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X‑ray emission during the ablative processing of biological materials by ultrashort laser pulses
(2023)
The ablative laser processing with ultrashort pulsed laser beams may cause secondary emission of hazardous X-rays. While the effect has recently been proven to be considered in working safety regulations when processing technical materials, such as metals, the X-ray emission rates during the ablative processing of biological tissue materials are widely unexplored yet.
Therefore, biological materials like water, isotonic saline solution, pig eyes, and human teeth were ablated with ultrashort laser pulses of 1030 nm wavelength, 600 fs pulse duration and 5 kHz pulse repetition rate, aiming to mimic typical surgery situations. Simultaneously, in-situ X-ray dose rate measurements were performed at a short distance from the plasma to display potential X-ray emission. For all four studied biological materials, our measurements prove the secondary emission of laser-induced X-rays.
Ultrashort pulse laser processing of materials allows for precise machining with high accuracy. By increasing the repetition rate to several 100 kHz, laser machining becomes quick and cost-effective. Ultrafast laser processing at high repetition rates and peak intensities above 10^13 W/cm^2 can cause a potential hazard by generation of unwanted x-ray radiation. Therefore, radiation protection must be considered. For 925 fs pulse duration at a center wavelength of 1030 nm, the x-ray emission in air at a repetition rate of 400 kHz was investigated up to a
peak intensity of 2.6 × 10^14 W/cm^2. Based on the presented measurements, the properties of potential shielding materials will be discussed. By extending our previous works, a scaling of the x-ray radiation emission to higher peak intensities up to 10^15 W/cm^2 is described, and emitted x-ray doses are predicted.
X-ray emission hazards from ultrashort pulsed laser material processing in an industrial setting
(2021)
Interactions between ultrashort laser pulses with intensities larger than 10^13 W/cm^2 and solids during material processing can lead to the emission of X-rays with photon energies above 5 keV, causing radiation hazards to operators. A framework for inspecting X-ray emission hazards during laser material processing has yet to be developed. One requirement for conducting radiation protection inspections is using a reference scenario, i.e., laser settings and process parameters that will lead to an almost constant and high level of X-ray emissions. To study the feasibility of setting up a reference scenario in practice, ambient dose rates and photon energies were measured using traceable measurement equipment in an industrial setting at SCHOTT AG. Ultrashort pulsed (USP) lasers with a maximum average power of 220 W provided the opportunity to measure X-ray emissions at laser peak intensities of up to 3.3 × 10^15 W/cm^2 at pulse durations of ~1 ps. The results indicate that increasing the laser peak intensity is insufficient to generate high dose rates. The investigations were affected by various constraints which prevented measuring high ambient dose rates. In this work, a list of issues which may be encountered when performing measurements at USP-laser machines in industrial settings is identified.
The industrial use of ultrashort laser pulses has made considerable progress in recent years. The reasons for this lie in the availability of high average powers at pulse repetition rates in the several 100 kHz range. The advantages of using ultrashort laser pulses in terms of processing precision can thus be fully exploited. However, high laser intensities on the workpiece can also lead to the generation of unwanted X-rays. Even if the emitted X-ray dose per pulse is low, the accumulated X-ray dose can become significant for high-repetition-rate laser systems so that X-ray exposure safety limits must be considered. The X-ray emission during ultrashort pulse laser processing was investigated for a pulse duration of 925 fs at 1030 nm wavelength and 400 kHz repetition rate. Industrially relevant materials such as steel,aluminum and glass were treated. Tungsten served as reference. X-ray spectra were recorded, and X-ray dose measurements were performed for laser treatment in air. For laser intensities > 2 × 10^13 W/cm2, X-ray doses exceeding the regulatory exposure limits for members of the public were found. Suitable X-ray protection strategies are proposed.
Ultrashort laser pulse micromachining features a high precision. By increasing the repetition rate of the applied laser to several 100 kHz, laser processing becomes quick and cost-effective and make
this method attractive for industrial applications. Upon exceeding a critical laser intensity, hard X-ray radiation is generated as a side effect. Even if the emitted X-ray dose per pulse is low, the accumulated X-ray dose becomes significant for high-repetition-rate laser systems so that radiation
safety must be considered.
Ultrashort laser pulse micromachining features a high precision. By increasing the repetition rate of the applied laser to several 100 kHz, laser processing becomes quick and cost-effective and make this method attractive for industrial applications. Upon exceeding a critical laser intensity, hard X-ray radiation is generated as a side effect. Even if the emitted X-ray dose per pulse is low, the accumulated X-ray dose becomes significant for high-repetition-rate laser systems so that radiation safety must be considered.
In laser machining with ultrashort laser pulses unwanted X-ray radiation in the keV range can be generated when a critical laser intensity is exceeded. Even if the emitted X-ray dose per pulse is low, high laser repetition rates can lead to an accumulation of X-ray doses beyond exposure safety limits. For 925 fs pulse duration at a center wavelength of 1030 nm, the X-ray emission was investigated up to an intensity of 2.6 × 10^14 W/cm2. The experiments were performed in air with a thin disk laser at a repetition rate of 400 kHz. X-ray spectra and doses were measured for various planar target materials covering a wide range of the periodic table from aluminum to tungsten. Without radiation shielding, the measured radiation doses at this high repetition rate clearly exceed the regulatory limits. Estimations for an adequate radiation shielding are provided.
Ultrashort pulse laser processing can result in the secondary generation of unwanted X-rays if a critical laser irradiance of about 10^13 W/cm^2 is exceeded. Spectral X-ray emissions were investigated during the processing of tungsten and steel using three complementary spectrometers (based on CdTe and silicon drift detectors) simultaneously for the identification of a worst-case spectral scenario. Therefore, maximum X-ray photon energies were determined, and corresponding dose equivalent rates were calculated. An ultrashort pulse laser workstation with a pulse duration of 274 fs, a center wavelength of 1030 nm, pulse repetition rates between 50 kHz and 200 kHz, and a Gaussian laser beam focused to a spot diameter of 33 µm was employed in a single pulse and burst laser operation mode. Different combinations of laser pulse energy and repetition rate were utilized, keeping the average laser power constant close to the maximum power of 20 W. Peak irradiances ranging from 7.3 × 10^13 W/cm^2 up to 3.0 × 10^14 W/cm^2 were used. The X-ray dose equivalent rate increases for lower repetition rates and higher pulse energy if a constant average power is used. Laser processing with burst mode significantly increases the dose rates and the X-ray photon energies. A maximum X-ray photon energy of about 40 keV was observed for burst mode processing of tungsten with a repetition rate of 50 kHz and a peak irradiance of 3 × 10^14 W/cm^2.
Unwanted X-ray emission in ultrashort pulse laser processing: From metallic to biological materials
(2023)
X-rays can be generated as an unwanted side effect during ultrashort pulse laser material processing of technical work pieces and even biological samples with laser intensities above 10^13 W/cm^2. First studies demonstrate the need to address this effect in industrial as well as in medical applications. This secondary hazard should be considered in work safety and risk assessment.
Ablation of indium phosphide wafers in air was performed with low repetition rate ultrashort laser pulses (130 fs, 10 Hz) of 800 nm wavelength. The relationships between the dimensions of the craters and the ablation parameters were analyzed. The ablation threshold fluence depends on the number of pulses applied to the same spot. The single-pulse ablation threshold value was estimated to be fth(1)=0.16 J/cm2. The dependence of the threshold fluence on the number of laser pulses indicates an incubation effect. Morphological and chemical changes of the ablated regions were characterized by means of scanning electron microscopy and Auger electron spectroscopy.
Ablation experiments with ultrashort laser pulses (pulse duration 150 fs, wavelength 800 nm) on polymers (PC, PMMA) relevant for biomedical technology have been performed in air. The lateral and vertical machining precision was evaluated by optical, atomic force and scanning electron microscopy. The ablation threshold reaches values in the range of 0.52.5 J/cm2 and depends significantly on the number of laser pulses applied to the same spot. The hole diameters are influenced by the laser fluence and the number of laser pulses. The relation between the ablation threshold and the number of laser pulses applied to the same spot is described in accordance with an incubation model.
Die Reduktion von Reibung und Verschleiß in technischen Systemen bietet ein großes Potenzial zur Reduktion von CO2-Emissionen. Dieser Beitrag diskutiert die Erzeugung und tribologische Charakterisierung von Ultrakurzpuls-generierten Nano- und Mikrostrukturen auf Metallen (Stahl, Titan). Besonderes Augenmerk wird dabei auf die Rolle der laserinduzierten Oxidschicht im Zusammenspiel mit verschleißreduzierenden Additiven in ölbasierten Schmiermitteln gerichtet.
Ultrafast laser processing can be used to realize various morphological surface transformations, ranging from direct contour shaping to large-area-surface functionalization via the generation of “self-ordered” micro- and nanostructures as well as their hierarchical hybrids. Irradiation with high-intensity laser pulses excites materials into extreme conditions, which then return to equilibrium through these unique surface transformations. In combination with suitable top-down or bottom-up manufacturing strategies, such laser-tailored surface morphologies open up new avenues toward the control of optical, chemical, and mechanical surface properties, featuring various technical applications especially in the fields of photovoltaics, tribology, and medicine. This article reviews recent efforts in the fundamental understanding of the formation of laser-induced surface micro- and nanostructures and discusses some of their emerging capabilities.
Long seen as “a solution seeking a problem,” laser pulses are nowadays – more than 60 years after their first practical demonstration – paramount in shaping and structuring matter. Harnessing their capabilities to direct intense beams of light, the number of scientific and technological developments and daily-life applications is continuously increasing. Today, the presence of lasers is ubiquitous in all sites of scientific and technological interest, from the most advanced research laboratories to industrial factories and medical hospitals.
The directionality of the laser beam determines equally a local character on lightmatter interaction and as such a local modification to a material target. Furthermore, the coherence of laser radiation enables near-field or far-field scattering and interference effects that widen significantly the capabilities of controlling and tracking laser-matter interactions in space and time. Already with the advent of lasers, powerful beams of light have been directed at solid materials for a variety of purposes, making this application as old as the laser itself. The roots of the major applications for laser structuring were developed already in the 1960s, setting the base of both theoretical and experimental studies on laser ablation, with the number of publications expanding explosively during the next decades.
Within this dynamic context, laser processing of materials experienced an impressive development over the years. Laser processing means specifically the capability to structure and tailor a material on its surface or even within its volume, rendering new functions and properties that are impacting the mechanical, electrical, or optical characteristics of the material. These properties are scaledependent, generating thus over the years an equally impressive quest for spatial or temporal resolution. Therefore, the developments in laser engineering with major breakthroughs, notably in pulse duration and power, always closely accompanied the efforts in material structuring with two milestones in sight: (1) yield and (2) resolution. Every step in shrinking the laser pulse duration led to a subsequent strong-impact development in process precision – particularly when the ultrashort pulse durations surpassed the fundamental electron-phonon relaxation times. Thus, minimizing heat diffusion, the advent of pulses with duration smaller than molecular or lattice vibration times has managed to confine the spatial resolution to the optical diffraction limit and sometimes even beyond. The nanoscale was already in sight at the turn of the millennium. An important question may be raised now; is there any fundamental limit in the processing resolution, a barrier defined by the intrinsic properties of light and matter? The answer has an inherently multidisciplinary nature, following the conversion of free-propagating electromagnetic radiation into
material-confined energy potentially usable to drive or transform matter, and will be the focus of the present book.
Relying on the experience and expertise of the leading researchers in the field, the present book intends to explore the current efforts in achieving laser processing resolution beyond the diffraction limit, laying down a perspective towards extreme laser nanostructuring. Following the most recent advances and developments, it puts forward a concept of extreme processing scales enabled by optical pulses that are able to bypass diffraction limits and achieve structuring characteristic scales beyond 100 nm. This objective can be achieved by a comprehensive understanding on how light can change matter and how, in turn, matter can change light, allowing jointly for actively controlling light and material processes. In order to give an extended perspective on the current state-of-the-art in the field of precision laser structuring, the book is divided into three main parts.
The first part of the book (Part I: Fundamental Processes) offers a perspective into the fundamentals of laser-matter interaction on extreme spatial scales, with a description of the most advanced modeling efforts in understanding energy deposition in matter, a plethora of material-relaxation pathways, as well as advanced concepts for probing and observing matter in motion. Roadmaps for energy localization will be developed, and the atomistic perspective of laser ablation visualized. Theoretical modelling enables in-depth insights on ultrafast quantum processes at the nanoscale. Laser-driven self-organization at surfaces will be dissected regarding the question of how light drives material periodic patterns down to the nanoscale, explored and transmitted to its ultimate limits of an atomic printer, and immediately complemented by the unprecedented capabilities of ultrafast in-situ observation approaches for tracking the laser-induced material response with extreme spatial and temporal resolution.
In the second part of the book (Part II: Concepts of Extreme Nanostructuring), distinct concepts will be developed and explored that allow confinement of light and harnessing of a material response restricted to nano- or mesoscopic scales at surfaces or in the volume of irradiated materials. A special focus will be on optical near-field related approaches for localizing light on scales even below the optical diffraction limit and plasmonic printing. Spatial and temporal beam-shaping and tailored interference techniques are discussed in the context of ultrashort laser pulses, and insights into some extreme states of matter realized by the tight confinement of laser energy are presented. The ultimate limits of writing waveguides in the bulk of dielectrics and for manifesting 3D-nanolithography are elucidated. Plasma-based surface treatments can significantly enhance the vertical precision of surface processing through etching processes.
Finally, the third part of the book (Part III: Applications) leads us to a number of resuming applications, unveiling the tremendous capabilities of surface functionalization through laser micro- and nanostructuring, assessing the 3D-writing of waveguides in the bulk of dielectrics or semiconductors for enabling new branches of integrated photonics, and summarizing related applications ranging from nanophotonics to nanofluidics and from optical sensing to biomedical applications, including the latest capabilities of refractive eye surgery. This part will analyze the applications’ compatibility in yield and reproducibility with current industrial requirements, costs, and intellectual property aspects. It expands the involved spatial scales by more than eight orders of magnitude, when extending extremely small structures featuring sizes of few tens of nanometers to larger dimensions in the meter range.
Thus, from surfaces to the bulk, from subtractive to additive manufacturing approaches, from advanced theoretical frames to practical technological processes – we invite the readers here to an exciting journey into the varicolored landscape of extreme laser nanostructuring.
The idea of this book project was seeded in early 2020. We were delighted about the numerous and extremely positive responses from the laser-processing community, quickly receiving commitments for more than 30 individual book chapters. About 2500 communications later, the book is published. We would like to thank all authors of this book project for their insightful and detailed chapters, reviewing and reporting on this fascinating topic of the pursuit of extreme scales in ultrafast laser nanostructuring. Moreover, we would like to acknowledge the professional help and guidance of the staff of Springer Nature. Finally, we hope you will enjoy reading this book as much as we have enjoyed putting it together.
Saint Etienne, France Razvan Stoian
Berlin, Germany Jörn Bonse
December 2022
Lasermaterialbearbeitung ist eine sich schnell entwickelnde Technologie, um eine Vielzahl von Oberflächenfunktionalitäten auf Basis optischer, mechanischer oder chemischer Eigenschaften zu realisieren. Die Verwendung ultrakurzer Laserimpulse mit Dauern im Femtosekundenbereich ermöglicht dabei neben einer herausragenden Bearbeitungs-präzision auch die selbstorganisierte Erzeugung verschiedener charakteristischer Ober-flächenstrukturen mit Größenskalen im Mikrometer- bis hinunter in den sub-100-nm-Bereich, z.B. sogenannte Ripples („Laser-Induced Periodic Surface Structures“, LIPSS), Grooves, oder Spikes.
In dem Vortrag wird ein Überblick über die in den vergangenen Jahren in Zusammenarbeit mit dem BAM Fachbereich 6.3 durchgeführten tribologischen Experimente gegeben. Besonderes Augenmerk liegt dabei auf den tribologischen Eigenschaften (Reibung und Verschleiß) der unterschiedlichen Femtosekunden-Laser-generierten Oberflächen-morphologien auf gängigen Metallen (z.B. Stahl, Titan). Einflüsse durch die Veränderungen der Härte des Werkstoffs infolge Laser-induzierter Oxidation, der Dicke und Struktur der Oxidschicht, und die Wirksamkeit unterschiedlicher Schmiermittel (z.B. additiviertes Motoröl) werden diskutiert.
Laser-induced periodic surface structures (LIPSS, ripples) were generated on steel and titanium surfaces upon irradiation with multiple linear polarized femtosecond laser pulses (pulse duration 30 fs, central wavelength 790 nm). The experimental conditions (laser fluence, spatial spot overlap) were optimized in a sample-scanning geometry for the processing of large surface areas covered homogeneously by the nanostructures. The irradiated surface regions were subjected to optical microscopy (OM), white light interference microscopy (WLIM) and scanning electron microscopy (SEM) revealing sub-wavelength spatial periods. The nanostructured surfaces were tribologically tested under reciprocal sliding conditions against a sphere of hardened 100Cr6 steel at 1 Hz using paraffin oil and engine oil as lubricants. After 1000 sliding cycles at a load of 1.0 N, the corresponding wear tracks were characterized by OM and SEM. For specific conditions the laser-generated nanostructures endured the tribological treatment. Simultaneously, a significant reduction of the friction coefficient was observed in the laser-irradiated (LIPSS-covered) areas when compared to the non-irradiated surface, indicating the potential benefit of laser surface structuring for tribological applications.
Commercial grade-1 titanium samples (Ti, 99.5% purity) were treated using three alternative methods, i.e., fs-laser processing in air, thermal heat treatment in an oven, or anodization in an electrochemical bath, all resulting in the formation of differently conditioned superficial oxide layers. The laser processing was carried out by a Ti:sapphire laser (pulse duration 30 fs, central wavelength 790 nm, pulse repetition rate 1 kHz) in a regime of generating laser-induced periodic surface structures (LIPSS). The experimental conditions (laser fluence, spatial spot overlap) were optimized in a sample-scanning geometry for the processing of several square-millimetres large surface areas covered homogeneously by these nanostructures. The thermal processing in the oven was done at two different temperatures, while the electrochemical anodization was performed at room temperature, aiming to generate different polymorphs of titanium oxide at similar oxide layer thickness. The irradiated surface regions were characterized by optical and scanning electron microscopy, and micro Raman spectroscopy. The tribological performance of the differently treated titanium surfaces was characterized in the regime of mixed friction by reciprocating sliding tests against a sphere of hardened steel in un-additivated paraffin oil and fully formulated engine oil as lubricants. The specific tribological performance of the differently treated surfaces is discussed on the basis of possible physical and chemical mechanisms.
Commercial grade-1 titanium samples (Ti, 99.6%) were treated using three alternative methods, (i) femtosecond laser processing, (ii) thermal heat treatment, and (iii) electrochemical anodization, respectively, resulting in the formation of differently conditioned superficial titanium oxide layers. The laser processing (i) was carried out by a Ti:sapphire laser (pulse duration 30 fs, central wavelength 790 nm, pulse repetition rate 1 kHz) in a regime of generating laser-induced periodic surface structures (LIPSS). The experimental conditions (laser fluence, spatial spot overlap) were optimized in a sample-scanning setup for the processing of several square-millimeters large surface areas covered homogeneously by these nanostructures. The differently oxidized titanium surfaces were characterized by optical microscopy, micro Raman spectroscopy, variable angle spectroscopic ellipsometry, and instrumented indentation testing. The tribological performance was characterized in the regime of mixed friction by reciprocating sliding tests against a sphere of hardened steel in fully formulated engine oil as lubricant. The specific tribological performance of the differently treated surfaces is discussed with respect to possible physical and chemical mechanisms.
Sub-100-nm laser-induced periodic surface structures (LIPSS) were processed on bulk titanium (Ti) surfaces by femtosecond laser pulse irradiation in air (30 fs pulse duration, 790 nm wavelength). The laser peak fluence, the spatial spot overlap, and the number of overscans were optimized in a sample-scanning geometry in order to obtain large surface areas (5 mm × 5 mm) covered homogeneously by the LIPSS. The laser-processed regions were characterized by optical microscopy (OM), white light interference microscopy (WLIM) and scanning electron microscopy (SEM). The friction coefficient of the nanostructured surfaces was tested during 1000 cycles under reciprocal sliding conditions (1 Hz, 1.0 N normal load) against a 10-mm diameter ball of hardened 100Cr6 steel, both in paraffin oil and in engine oil used as lubricants. Subsequently, the corresponding wear tracks were qualified by OM, SEM, and energy dispersive X-ray analyses (EDX). The results of the tribological tests are discussed and compared to that obtained for near wavelength-sized fs-LIPSS, processed under somewhat different irradiation conditions. Some constraints for a beneficial effect of LIPSS on the tribological performance are provided.
The impact of femtosecond (fs) laser-induced periodic surface structures (LIPSS) on tribological properties was investigated for metal-reinforced ceramic composites (Al2O3-ZrO2-Nb). For this purpose, the metallic niobium (Nb) phase was selectively structured with LIPSS in an air environment with different values of the fs-laser peak fluence by near-infrared fs-laser radiation (λ = 1025 nm, τ = 300 fs, frep = 1 kHz), taking advantage of the different light absorption behavior of ceramic and metal. The tribological performance was evaluated by reciprocating sliding tests in a ball-on-disc configuration using Ringer's solution as lubricant. The surfaces were characterized before and after laser irradiation by optical microscopy, scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy and by measuring the contact angle with Ringer's solution. The LIPSS formation resulted in an increased wetting of the surface with the lubricant. Moreover, the selectively structured composite surfaces revealed a coefficient of friction significantly reduced by a factor of ~3 when compared to the non-irradiated surface. Furthermore, the formation of a laser-induced oxidation layer was detected with NbO as the most prominent oxidation state. Selectively structured composites with outstanding mechanical properties and enhanced tribological performance are of particular interest for biomedical applications.
Laser-induced periodic surface structures (LIPSS, ripples) were processed on steel (X30CrMoN15-1) and titanium (Ti) surfaces by irradiation in air with linear polarized femtosecond laser pulses with a pulse duration of 30 fs at 790 nm wavelength. For the processing of large LIPSS covered surface areas (5 mm × 5 mm), the laser fluence and the spatial spot overlap were optimized in a sample-scanning geometry. The laser-processed surfaces were characterized by optical microscopy (OM), white light interference microscopy (WLIM) and scanning electron microscopy (SEM). Spatial LIPSS periods between 450 and 600 nm were determined. The nanostructured surface regions were tribologically tested under reciprocal sliding conditions against a 10-mm diameter ball of hardened 100Cr6 steel. Paraffin oil and engine oil were used as lubricants for 1000 sliding cycles at 1 Hz with a normal load of 1.0 N. The corresponding wear tracks were analyzed by OM and SEM. In particular cases, the laser-generated nanostructures endured the tribological treatment. Simultaneously, a significant reduction of the friction coefficient and the wear was observed in the laser-irradiated (LIPSS-covered) areas when compared to the non-irradiated surface. The experiments reveal the potential benefit of laser surface structuring for tribological applications.
Tribological performance of femtosecond laser-induced periodic surface structures on titanium
(2015)
Towards an industrial laser doping process for the selective emitter using phosphoric acid as dopant
(2011)
Different laser supported approaches have already been realized, proving the great potential of laserdoped selective emitters (LDSE). However, it is challenging to establish a low-cost process by using pulsed laser tools. So far a single-step process only leads to satisfying results utilizing cw-lasers. In this paper we have examined a two-step process to produce laser-doped selective emitters on multicrystalline textured standard silicon photovoltaic wafers (90-Ω/sq-Emitter, SiN-antireflection coating (ARC)). The precise ARC removal by near-infrared fs-laser pulses (30 fs, 800 nm), and the doping of uncoated silicon wafers by ns-laser pulses (8 ns, 532 nm) were systematically investigated. In the fs-experiment, optimum conditions for ARC removal were identified. In the nsexperiments under suitable conditions (melting regime), the phosphorous concentration underneath the wafer surface was significantly increased and the sheet resistance was reduced by nearly a factor of two. Moreover, electrical measurements on fired metallization fingers deposited on the laser processed wafers showed low contact resistances. Hence, wafer conditioning with combined fs-laser- and ns-laser-processes are expected to be a promising technology for producing selective emitters.
Time-resolved phase-contrast microscopy is employed to visualize spatio-temporal thermal transients induced by tight focusing of a single Ti:sapphire fs-laser pulse into a solid dielectric sample. This method relies on the coupling of the refractive index change and the sample temperature through the thermo-optic coefficient dn/dT. The thermal transients are studied on a timescale ranging from 10 ns up to 0.1 ms after laser excitation. Beyond providing direct insights into the laser–matter interaction, analyzing the results obtained also enables quantifying the local thermal diffusivity of the sample on a micrometer scale. Studies conducted in different solid dielectrics, namely amorphous fused silica (a-SiO2), a commercial borosilicate glass (BO33, Schott), and a custom alkaline earth silicate glass (NaSi66), illustrate the applicability of this approach to the investigation of various glassy materials.
We describe a method to visualize ultrafast laser-induced refractive index changes in transparent materials with a 310 fs impulse response and a submicrometer spatial resolution. The temporal profile of the laser excitation sequence can be arbitrarily set on the subpicosecond and picosecond time scales with a pulse shaping unit, allowing for complex laser excitation. Time-resolved phase contrast microscopy reveals the real part of the refractive index change and complementary time-resolved optical transmission microscopy measurements give access to the imaginary part of the refractive index in the irradiated region. A femtosecond laser source probes the complex refractive index changes from the excitation time up to 1 ns, and a frequency-doubled Nd:YAG laser emitting 1 ns duration pulses is employed for collecting data at longer time delays, when the evolution is slow. We demonstrate the performance of our setup by studying the energy relaxation in a fused silica sample after irradiation with a double pulse sequence. The excitation pulses are separated by 3 ps. Our results show two dimensional refractive index maps at different times from 200 fs to 100 µs after the laser excitation. On the subpicosecond time scale we have access to the spatial characteristics of the energy deposition into the sample. At longer times (800 ps), time-resolved phase contrast microscopy shows the appearance of a strong compression wave emitted from the excited region. On the microsecond time scale, we observe energy transfer outside the irradiated region.
Ultrashort pulses lasers are tools of choice for functionalizing the bulk of transparent materials. In particular, direct photoinscription of simple photonic functions have been demonstrated. Those elementary functions rely on the local refractive index change induced when focusing an ultrashort pulse in the volume of a transparent material. The range of possibilities offered by direct photoinscription is still under investigation. To help understanding, optimizing and assessing the full potential of this method, we developed a time-resolved phase contrast microscopy setup. The imaginary part (absorption) and the real part of the laser-induced complex refractive index can be visualized in the irradiated region. The setup is based on a commercially available phase contrast microscope extended into a pump-probe scheme. The originality of our approach is that the illumination is performed by using a pulsed laser source (i.e. a probe beam). Speckle-related issues are solved by employing adequate sets of diffusers. This laser-microscopy technique has a spatial resolution of 650 nm, and the impulse response is about 300 fs. The laser-induced refractive index changes can be tracked up to milliseconds after the energy deposition. The excitation beam (the pump) is focused with a microscope objective (numerical aperture of 0.45) into the bulk of an a-SiO2 sample. The pump beam can be temporally shaped with a SLM-based pulse shaping unit. This additional degree of flexibility allows for observing different interaction regimes. For instance, bulk material processing with femtosecond and picosecond duration pulses will be studied.
Single-pulse femtosecond (fs) (pulse duration ~200 fs, wavelength 398 nm) and nanosecond (ns) (pulse duration 4 ns, wavelength 355 nm) laser ablation have been applied in combination with time-of-flight mass spectrometer (TOFMS) to analyze the elemental composition of the plasma plume of single-crystalline telluria (c-TeO2, grown by the balance controlled Czochralski growth method). Due to the three-order difference of the peak intensities of the ns and fs-laser pulses, significant differences were observed regarding the laser-induced species in the plasma plume. Positive singly, doubly and triply charged Te ions (Te+, Te2+, Te3+) in the form of their isotopes were observed in case of both irradiations. In case of the ns-laser ablation the TeO+ formation was negligible compared to the fs case and there was no Te trimer (Te3+) formation observed. It was found that the amplitude of Te ion signals strongly depended on the applied laser pulse energy. Singly charged oxygen ions (O+) are always present as a byproduct in both kinds of laser ablation.
Femtosecond time-resolved microscopy has been used to analyze the structural transformation dynamics (melting, ablation, and solidification phenomena) induced by single intense 130 fs laser pulses in single-crystalline (100)-indium phosphide wafers in air on a time scale from ~100 fs up to 8 ns. In the ablative regime close to the ablation threshold, transient surface reflectivity patterns are observed by fs microscopy on a ps to ns time scale as a consequence of the complex spatial density structure of the ablating material (dynamic Newton fringes). At higher fluences, exceeding six times the ablation threshold, optical breakdown causes another, more violent ablation regime, which reduces the energy deposition depth along with the time of significant material removal. As a consequence, ablation lasts longer in a ring-shaped region around the region of optical breakdown. This leads to the formation of a crater profile with a central protrusion. In the melting regime below the ablation threshold, the melting dynamics of indium phosphide has been quantified and subsequent superficial amorphization has been observed upon solidification on the ns time scale leading to amorphous layer thicknesses of the order of a few tens of nanometers.
Laser-induced periodic surface structures (LIPSS) are often present when processing solid targets with linearly polarized ultrashort laser pulses. The different irradiation parameters to produce them on metals, semiconductors and dielectrics have been studied extensively, identifying suitable regimes to tailor its properties for applications in the fields of optics, medicine, fluidics and tribology, to name a few. One important parameter widely present when exposing the samples to the high intensities provided by these laser pulses in air environment, that generally is not considered, is the formation of a superficial laser-induced oxide layer. In this paper, we fabricate LIPSS on a layer of the oxidation prone hard-coating material chromium nitride in order to investigate the impact of the laser-induced oxide layer on its formation. A variety of complementary surface analytic techniques were employed, revealing morphological, chemical and structural characteristics of well-known high-spatial frequency LIPSS (HSFL) together with a new type of low-spatial frequency LIPSS (LSFL) with an anomalous orientation parallel to the laser polarization. Based on this input, we performed finite-difference time-domain calculations considering a layered system resembling the geometry of the HSFL along with the presence of a laser-induced oxide layer. The simulations support a scenario that the new type of LSFL is formed at the interface between the laser-induced oxide layer and the non-altered material underneath. These findings suggest that LSFL structures parallel to the polarization can be easily induced in materials that are prone to oxidation.
Laser-induced periodic surface structures (LIPSS) are a universal phenomenon that is accompanying laser materials processing. These surface nanostructures pave a simple way for surface functionalization with numerous applications in optics, fluidics, tribology, medicine, etc. This contribution reviews the current view on the role of electromagnetic scattering in the formation of LIPSS.
The role of electromagnetic scattering in the formation of laser-induced periodic surface structures
(2021)
Laser-induced periodic surface structures (LIPSS) are a universal phenomenon that is accompanying laser materials processing. These surface nanostructures pave a simple way for surface functionalization with numerous applications in optics, fluidics, tribology, medicine, etc. During the last decade remarkable experimental and theoretical improvements in understanding of their formation mechanisms were obtained - all pointing toward polarization-dependent energy deposition by absorption of optical radiation that is scattered at the surface roughness and interfering with the laser beam. This contribution reviews the current state-of-the-art on the role of electromagnetic scattering in the formation of LIPSS by ultrashort laser pulses. Special attention is drawn to recent finite-difference time-domain (FDTD) calculations that allow to visualize the radiation patterns formed in the vicinity of the sample surface and to the impact of a thin superficial laser-induced oxidation layer.
Ti:sapphire laser pulses of 130 fs and 800 nm were focused on 3.2-7m-thick TiN films by a 60-mm focal length lens in air. The morphology of the ablated areas generated by laser pulses at a fluence slightly above the ablation threshold was characterized in dependence on the pulse number by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The cavity profiles, depths, diameters, and volumes were quantitatively evaluated by AFM. The polarization state of the laser light is discussed as a further parameter, in addition to fluence and pulse number, that influences and controls the ablation precision of these materials. It was observed that circularly polarized radiation enhances the average ablation rates and reduces the roughness in the cavities by a factor of 2-3 as compared to linearly polarized radiation of the same incident laser fluence. Special attention was paid to the interfacial region between the coating and substrate. Ultrashort-pulse laser drilling into the Si substrate revealed the generation of columnar features which even may surmount the original coating under laser conditions.
The influence of processing parameters on X‑ray emission during ultra‑short pulse laser machining
(2019)
During ultra-short laser material processing at high laser pulse repetition rates unwanted X-ray radiation can be generated in a quantity that may constitute a potential risk for health. An adequate X-ray radiation protection requires a thoroughly understanding of the influence of the laser processing parameters on the generation of X-ray radiation. In the present work, the generated X-ray dose during laser machining was investigated in air for varying beam scanning conditions at a pulse duration of 925 fs, a center wavelength of 1030 nm and a laser peak intensity of 2.6 × 10^14 W/cm^2. The X-ray radiation dose and the corresponding spectral X-ray emission were investigated in dependence on the laser’s pulse repetition rate and on the beam scanning speed. The results show a strong dependence of the X-ray emission on these laser processing parameters.
Laser-induced periodic surface structures (LIPSS) are a simple and robust route for the nanostructuring of solids that can create various surface functionalities featuring applications in optics, medicine, tribology, energy technologies, etc. While the current laser technologies already allow surface processing rates at the level of m2/min, industrial applications of LIPSS are sometimes hampered by the complex interplay between the nanoscale surface topography and the specific surface chemistry, as well as by limitations in controlling the processing of LIPSS and in the long-term stability of the created surface functions. This Perspective article aims to identify some open questions about LIPSS, discusses the pending technological limitations, and sketches the current state of theoretical modelling. Hereby, we intend to stimulate further research and developments in the field of LIPSS for overcoming these limitations and for supporting the transfer of the LIPSS technology into industry.
Irradiation with ultra-short (femtosecond) laser beams enables the generation of sub-wavelength laser-induced periodic surface structures (LIPSS) over large areas with controlled spatial periodicity, orientation, and depths affecting only a material layer on the sub-micrometer scale. This study reports on how fs-laser irradiation of commercially available Nb foil samples affects their superconducting behavior. DC magnetization and AC susceptibility measurements at cryogenic temperatures and with magnetic fields of different amplitude and orientation are thus analyzed and reported. This study pays special attention to the surface superconducting layer that persists above the upper critical magnetic field strength Hc2, and disappears at a higher nucleation field strength Hc3. Characteristic changes were distinguished between the surface properties of the laser-irradiated samples, as compared to the corresponding reference samples (non-irradiated). Clear correlations have been observed between the surface nanostructures and the nucleation field Hc3, which depends on the relative orientation of the magnetic field and the surface patterns developed by the laser irradiation.
Different types of laser-generated surface structures, i.e., Laser-induced Periodic Surface Structures (LIPSS, ripples), Grooves, and Spikes are generated on titanium and Ti6Al4V surfaces by means of femtosecond (fs) laser scan processing (790 nm, 30 fs, 1 kHz) in ambient air. Morphological, chemical and structural properties of the different surface structures are characterized by various surface analytical techniques, including scanning electron microscopy (SEM), X-ray diffraction (XRD), Glow discharge optical emission spectroscopy (GD-OES), and depth-profiling Auger electron spectroscopy (AES). It is revealed that the formation of near-wavelength sized LIPSS is accompanied by the formation of a graded oxide extending several tens to a few hundreds of nanometers into depth. GD-OES performed on other superficial fs-laser generated structures produced at higher fluences and effective number of pulses per spot area such as periodic Grooves and irregular Spikes indicate even thicker graded oxide layers. These graded layers may be suitable for applications in prosthetics or tribology.
The presentation reviews the BAM activities in the field of surface processing by ultrashort laser pulses. A focus is laid on the generation of laser-induced periodic surface structures (LIPSS) which allow various surface functionalizations for applications in optics, tribology, liquid management, and medicine.
In this contribution the mechanisms of formation and current applications of LIPSS are reviewed, including the colorization of technical surfaces, the control of surface wetting properties, the mimicry of the natural texture of animal integuments, the tailoring of surface colonization by bacterial biofilms, and the improvement of the tribological performance of nanostructured metal surfaces.
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 femtosecond to picosecond 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, biological, or chemical surface properties. In this contribution the mechanisms of formation and current applications of LIPSS are reviewed, including the colorization of technical surfaces, the control of surface wetting properties, the mimicry of the natural texture of animal integuments, the tailoring of surface colonization by bacterial biofilms, the advancement of medical pacemakers, and the improvement of the tribological performance of nanostructured metal surfaces.
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 wavelengths, pulse durations and repetition rate laser sources, customized micro- and nanometric spatial resolutions, and 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.
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.
The formation of laser-induced periodic surface structures (LIPSS) on titanium upon irradiation with linearly polarized femtosecond (fs) laser pulses (τ = 30 fs, λ = 790 nm) in an air environment is studied experimentally and theoretically. In the experiments, the dependence on the laser fluence and the number of laser pulses per irradiation spot has been analyzed. For a moderate number of laser pulses (N < 1000) and at fluences between ~0.09 and ~0.35 J/cm², predominantly low-spatial-frequency-LIPSS with periods between 400 nm and 800 nm are observed perpendicular to the polarization. In a narrow fluence range between 0.05 and 0.09 J/cm², high-spatial-frequency-LIPSS with sub-100-nm spatial periods (~λ/10) can be generated with an orientation parallel to the polarization (N = 50). These experimental results are complemented by calculations based on a theoretical LIPSS model and compared to the present literature.
Modern life and global communication would not be possible without technologically tailored thin films; they are omnipresent in daily life applications. In most cases, the films are deposited entirely at the carrying substrates in a specific processing step of the device or sample. In some cases, however, removal or modification must be performed locally, i.e., site-controlled and material selective through an additional laser processing step. For that ultrashort laser pulses with durations in the femtosecond and picosecond range can provide unique advantages and capabilities in industrially scalable schemes. This article reviews the current state of the research and corresponding industrial transfer related to the structuring of thin films by ultrashort pulsed lasers. It focuses on the pertinent historic developments, reveals the relevant physical and chemical effects, explores the ultimate limits, and discusses selected industrial and scientific applications.
Structure formation on the surface of indium phosphide irradiated by femtosecond laser pulses
(2005)
Laser-induced periodic surface structures (LIPSS; ripples) with different spatial characteristics have been observed after irradiation of single-crystalline indium phosphide (c-InP) with multiple linearly polarized femtosecond pulses (130 fs, 800 nm) in air. With an increasing number of pulses per spot, N, up to 100, a characteristic evolution of two different types of ripples has been observed, i.e., (i) the growth of a grating perpendicular to the polarization vector consisting of nearly wavelength-sized periodic lines and (ii), in a specific pulse number regime (N = 5-30), the additional formation of equally oriented ripples with a spatial period close to half of the laser wavelength. For pulse numbers higher than 50, the formation of micrometer-spaced grooves has been found, which are oriented perpendicular to the ripples. These topographical surface alterations are discussed in the frame of existing LIPSS theories.