6.2 Material- und Oberflächentechnologien
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Laser-induced periodic surface structures (LIPSS) have gained remarkable attention as they represent a universal phenomenon that is often accompanying laser-processing. Such LIPSS enable a large variety of different surface functionalizations for applications in the fields of optics, fluidics, tribology, or medicine. Moreover, so-called “high spatial frequency LIPSS” (HSFL) provide an appealing and straightforward way for surface nanostructuring featuring spatial periods even below 100 nm – far beyond the optical diffraction limit. However, the imposed surface functionalities are usually caused by both, topographic and chemical surface alterations. For exploring these effects in detail, multi-method characterizations were performed here for HSFL on processed Ti- 6Al- 4V alloy upon irradiation with near-infrared ps-laser pulses (1030 nm wavelength, ~1 ps pulse duration, 1 – 400 kHz pulse repetition rate) under different laser and scan processing conditions. The sample characterization involved morphological and topographical investigations by scanning electron microscopy (SEM), atomic force microscopy (AFM), stylus profilometry (SP), and white light interference microscopy (WLIM), as well as near-surface chemical analyses by X-ray photoelectron spectroscopy (XPS), hard X-ray photoelectron spectroscopy (HAXPES) and depth-profiling time-of-flight secondary ion mass spectrometry (TOF-SIMS). The results allow to qualify the laser ablation depth, the geometrical HSFL characteristics and provide detailed insights into the depth extent and the nature of the ps-laser-induced near-surface oxidation. Significance for medical applications will be outlined.
Irradiation of solid surfaces with intense ultrashort laser pulses represents a unique way of depositing energy into materials. It allows to realize states of extreme electronic excitation and/or very high temperature and pressure and to drive materials close to and beyond fundamental stability limits. As a consequence, structural changes and phase transitions often occur along unusual pathways and under strongly nonequilibrium conditions. Due to the inherent multiscale nature — both temporally and spatially—of these irreversible processes, their direct experimental observation requires techniques that combine high temporal resolution with the appropriate spatial resolution and the capability to obtain good quality data on a single pulse/event basis. In this respect, fourth-generation light sources, namely, short wavelength and short pulse free electron lasers (FELs), are offering new and fascinating possibilities. As an example, this talk will discuss the results of scattering experiments carried out at the FLASH free electron laser at DESY (Hamburg, Germany), which allowed us to resolve laser-induced structure formation at surfaces on the nanometer to submicron length scale and in temporal regimes ranging from picoseconds to several nanoseconds with sub-picosecond resolution. The current status and future perspectives in this field via exploiting the unique possibilities of these 4th-generation light sources will be discussed.
Surfaces of metallic copper and copper alloys effectively inactivate microorganisms and viruses. However, the exact inactivation mode is still under debate. Main factors are assumed to include direct contact with the metallic surface, influx of Cu(I)/Cu(II) ions and the generation of reactive oxygen species (ROS). Laser-induced periodic surface structures (LIPSS) are frequently reported to act antibacterial, mainly by prevention of bacterial adhesion due to a limited number of possible adhesion points or by increasing the overall surface of intrinsically antibacterial materials. In time-kill experiments with E. coli and S. aureus we analyzed the impact of LIPSS on the toxicity of metallic copper and brass. We also conducted ROS accumulation assays and conclude that the application of LIPSS is not generally straight forward to obtain or improve antibacterial surfaces. Thus, the antibacterial effects of LIPPS.
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.
Bulk metallic glasses (BMG) are amorphous metal alloys known for their unique physical and mechanical properties. In the present study, the formation of femtosecond (fs) laser-induced periodic surface structures (LIPSS) on the Zr-based BMGs Zr46Cu46Al8, Zr61Cu25Al12Ti2, Zr52.5Cu17.9Al10Ni14.6Ti5 (Vit105) and Zr57Cu15.4Al10Ni12.6Nb5 (Vit106) was investigated as a function of their different chemical composition. For this purpose, LIPSS were generated on the sample surfaces in an air environment by fs-laser irradiation (λ = 1025 nm, τ = 300 fs, frep = 100 kHz). The surface topography was characterized by scanning electron microscopy and atomic force microscopy. Moreover, the impact of LIPSS formation on the structure and chemical surface composition was analyzed before and after fs-laser irradiation by X-ray diffraction and X-ray photoelectron spectroscopy as well as by transmission electron microscopy in combination with energy dispersive X-ray spectroscopy. Despite the different chemical composition of the investigated BMGs, the fs-laser irradiation resulted in almost similar properties of the generated LIPSS patterns. In the case of Zr61Cu25Al12Ti2, Vit105 and Vit106, the surface analysis revealed the preservation of the amorphous state of the materials during fs-laser irradiation. The study demonstrated the presence of a native oxide layer on all pristine BMGs. In addition, fs-laser irradiation results in the formation of laser-induced oxide layers of larger thickness consisting of an amorphous ZrAlCu-oxide. The precise laser-structuring of BMG surfaces on the nanoscale provides a versatile alternative to thermoplastic forming of BMG surfaces and is of particular interest for the engineering of functional material surfaces.
Laser-induced periodic surface structures (LIPSS) have gained remarkable attention as they represent a universal phenomenon that is often accompanying laser-processing. Such LIPSS enable a plethora of different surface functionalizations for applications in the fields of optics, fluidics, tribology, or medicine. Due to the inherent multiscale nature of processes involved in LIPSS formation, their in-situ observation requires experimental techniques that combine high temporal resolution with the appropriate spatial resolution. In this respect fourth generation light sources, namely short wavelength, short pulse free electron lasers (FELs) are offering new and fascinating possibilities. This work contribution will discuss the results of scattering experiments carried at the FLASH free electron laser at DESY (Hamburg, Germany), which allowed us to resolve laser-induced structure formation at surfaces on the nm to sub-µm length scale and in temporal regimes ranging from ps to several ns with sub-ps resolution. A ps-optical pump / fs-XUV scattering probe scheme was employed to 100 nm thick laser-excited silicon films, while recording snapshots of the transmitted XUV scattering patterns at various delay times after the laser pulse impact. On timescales ranging from hundred ps until several ns almost quantitative agreement was observed between certain features of the recorded scattering patterns and predictions of the first-principles theory of J.E. Sipe and coworkers. Other scattering features appearing with a delay of ~100 ps and lasting for ~1 ns are attributed to capillary surface waves being excited at the laser-melted film material while ablation proceeds. Our superior spatio-temporal resolution experiments allow to uniquely reveal and distinguish early signatures of coherent/plasmonic electromagnetic scattering effects, separately followed by hydrodynamic matter reorganization.
Irradiation of solid surfaces with intense ultrashort laser pulses represents a unique way of depositing energy into materials. It allows to realize states of extreme electronic excitation and/or very high temperature and pressure and to drive materials close to and beyond fundamental stability limits. As a consequence, structural changes and phase transitions often occur along unusual pathways and under strongly nonequilibrium conditions. Due to the inherent multiscale nature—both temporally and spatially—of these irreversible processes, their direct experimental observation requires techniques that combine high temporal resolution with the appropriate spatial resolution and the capability to obtain good quality data on a single pulse/event basis. In this respect, fourth-generation light sources, namely, short wavelength and short pulse free electron lasers (FELs), are offering new and fascinating possibilities. As an example, this chapter will discuss the results of scattering experiments carried out at the FLASH free electron laser at DESY (Hamburg, Germany), which allowed us to resolve laser-induced structure formation at surfaces on the nanometer to submicron length scale and in temporal regimes ranging from picoseconds to several nanoseconds with sub-picosecond resolution.
Surface nanostructures provide the possibility to create and tailor surface functionalities mainly via controlling their topography along with other chemical and physical material properties. One of the most appealing technologies for surface functionalization via micro- and nanostructuring is based on laser processing. This can be done either via direct contour-shaping of the irradiated material using a tightly focused laser beam or in a self-ordered way that allows employing larger laser beam diameters along with areal scanning to create a variety of laser-induced periodic surface structures (LIPSS). For the latter approach, particularly ultrashort pulsed lasers have recently pushed the borders across long-lasting limitations regarding the minimum achievable feature sizes and additionally boosted up the production times. This chapter reviews the plethora of recently investigated applications of LIPSS—for example, via imposing diffractive or plasmonic structural colors, the management of liquids and surface wetting properties, biomedical and bioinspired functionalities, beneficial effects in tribology for reducing friction and wear, the manipulation of optical scattering and absorption in photovoltaics, or the modification of magnetic or superconducting surface properties in other energy applications. The footprint of the LIPSS-based technology is explored in detail regarding the current state of industrialization, including an analysis of the market and associated LIPSS production costs.
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