Hu, Meilin (2022) Laser induced periodic surface structures: influence of materials and processing conditions. [Tesi di dottorato]

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Abstract

The research on the micro-nano structures formed on the surface of a target irradiated by laser beams has been an evergreen scientific topic. Research on the micro-nano structures formed on a laser-irradiated target has been a topic of constant interest for the past few decades. The non-contact nature of laser processing along with its high energy localization make it widely applicable to manufacturing, tribology, biomedicine, chemistry, as well as several other disciplines. After T. H. Maiman et al. made the breakthrough with the first pulsed laser in 1960, laser technology continued to develop rapidly, strengthening the field of laser materials processing. In the last decades, interest in laser-solid irradiation has gradually shifted toward ultrashort light pulses. Due to the ultrashort duration, femtosecond (fs) lasers pulses have the ability to induce a considerably less heat-affected zone, which in turn provides a greater spatial resolution in material processing. This makes the fs laser a precise and versatile tool for micro- and nano-fabrication. Since many surface properties of solids are closely related to their morphology, the possibility to modify and tailor the material surface can have a great impact on various features, such as the optical, wetting, mechanical responses, etc. Further applications of laser processing include the creation of microfluidic channels for biological applications and optical integrated circuits for quantum applications. The formation of laser-induced periodic surface structures (LIPSS) is a phenomenon that is almost ubiquitous to any material. The aim of this work is to experimentally analyze the role of various processing parameters (e.g. laser repetition rate, wavelength, scanning speed, etc.) taking also into account the materials properties by selecting few different targets. In this thesis, the LIPSS generated on various targets like silicon, copper, semiconductive films and topological insulators are investigated by a combination of different microscopy characterization techniques (i.e. Optical Microscopy, Scanning Electron Microscopy, Atomic Force Microscopy) and image processing analyses (ImageJ, Gwyddion) in different experimental configurations such as static spots, dynamic lines as well as large areas processing. We found that the morphological characteristics of LIPSS can be controlled by the influence of various experimental parameters, e.g., laser pulse wavelength, fluence, pulse repetition rate, beam or target scanning speed, processing ambient, etc. The characteristics can be illustrated by the periods and shapes of the ripples, and the strength of the nanoparticles. This thesis is composed of six chapters. Chapter 1 mainly introduces the characteristics of LIPSS, related applications and generation mechanisms, which provide a theoretical basis for the research in the subsequent chapters. In Chapter 2, the laser systems used and the image processing methods are reported in detail. The description of circular and elliptical craters generated by focusing Gaussian beams with different lenses is also theoretically analyzed. The specific study of fs laser surface properties on silicon targets is presented in Chapter 3. First, we find that in the dynamic configuration, the scanning speed and laser pulse energy have a great influence on the properties of subwavelength ripples. We also compare laser ablation of silicon samples in air and vacuum at high repetition rates, addressing the very interesting aspect that the influence of plume shielding is negligible in vacuum but becomes important in air at repetition rates larger than about 10 kHz. Large-area irradiation of copper is presented in Chapter 4, where the ripples generated at different laser intensities are analyzed in detail. The different zones of the spatial distribution of the laser intensity will have a different degree of overlap of the Gaussian laser intensity distribution, resulting in different morphological characteristics in each zone. We report in Chapter 5 that the ripples orientation follows the local direction of the laser polarization in a semiconductor polymer PH3T and a fullerene derivative PC71BM. Chapter 6 mainly summarizes the formation of periodic ripples on a topological insulator, Bi2Te3, induced by fs laser ablation. This work is devoted to the morphological characterization of laser-induced surface structures on various types of materials with different laser parameters. These challenging studies provide valuable and interesting results that contribute to a deeper understanding and greater control of laser-induced surface structures.

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