samples provided insight into the remaining soiling on the surface and in the bulk of the paper material.
Single and multi pulse laser-induced damage thresholds (LIDT) of core, cladding, and coating materials of high-power
optical multimode fibers were determined in accordance with ISO 11254 for 532 nm and 1064 nm wavelength in the 10-ns
pulse duration regime with spatial Gaussian beam shape. For all-silica fibers, LIDT increases with rising core diameter in a range between 100-600 µm for a constant cladding-core ratio of 1.2. The damage resistance of the low refracting cladding (0.3 % fluorine doped fused silica) is comparable to the undoped SiO2 core. Coating materials show significantly lower LIDT than light-guiding parts of the fibers.
Femtosecond lasers provide a novel method of attaching bioceramic material to a titanium alloy, thereby improving the quality of bone implants. The ultrashort 30 fs laser pulses (790 nm wavelength) penetrate a thin dip-coated layer of fine ceramic powder, while simultaneously melting a surface layer of the underlying metal. The specific adjustment of the laser parameters (pulse energy and number of pulses per spot) avoids unnecessary melting of the bioactive calcium phosphate, and permits a defined thin surface melting of the metal, which in turn is not heated throughout, and therefore maintains its mechanical stability. It is essential to choose laser energy densities that correspond to the interval between the ablation fluences of both materials involved: about 0.1-0.4 J cm-2. In this work, we present the first results of this unusual technique, including laser ablation studies, scanning electron microscopy and optical microscope images, combined with EDX data.
The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of silicon wafer surfaces by linearly polarized Ti:sapphire femtosecond laser pulses (pulse duration 130 fs, central wavelength 800 nm) is studied experimentally and theoretically. In the experiments, so-called low-spatial frequency LIPSS (LSFL) were found with periods smaller than the laser wavelength and an orientation perpendicular to the polarization. The experimental results are analyzed by means of a new theoretical approach, which combines the widely accepted LIPSS theory of Sipe et al. with a Drude model, in order to account for transient (intra-pulse) changes of the optical properties of the irradiated materials. It is found that the LSFL formation is caused by the excitation of surface plasmon polaritons, SPPs, once the initially semiconducting material turns to a metallic state upon formation of a dense free-electron-plasma in the material and the subsequent interference between its electrical field with that of the incident laser beam resulting in a spatially modulated energy deposition at the surface. Moreover, the influence of the laser-excited carrier density and the role of the feedback upon the multi-pulse irradiation and its relation to the excitation of SPP in a grating-like surface structure is discussed.
Bone implants made of metal, often titanium or the titanium alloy Ti6Al4V, need to be surface treated to become bioactive. This enables the formation of a firm and durable connection of the prosthesis with the living bone. We present a new method to uniformly cover Ti6Al4V with a thin layer of ceramics that imitates bone material. These calcium alkali phosphates, called GB14 and Ca10, are applied to the metal by dip coating of metal plates into an aqueous slurry containing the fine ceramic powder. The dried samples are illuminated with the 790 nm radiation of a pulsed femtosecond laser. If the laser fluence is set to a value just below the ablation threshold of the ceramic (ca. 0.4 J/cm²) the 30 fs laser pulses penetrate the partly transparent ceramic layer of 20–40 µm thickness. The remaining laser fluence at the ceramic–metal interface is still high enough to generate a thin metal melt layer leading to the ceramic fixation on the metal. The laser processing step is only possible because Ti6Al4V has a lower ablation threshold (between 0.1 and 0.15 J/cm²) than the ceramic material. After laser treatment in a fluence range between 0.1 and 0.4 J/cm², only the particles in contact with the metal withstand a post-laser treatment (ultrasonic cleaning). The non-irradiated rest of the layer is washed off. In this work, we present results of a successful ceramic fixation extending over larger areas. This is fundamental for future applications of arbitrarily shaped implants.
The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of semiconductors and dielectrics by linearly polarized high-intensity Ti:sapphire fs-laser pulses (τ ~100 fs, λ ~800 nm) is studied experimentally and theoretically. In the experiments, two different types of LIPSS exhibiting very different spatial periods are observed (socalled LSFL low spatial frequency LIPSS, and HSFL - high spatial frequency LIPSS), both having a different dependence on the incident laser fluence and pulse number per spot. The experimental results are analyzed by means of a new theoretical approach, which combines the generally accepted LIPSS theory of J. E. Sipe and co-workers [Phys. Rev. B 27, 1141-1154 (1983)] with a Drude model, in order to account for transient changes of the optical properties of the irradiated materials. The joint Sipe-Drude model is capable of explaining numerous aspects of fs-LIPSS formation, i.e., the orientation of the LIPSS, their fluence dependence as well as their spatial periods. The latter aspect is specifically demonstrated for silicon crystals, which show experimental LSFL periods Λ somewhat smaller than λ. This behaviour is caused by the excitation of surface plasmon polaritons, SPP, (once the initially semiconducting material turns to a metallic state upon formation of a dense free-electron-plasma in the material) and the subsequent interference between its electrical fields with that of the incident laser beam, resulting in a spatially modulated energy deposition at the surface.
Upon multi-pulse irradiation, a feedback mechanism, caused by the redshift of the resonance in a grating-assisted SPP excitation, is further reducing the LSFL spatial periods. The SPP-based mechanism of LSFL successfully explains the remarkably large range of LSFL periods between ~0.6 λ and λ.
The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of fused silica with multiple irradiation sequences consisting of five Ti:sapphire femtosecond (fs) laser pulse pairs (150 fs, 800 nm) is studied experimentally. A Michelson interferometer is used to generate near-equal-energy double-pulse sequences with a temporal pulse delay from -20 to +20 ps between the cross-polarized individual fs-laser pulses (~0.2 ps resolution). The results of multiple double-pulse irradiation sequences are characterized by means of Scanning Electron and Scanning Force Microscopy. Specifically in the sub-ps delay domain striking differences in the surface morphologies can be observed, indicating the importance of the laser-induced free-electron plasma in the conduction band of the solids for the formation of LIPSS.
The influence of paper type and state of degradation on laser cleaning of artificially soiled paper
(2011)
Lasers can be a supplemental tool for restorers to overcome some of the limitations of traditional dry cleaning techniques for works of art on paper. The laser working range has to be optimized allowing for safe removal of contamination and limitation of damage to the substrate. This paper addresses the influence of paper type and state of degradation on laser working range. Three types of new paper (pure cellulose, bleached pulp paper, rag paper) were degraded and characterized with respect their degree of polymerization. Laser-induced damage thresholds of new and degraded paper were determined using SEM and viscometry. Additionally, artificially soiled model samples were made using two kinds of soiling, namely pulverized charcoal and soot-blackened standard test dust. Cleaning thresholds of soiled paper samples were evaluated. A working range for all combinations of paper and soiling between 0.05 J/cm2 and 0.5 J/cm2 was found for the application of 8-ns laser pulses at 532 nm wavelength.
Diese Arbeit beschreibt die Laserreinigung von gebräuchlichen Papieren des 19. Jahrhunderts in Kombination mit zwei Schmutzvarianten. Aufgrund der weiten Verbreitung in Bibliotheken und Kunstsammlungen wurden Hadernpapier und holzhaltiges Papier ausgewählt. Als Referenz diente Whatman-Filterpapier. Zur künstlichen Verschmutzung wurden Zeichenkohle oder Standardschmutz (Standard Test Dust SAE J 726 fine, mit Flammruß im Verhältnis 20:1 gemischt) mit Hilfe von Pinsel und Staubsauger jeweils auf die drei Papierarten aufgetragen und in den Faserverbund eingesaugt. Die Nutzung von Modellproben erweist sich für grundlegende Untersuchungen als vorteilhaft, da das Vorhandensein hinreichend großer Flächen (gleichartiger Objekte) für einen Vergleich mit Ergebnissen klassischer Reinigungstechniken unabdingbar ist. Die preiswerten Proben erlauben darüber hinaus auch die Anwendung zerstörender Analysetechniken.
Für die Laserreinigungsexperimente wurde ein Nd:YAG-Laser mit Impulsdauern in der Größenordnung von 10 ns bei 532 nm Wellenlänge und einer Repetitionsrate von 500 Hz verwendet. Die Laser-Bearbeitungsstation erlaubt die flächige Bearbeitung von Objekten, wobei der Laserstrahl bewegt wird. Der Laser wird mittels Computer ferngesteuert und unter Klasse-1-Sicherheitsbedingungen betrieben. Bei der Untersuchung der Modellproben wurde jeweils die Anzahl der Impulse pro Probenstelle sowie die Laser-Energiedichte variiert. Ziel der Arbeiten war die Ermittlung eines Laser-Arbeitsbereiches für die Reinigung der verschiedenen Papierproben. Die Modellobjekte wurden vor und nach der Laserbehandlung mittels lichtmikroskopischer, rasterelektronenmikroskopischer und colorimetrischer Analysemethoden untersucht.
This contribution investigates laser-induced damage of thin film and bulk polymer
samples, with the focus on physical processes occurring close to the damage threshold. In-situ
real-time reflectivity (RTR) measurements with picosecond (ps) and nanosecond (ns) temporal
resolution were performed on thin polymer films on a timescale up to a few microseconds (µs).
A model for polymer thin film damage is presented, indicating that irreversible chemical
modification processes take place already below the fluence threshold for macroscopic damage.
On dye-doped bulk polymer filters (as used for laser goggles), transmission studies using fs-and
ps-laser pulses reveal the optical saturation behavior of the material and its relation to the
threshold of permanent damage. Implications of the sub-threshold processes for laser safety
applications will be discussed for thin film and bulk polymer damage.
Cleaning of artificially soiled paper using nanosecond, picosecond and femtosecond laser pulses
(2010)
Cleaning of cultural assets, especially fragile
organic materials like paper, is a part of the conservation
process. Laser radiation as a non-contact tool offers
prospects for that purpose. For the studies presented here,
paper model samples were prepared using three different paper
types (pure cellulose, rag paper, and wood-pulp paper).
Pure cellulose serves as reference material. Rag and woodpulp
paper represent essential characteristics of the basic
materials of real-world artworks. The papers were mechanically
soiled employing pulverized charcoal. Pure and artificially soiled paper samples were treated with laser pulses of
28 fs (800 nm wavelength) and 8–12 ns (532 nm) duration in
a multi pulse approach. Additionally, the cellulose reference
material was processed with 30 ps (532 nm) laser pulses.
Damage and cleaning thresholds of pure and soiled paper
were determined for the different laser regimes. Laser working
ranges allowing for removal of contamination and avoiding
permanent modification to the substrate were found.
The specimens prior and after laser illumination were characterized
by light-optical microscopy (OM) and scanning
electron microscopy (SEM) as well as multi spectral imaging
analysis. The work extends previous nanosecond laser
cleaning investigations on paper into the ultra-short pulse
duration domain.
Paper is one of the most important materials representing and witnessing human culture particularly as a carrier medium for text and image. As soiling hampers the reception of information, paper cleaning techniques are needed. Traditional mechanical and chemical cleaning methods are used by conservator-restorers. In some cases, a classical cleaning procedure of paper objects yields unsatisfactory results or a conventional treatment is even impossible. Especially, fragile paper objects cause problems due to mechanical instabilities. Laser cleaning as a non-contact method might be a way to overcome some of the limitations of classical cleaning techniques. Laser parameters have to be chosen to achieve removal of the soiling without influencing the artwork. Any immediate as well as long-term effects causing an irreversible change of the artwork have to be avoided. At present, most laser applications are found in stone and metal conservation, while laser treatment of complex organic materials like paper is still not fully developed for application in conservators' workshops. This contribution describes recent work of pulsed laser cleaning of soiled model samples. Pure cellulose, rag paper and wood-pulp paper were mechanically soiled with pulverized charcoal in a standardized procedure to make model samples representing essential characteristics of contaminated real-world artworks. Afterwards, model samples were cleaned using short and ultrashort laser pulses in the nanosecond and femtosecond time domain, respectively. An extensive analysis of the model samples after laser treatment using an optical microscope and a multi-spectral imaging system allows a comparison of the cleaning results obtained with both laser sources.
The possibility to excite surface plasmon polaritons (SPPs) at the interface between two media depends on the optical properties of both media and geometrical aspects. Specific conditions allowing the coupling of light with a plasmon-active interface must be satisfied. Plasmonic effects are well described in noble metals where the imaginary part of the dielectric permittivity is often neglected ('perfect medium approximation (PMA)'). However, some systems exist for which such approximation cannot be applied, hence requiring a refinement of the common SPP theory. In this context, several properties of SPPs such as excitation conditions, period of the electromagnetic field modulation and SPP lifetime then may strongly deviate from that of the PMA. In this paper, calculations taking into account the imaginary part of the dielectric permittivities are presented. The model identifies analytical terms which should not be neglected in the mathematical description of SPPs on lossy materials. These calculations are applied to numerous material combinations resulting in a prediction of the corresponding SPP features. A list of plasmon-active interfaces is provided along with a quantification of the above mentioned SPP properties in the regime where the PMA is not applicable.
The dynamics of the formation of laser-induced periodic surface structures (LIPSS) on fused silica upon irradiation with linearly polarized fs-laser pulses (50 fs pulse duration) is studied by cross-polarized two-color double-fs-pulse experiments. In order to analyze the relevance of temporally distributed energy deposition in the early stage of LIPSS formation, a Mach-Zehnder interferometer was used for generating multiple double-pulse sequences at two different wavelengths (400 and 800 nm). The inter-pulse delay between the individual cross-polarized pulses of each sequence was systematically varied in the sub-ps range and the resulting LIPSS morphologies were characterized by scanning electron microscopy. It is found that the polarization of the first laser pulse arriving to the surface determines the orientation and the periodicity of the LIPSS. These two-color experiments further confirm the importance of the ultrafast energy deposition to the silica surface for LIPSS formation, particularly by the first laser pulse of each sequence. The second laser pulse subsequently reinforces the previously seeded spatial LIPSS characteristics (period, orientation).
Wer mit Laserstrahlung zu tun hat, sollte vor allem auf den Schutz seiner Augen achten, denn die Netzhaut ist eines der empfindlichsten Gewebe des menschlichen Körpers. Auch die Hornhaut des Auges kann durch Laserstrahlung gefährdet sein. Sie hat eine Dicke von nur etwa 0,5 mm. Der Schutz der Augen vor Laserstrahlung ist durch verschiedene Maßnahmen möglich.
Single- and two-color double-fs-pulse experiments were performed on titanium to study the dynamics of the formation of laser-induced periodic surface structures (LIPSS). A Mach-Zehnder inter-ferometer generated polarization controlled (parallel or cross-polarized) double-pulse sequences in two configurations – either at 800 nm only, or at 400 and 800 nm wavelengths. The inter-pulse delays of the individual 50-fs pulses ranged up to some tens of picoseconds. Multiple of these single- or two-color double-fs-pulse sequences were collinearly focused by a spherical mirror to the sample surface. In both experimental configurations, the peak fluence of each individual pulse was kept below its respective ablation threshold and only the joint action of both pulses lead to the formation of LIPSS. Their resulting characteristics were analyzed by scanning electron microscopy and the periods were quantified by Fourier analyses. The LIPSS periods along with the orientation allow a clear identification of the pulse which dominates the energy coupling to the material. A plasmonic model successfully explains the delay-dependence of the LIPSS on titanium and confirms the importance of the ultrafast energy deposition stage for LIPSS formation.
The European laser safety standards EN 207, EN 208, and EN 12254 each contain an annex B, which serves as a guidance for the selection of products. These annexes are informative only and are therefore not binding. As there are a variety of hazard scenarios, it is not recommended to change these annexes to a normative status, through which they would become mandatory. Instead, it is recommended to allow users to apply their own skills and know-how in selecting appropriate products, justifying where and why they deviate from the guidance in the standards. This paper explains the background on which the guidance for selection in the annexes of the standards is based and shows physically meaningful leeway.
Two-color double-fs-pulse experiments were performed on silicon wafers to study the temporally distributed energy deposition in the formation of laser-induced periodic surface structures (LIPSS). A Mach-Zehnder interferometer generated parallel or cross-polarized double-pulse sequences at 400 and 800 nm wavelength, with inter-pulse delays up to a few picoseconds between the sub-ablation 50-fs-pulses. Multiple two-color double-pulse sequences were collinearly focused by a spherical mirror to the sample. The resulting LIPSS characteristics (periods, areas) were analyzed by scanning electron microscopy. A wavelength-dependent plasmonic mechanism is proposed to explain the delay-dependence of the LIPSS. These two-color experiments extend previous single-color studies and prove the importance of the ultrafast energy deposition for LIPSS formation.
The formation of laser-induced periodic surface structures (LIPSS) on different materials (silicon, fused silica, quartz) with linearly polarized fs-laser irradiation is studied experimentally. In dielectrics, the importance of transient excitation stages in the LIPSS formation is demonstrated by using (multiple) cross-polarized double-fs-laser-pulse irradiation sequences. A characteristic decrease of the spatial LIPSS periods is observed for double-pulse delays of less than 2 ps along with a characteristic 90°-rotation of the LIPSS orientation.
Laser-induced periodic surface structures (LIPSS, ripples) are a universal phenomenon and can be generated on almost any material upon irradiation with linearly polarized radiation. With the availability of ultrashort laser pulses, LIPSS have gained an increasing attraction during the past decade, since these structures can be generated in a simple single-step process, which allows a surface nanostructuring for tailoring optical, mechanical, and chemical surface properties. In this study, the current state in the field of LIPSS is reviewed. Their formation mechanisms are analyzed in ultrafast time-resolved scattering, diffraction, and polarization constrained double-pulse experiments. These experiments allow us to address the question whether the LIPSS are seeded via ultrafast energy deposition mechanisms acting during the absorption of optical radiation or via self-organization after the irradiation process. Relevant control parameters of LIPSS are identified, and technological applications featuring surface functionalization in the fields of optics, fluidics, medicine, and tribology are discussed.
Der Vortrag gibt einen Überblick zu aktuellen Projektaktivitäten im BAM-Fachbereich 6.4 Technologien mit Nanowerkstoffen. Der Einsatz von Femtosekunden-Laserpulsen zur Mikro- und Nanostrukturierung, für Prozessschritte in der Photovoltaik und zur Reinigung und Dekontamination von Oberflächen wird beschrieben. Darüber hinaus werden Laser-Zerstörschwellenmessungen erläutert.
In order to study the temporally distributed energy deposition in the formation of laser-induced periodic surface structures (LIPSS) on single-crystalline zinc oxide (ZnO), two-colour double-fs-pulse experiments were performed. Parallel or cross-polarised double-pulse sequences at 400 and 800 nm wavelength were generated by a Mach–Zehnder interferometer, exhibiting inter-pulse delays up to a few picoseconds between the sub-ablation 50-fs-pulses. Twenty two-colour double-pulse sequences were collinearly focused by a spherical mirror to the sample surface. The resulting LIPSS periods and areas were analysed by scanning electron microscopy. The delay-dependence of these LIPSS characteristics shows a dissimilar behaviour when compared to the semiconductor silicon, the dielectric fused silica, or the metal titanium. A wavelength-dependent plasmonic mechanism is proposed to explain the delay-dependence of the LIPSS on ZnO when considering multi-photon excitation processes. Our results support the involvement of nonlinear processes for temporally overlapping pulses. These experiments extend previous two-colour studies on the indirect semiconductor silicon towards the direct wide band-gap semiconductor ZnO and further manifest the relevance of the ultrafast energy deposition for LIPSS formation.
A laser-based bottom-up technique for the fabrication of Cu(In,Ga)Se2 (CIGSe) micro solar cells is presented.
We use femtosecond laser-induced forward transfer (LIFT) to transport a metallic precursor composed of copper, indium, and gallium onto a molybdenum back contact layer on a glass substrate. A CIGSe absorber forms by subsequent selenization. An array of micro absorbers with defined spacing is fabricated to solar cells and characterized under concentrated light illumination. The solar cell array exhibited a conversion efficiency of 1.4‰ at 1 sun as well as a significant efficiency enhancement of 68% rel. under 20-fold concentration. This work demonstrates the possibility of directly grown micrometer-sized solar cells based on chalcogenide absorber layers, enabling effective material usage.
Laser texturing is an emerging technology for generating surface functionalities on basis of optical, mechanical, or chemical properties. Taking benefit of laser sources with ultrashort (fs) pulse durations features outstanding precision of machining and negligible rims or burrs surrounding the laser-irradiation zone. Consequently, additional mechanical or chemical post-processing steps are usually not required for fs-laser surface texturing (fs-LST). This work aimed to provide a bridge between research in the field of tribology and laser materials processing. The paper reviews the current state-of-the-art in fs-LST, with a focus on the tribological performance (friction and wear) of specific self-organized surface structures (so-called ripples, grooves, and spikes) on steel and titanium alloys. On the titanium alloy, specific sickle-shaped hybrid micro-nanostructures were also observed and tribologically tested. Care is taken to identify accompanying effects affecting the materials hardness, superficial oxidation, nano- and microscale topographies, and the role of additives contained in lubricants, such as commercial engine oil.
Laser-induced periodic surface structures (LIPSS) are a universal phenomenon and can be generated on almost any material by irradiation with linearly polarized radiation. This chapter reviews the current state in the field of LIPSS, which are formed in a “self-ordered” way and are often accompanying materials processing applications. LIPSS can be produced in a single-stage process and enable surface nanostructuring and, in turn, adaption of optical, mechanical, and chemical surface properties. Typically, they feature a structural size ranging from several micrometers down to less than 100 nm and show a clear correlation with the polarization direction of the light used for their generation. Various types of LIPSS are classified, relevant control parameters are identified, and their material-specific formation mechanisms are analyzed for different types of inorganic solids, i.e., metals, semiconductors, and dielectrics. Finally, technological applications featuring surface functionalization in the fields of optics, fluidics, medicine, and tribology are discussed.
The exciting properties of micro- and nano-patterned surfaces found in natural species hide a virtually endless potential of technological ideas, opening new opportunities for innovation and exploitation in materials science and engineering. Due to the diversity of biomimetic surface functionalities, inspirations from natural surfaces are interesting for a broad range of applications in engineering, including phenomena of adhesion, friction, wear, lubrication, wetting phenomena, self-cleaning, antifouling, antibacterial phenomena, thermoregulation and optics. Lasers are increasingly proving to be promising tools for the precise and controlled structuring of materials at micro- and nano-scales. When ultrashort-pulsed lasers are used, the optimal interplay between laser and material parameters enables structuring down to the nanometer scale. Besides this, a unique aspect of laser processing technology is the possibility for material modifications at multiple (hierarchical) length scales, leading to the complex biomimetic micro- and nano-scale patterns, while adding a new dimension to structure optimization. This article reviews the current state of the art of laser processing methodologies, which are being used for the fabrication of bioinspired artificial surfaces to realize extraordinary wetting, optical, mechanical, and biological-active properties for numerous applications. The innovative aspect of laser functionalized biomimetic surfaces for a wide variety of current and future applications is particularly demonstrated and discussed. The article concludes with illustrating the wealth of arising possibilities and the number of new laser micro/nano fabrication approaches for obtaining complex high-resolution features, which prescribe a future where control of structures and subsequent functionalities are beyond our current imagination.
A great number of Central Asian wall paintings, archeological materials, architectural fragments, and textiles, as well as painting fragments on silk and paper, make up the so called Turfan Collection at the Asian Art Museum in Berlin. The largest part of the collection comes from the Kucha region, a very important cultural center in the third to ninth centuries. Between 1902 and 1914, four German expeditions traveled along the northern Silk Road. During these expeditions, wall paintings were detached from their original settings in Buddhist cave complexes. This paper reports a technical study of a wall painting, existing in eight fragments, from the Buddhist cave no. 40 (Ritterhöhle). Its original painted surface is soot blackened and largely illegible. Grünwedel, leader of the first and third expeditions, described the almost complete destruction of the rediscovered temple complex and evidence of fire damage. The aim of this case study is to identify the materials used for the wall paintings. Furthermore, soot deposits as well as materials from conservation interventions were of interest. Non-invasive analyses were preferred but a limited number of samples were taken to provide more precise information on the painting technique. By employing optical and scanning electron microscopy, energy dispersive X-ray spectroscopy, micro X-ray fluorescence spectroscopy, X-ray diffraction analysis, and Raman spectroscopy, a layer sequence of earthen render, a ground layer made of gypsum, and a paint layer containing a variety of inorganic pigments were identified.
A bottom-up approach is presented for the production of arrays of indium islands on a molybdenum layer on glass, which can serve as micro-sized precursors for indium compounds such as copper-indium-gallium-diselenide used in photovoltaics. Femtosecond laser ablation of glass and a subsequent deposition of a molybdenum film or direct laser processing of the molybdenum film both allow the preferential nucleation and growth of indium islands at the predefined locations in a following indium-based physical vapor deposition (PVD) process. A proper choice of laser and deposition parameters ensures the controlled growth of indium islands exclusively at the laser ablated spots. Based on a statistical analysis, these results are compared to the non-structured molybdenum surface, leading to randomly grown indium islands after PVD.
In order to address the dynamics and physical mechanisms of LIPSS formation for three different classes of materials (metals, semiconductors, and dielectrics), two-color double-fs-pulse experiments were performed on Titanium, Silicon and Fused Silica. For that purpose a Mach–Zehnder interferometer generated polarization controlled (parallel or cross-polarized) double-pulse sequences at 400 nm and 800 nm wavelength, with inter-pulse delays up to a few picoseconds. Multiple of these two-color double-pulse sequences were collinearly focused by a spherical mirror to the sample surfaces. The fluence of each individual pulse (400 nm and 800 nm) was always kept below its respective ablation threshold and only the joint action of both pulses lead to the formation of LIPSS. Their resulting characteristics (periods, areas) were analyzed by scanning electron microscopy. The periods along with the LIPSS orientation allow a clear identification of the pulse which dominates the energy coupling to the material. For strong absorbing materials (Silicon, Titanium), a wavelength-dependent plasmonic mechanism can explain the delay-dependence of the LIPSS. In contrast, for dielectrics (Fused Silica) the first pulse always dominates the energy deposition and LIPSS orientation, supporting a non-plasmonic formation scenario. For all materials, these two-color experiments confirm the importance of the ultrafast energy deposition stage for LIPSS formation.
In recent years, the improved understanding of the formation of laser-induced periodic surface structures (LIPSS) has led to an emerging variety of applications that modify the optical, mechanical and chemical properties of many materials. Such structures strongly depend on the laser beam polarization and are formed usually after irradiation with ultrashort linearly polarized laser pulses. The most accepted explanation for the origin of the structures is based on the interference of the incident laser radiation with electromagnetic surface waves that propagate or scatter at the surface of the irradiated materials. This leads to an intensity modulation that is finally responsible for the selective ablation in the form of parallel structures with periods ranging from hundreds of nanometers up to some micrometers. The versatility when forming such structures is based on the high reproducibility with different wavelength, pulse duration and repetition rate laser sources, customized micro- and nanometric spatial resolutions, and the compatibility with industrially relevant processing speeds when combined with fast scanning devices. In this contribution, we review the latest applications in the rapidly emerging field of surface functionalization through LIPSS, including biomimetic functionalities on fluid transport, control of the wetting properties, specific optical responses in technical materials, improvement of tribological performance on metallic surfaces and bacterial and cell growth for medical devices, among many others.
We investigate the periodic structure formation upon intense femtosecond pulsed irradiation of chrome steel (100Cr6) for linearly polarised laser beams. The underlying physical mechanism of the laser-induced periodic structures is explored, their spatial frequency is calculated and theoretical results are compared with experimental observations. The proposed theoretical model comprises estimations of electron excitation, heat transfer, relaxation processes, and hydrodynamics-related mass transport. Simulations describe the sequential formation of sub-wavelength ripples and supra-wavelength grooves. In addition, the influence of the laser wavelength on the periodicity of the structures is discussed. The proposed theoretical investigation offers a systematic methodology towards laser processing of steel surfaces with important applications.
Many wooden artworks are contaminated by DDT (dichlorodiphenyltrichloroethane) as a result of a surface treatment by means of the liquid preservative Hylotox-59©. It was used until the end of the 1980s. DDT crystal structures are formed on the wood surfaces by the "blooming" of chlorine compounds. In addition to an aesthetic disturbance, it is assumed that DDT represents a health risk. Even decades after applying, the toxins in the wood preservatives are still detectable because they are of low volatility in many wood samples. Contaminated waste wood with natural biocide ageing, gilded and wood carved elements of an old picture frame and wooden samples with paint layers were provided by the Schlossmuseum Sondershausen. Non-contact procedures using laser and plasma appear reasonable to remove the DDT crystals. During the experiments, health and safety issues for the operator have to be taken into account.
The removal of DDT was evaluated employing femtosecond and nanosecond laser radiation and cold atmospheric plasma techniques with different working gases (air, nitrogen, and argon). Before laser application, a chlorine measurement representing the DDT density on the wooden surface is done by X-ray fluorescence (XRF) analysis as reference. After laser processing, the XRF analysis is used again at the same surface position to determine the depletion rate. Additionally, a documentation and characterization of the sample surface is performed before and after laser and plasma treatment using optical microscopy (OM). For plasma processing with various systems a chlorine measurement is done by gas chromatographic-mass spectrometry (GCMS) analysis.
A bottom-up approach to produce arrays of indium islands on a molybdenum layer on glass using 30-fs laser pulses at 790 nm wavelength is presented. These islands can serve as micro-sized precursors for indium compounds such as copper-indium-gallium-diselenide (CIGSe) used in photovoltaics. Molybdenum is the standard back contact material of CIGSe solar cells. Femtosecond laser ablation of glass and a subsequent deposition of a molybdenum film or direct laser processing of the molybdenum film both allow the preferential nucleation and growth of indium islands at the predefined locations in a following indium-based physical vapor deposition (PVD) process. A proper choice of laser and deposition parameters ensures the controlled growth of indium islands exclusively at the laser ablated spots. Based on a statistical analysis, these results are compared to the non-structured molybdenum surface, leading to randomly grown indium islands after PVD.
A promising technology in photovoltaics is based on micro-concentrator solar cells, where the photovoltaic active area is realized as an array of sub-millimeter sized cells onto which the incident light is focused via microlenses. This approach allows to increase the cell efficiency and to realize much more compact modules compared to macroscopic concentrator devices. At the same time, expensive raw materials can be saved, which is of interest, for example, with respect to indium in the case of copper-indium-gallium-diselenide (CIGSe) thin film solar cells. Two methods to produce micro-sized precursors of CIGSe absorbers on molybdenum are presented using 30-fs laser pulses at 790 nm wavelength. On the one hand, a multi pulse surface structuring of the molybdenum film or the underlying glass substrate and a subsequent physical vapor deposition were used for a site-selective aggregation of indium droplets. On the other hand, a single pulse laser-induced forward transfer was utilized to selectively deposit combined copper-indium precursor pixels on the molybdenum back contact of the solar cell. Post-processing (selenization, isolation, contacting) of the laser-generated micro-sized precursors results in functional CIGSe solar cells.
A promising technology in photovoltaics is based on micro-concentrator solar cells, where the photovoltaic active area is realized as an array of sub-millimeter sized thin-film solar cells. For copper-indium-gallium-diselenide (CIGSe), the solar cells can be arranged in the foci of a regular arrangement of micro-lenses to enhance their efficiency by light concentration, to allow a better heat dissipation and to save expensive raw material (indium). Different approaches to produce micro-sized precursors of CIGSe absorbers on molybdenum are presented using 30-fs laser pulses at 790 nm wavelength. On the one hand, a multi pulse surface structuring of the molybdenum or the underlying glass substrate and a subsequent physical vapor deposition (PVD) process were used for a site-selective aggregation of indium droplets. On the other hand, a single pulse laser-induced forward transfer (LIFT) was utilized to selectively deposit combined copper/indium/gallium precursor pixels on the molybdenum back contact of the solar cell. It was demonstrated that a postprocessing (selenization, isolation, contacting) of the laser-generated micro-sized precursors results in an array of working CIGSe solar cells with an efficiency of 2.8% for 1 sun illumination.
Decontamination of biocidal loaded wooden artworks
using femtosecond and nanosecond laser processing
(2017)
Until the end of the 1980s many wooden artworks underwent surface treatment by liquid preservatives, e.g. Hylotox-59. As a result, DDT (dichlorodiphenyltrichloroethane) crystal structures are formed on the wood surfaces by the "blooming" of chlorine compounds. In addition to an aesthetic disturbance, it is assumed that DDT represents a health risk. Even decades after applying, the toxins in the wood preservatives are still detectable. Contaminated waste wood with natural biocide ageing, gilded and wood carved elements of an old picture frame and wooden samples with paint layers were provided by the Schlossmuseum Sondershausen, Germany.
Laser cleaning of areas of some square millimeters on the surfaces of the objects was done by means of femtosecond and nanosecond laser pulses. For 30-fs laser pulses at 800 nm wavelength a line-wise meandering movement of the object under the focused beam was performed. 10-ns laser pulses at 1064 nm and 7-ns laser pulses at 532 nm wavelength were applied to the sample surface using a scanner. Before laser application, a chlorine measurement was done by X-ray fluorescence analysis (XRF) as reference. After laser processing, the XRF analysis was used again at the same surface position to determine chlorine depletion rates of up to 75% (30 fs, 800 nm), 70% (10 ns, 1064 nm), and 22% (7 ns, 532 nm). For the application of 30-fs laser pulses on waste wood, no crystalline DDT residues remain on the sample surface observed utilizing optical microscopy.
Laser-induced periodic surface structures (LIPSS, ripples) are a universal phenomenon that can be observed on almost any material after the irradiation by linearly polarized laser beams, particularly when using ultrashort laser pulses with durations in the picosecond to femtosecond range. During the past few years significantly increasing research activities have been reported in the field of LIPSS, since their generation in a single-step process provides a simple way of nanostructuring and surface functionalization towards the control of optical, mechanical or chemical properties. In this contribution current applications of LIPSS are reviewed, including the colorization of technical surfaces, the control of surface wetting, the tailoring of surface colonization by bacterial biofilms, and the improvement of the tribological performance of nanostructured metal surfaces.
This Conference Proceedings volume contains a selection of the contributions presented in Symposium C “Laser-material interactions for tailoring future applications” organized during the annual Spring Meeting of the European Materials Research Society (E-MRS) held from May 2nd to 6th 2016 in the Lille Grand Palais, France.