Maxwell meets marangoni -: a review of theories on laser‐induced periodic surface structures

Surface nanostructuring enables the manipulation of many essential surface properties. With the recent rapid advancements in laser technology, a contactless large‐area processing at rates of up to m 2 s −1 becomes feasible that allows new industrial applications in medicine, optics, tribology, biology, etc. On the other hand, the last two decades enable extremely successful and intense research in the field of so‐called laser‐induced periodic surface structures (LIPSS, ripples). Different types of these structures featuring periods of hundreds of nanometers only—far beyond the optical diffraction limit—up to several micrometers are easily manufactured in a single‐step process and can be widely controlled by a proper choice of the laser processing conditions. From a theoretical point of view, however, a vivid and very controversial debate emerges, whether LIPSS originate from electromagnetic effects or are caused by matter reorganization. This article aims to close a gap in the available literature on LIPSS by reviewing the currently existent theories of LIPSS along with their numerical implementations and by providing a comparison and critical assessment of these approaches

Untersuchungen zum Laserstrahlschweißen mit Dynamischer Polarisation

In der vorliegenden Arbeit wird der Einfluss einer schnellen, räumlich und zeitlich definierten Änderung (bis in den kHz-Bereich) des Polarisationszustandes der CO2-Laserstrahlung („Dynamische Polarisation“ DP) auf die dynamischen Prozesse im Wechselwirkungsvolumen beim CO2-Laserstrahlschweißen untersucht. Die Umsetzung dieser Methode basiert auf einer speziellen Anordnung aus einem Interferenz-Laserstrahlungs-Modulator (ILM-H) und einer λ 2 -Einheit, die die Erzeugung eines Strahls mit konstanter Gesamtleistung aber zeitlich variablem Polarisationszustand ermöglicht. Am Beispiel exemplarischer Schweißversuche an den Materialien Ck45, St37 und 22MnB5 (USIBOR) sowie an St37 unter Anwendung von Kontrastwerkstoffen mit kontinuierlichen Leistungen bis etwa 3 kW wurden die zu erwartenden DP-Effekte experimentell untersucht und mittels modellhafter Annahmen und Abschätzungen qualitativ erläutert. Die dabei verwendete Anordnung des ILM-H in Doppeltransmission ist die Grundvoraussetzung für die DP, die mittels anderer Modulatortypen (z.B. elektro- und akustooptische Modulatoren) nicht realisierbar ist. Der ILM-H basiert auf bisherigen ILM-Modellen und wurde im Rahmen dieser Arbeit für Leistungen im Multi-kWBereich weiterentwickelt. Die Charakterisierung des experimentellen Verhaltens des ILM-H zeigt, dass die Strahlteilungsfunktion für kontinuierliche (cw) Leistungen bis 2 kWnahezu der Theorie des idealen Fabry-Pérot-Interferometers folgt. Die thermische Drift des ILM-H beträgt im untersuchten Leistungsbereich zwischen 600W und 1800W mit 0.8 μm lediglich einen kleinen Bruchteil der Laserwellenlänge von 10.6 μm. Aus diesen experimentellen Aussagen, insbesondere zur thermischen Stabilität, lässt sich ableiten, dass der ILM-H bis zu cw-Leistungen von etwa 4 kW problemlos eingesetzt werden kann

Dual Laser Beam Processing of Semiconducting Thin Films by Excited State Absorption

We present a unique dual laser beam processing approach based on excited state absorption by structuring 200 nm thin zinc oxide films sputtered on fused silica substrates. The combination of two pulsed nanosecond-laser beams with different photon energies—one below and one above the zinc oxide band gap energy—allows for a precise, efficient, and homogeneous ablation of the films without substrate damage. Based on structuring experiments in dependence on laser wavelength, pulse fluence, and pulse delay of both laser beams, a detailed concept of energy transfer and excitation processes during irradiation was developed. It provides a comprehensive understanding of the thermal and electronic processes during ablation. To quantify the efficiency improvements of the dual-beam process compared to single-beam ablation, a simple efficiency model was developed

Ten Open Questions about Laser-Induced Periodic Surface Structures

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

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Formation and Properties of Laser-Induced Periodic Surface Structures on Different Glasses

The formation and properties of laser-induced periodic surface structures (LIPSS) was investigated on different technically relevant glasses including fused silica, borosilicate glass, and soda-lime-silicate glass under irradiation of fs-laser pulses characterized by a pulse duration τ = 300 fs and a laser wavelength λ = 1025 nm. For this purpose, LIPSS were fabricated in an air environment at normal incidence with different laser peak fluence, pulse number, and repetition frequency. The generated structures were characterized by using optical microscopy, scanning electron microscopy, focused ion beam preparation and Fast-Fourier transformation. The results reveal the formation of LIPSS on all investigated glasses. LIPSS formation on soda-lime-silicate glass is determined by remarkable melt-formation as an intra-pulse effect. Differences between the different glasses concerning the appearing structures, their spatial period and their morphology were discussed based on the non-linear absorption behavior and the temperature-dependent viscosity. The findings facilitate the fabrication of tailored LIPSS-based surface structures on different technically relevant glasses that could be of particular interest for various applications

Bio-Inspired Functional Surfaces Based on Laser-Induced Periodic Surface Structures

Nature developed numerous solutions to solve various technical problems related to material surfaces by combining the physico-chemical properties of a material with periodically aligned micro/nanostructures in a sophisticated manner. The utilization of ultra-short pulsed lasers allows mimicking numerous of these features by generating laser-induced periodic surface structures (LIPSS). In this review paper, we describe the physical background of LIPSS generation as well as the physical principles of surface related phenomena like wettability, reflectivity, and friction. Then we introduce several biological examples including e.g., lotus leafs, springtails, dessert beetles, moth eyes, butterfly wings, weevils, sharks, pangolins, and snakes to illustrate how nature solves technical problems, and we give a comprehensive overview of recent achievements related to the utilization of LIPSS to generate superhydrophobic, anti-reflective, colored, and drag resistant surfaces. Finally, we conclude with some future developments and perspectives related to forthcoming applications of LIPSS-based surfaces