Surface micro-structuring with ultrashort laser pulses

The field of laser ablation was born with the invention of laser and it did not take long to be present in a variety of applications. The number of studies in this field grew very fast and it became increasingly more popular for manufacturing, chemical analysis, biology, medicine and so on. For many years the researchers explored the basic scientific understanding of the involved processes, meanwhile improving the instrumentations used and expanding the type of laser sources. With the advent of femtosecond (fs) pulse lasers the interests in laser-solid irradiation and ablation moved towards the use of ultrashort light pulses. Among the various phenomena involved in the interaction of ultrashort laser pulses and solid targets, the appearance of tiny reproducible surface patterns made this field much more popular and of interest also for technological and industrial applications. This thesis explores the formation of fs laser induced periodic surface structures (LIPSS), their morphological features, possible ways to control them by exploiting the influence of experimental parameters (wavelength, polarization, energy, etc. of the laser pulses) and illustrates few examples of interest for possible applications. The surface morphologies were fabricated experimentally and analyzed by different imaging characterization techniques using as targets silicon and copper plates as well as gold thin film on a silicon substrate. The thesis starts with a brief introductory chapter (Chapter 1) presenting the history of this innovative field of research, fs laser surface structuring. In addition, the essential models and theories along with experimental approaches used to realize the LIPSS and investigate their formation mechanisms are also summarized. Chapter 2 illustrates the experimental setups and techniques used to carry out the experiments as well as methods for the measurement of physical parameters such as threshold fluence and spot size of the laser beams exploited in the course of the thesis, namely the Gaussian and the vector vortex beams. In particular, the standard Gaussian beam and the vector vortex more complex spatial intensity profiles are considered. The optical vector vortex beams are generated by using a beam converter based on a q-plate that modifies the original Gaussian beam. The complex spatial structure of the state of polarization and shape of these complex light fs laser beams when fired on a silicon substrate, in air, allows producing very complex LIPSS patterns and shaped craters, as discussed in Chapter 3. The characteristics of such surfaces and the possibility to fabricate unconventional surfaces are discussed in detail. So far, the formation mechanisms of LIPSS with sub-wavelength spatial period, named as ripples, are well established. Within the framework of this thesis, I tried to gain more information on the physical processes involved in the generation of LIPSS with supra-wavelength period, named grooves, on a silicon target. In Chapter 4, the influence of experimental parameters like laser pulse repetition rate, laser wavelength and ambient pressure on the surface structures with special emphasis on the grooves is addressed. The results evidence that the appearance of “grooves” is very much dependent on the experimental conditions and provide in some cases (namely, low ambient pressure and high repetition rate) information that can be useful to unveil certain mechanisms involved in supra-wavelength LIPSS formation. Finally, in Chapter 5, I summarized results of experiments carried out on other materials, e.g. copper plate and gold thin film on silicon substrate, in the course of my thesis work. The main aim here is on the possibility of modifying the response of the material through laser processing. As a first example, texturing of copper surfaces, through scanning of the fs laser beam in ablation regime, brought up very interesting surface features composed of induced ripples and random oriented nanoparticles. The manufactured copper surfaces showed a degree of freedom in controlling the wetting response of the material. The second example concerns the possibility to fabricate two-dimensional THz metasurfaces by means of the mask-free fs laser surface structuring technique. The THz transmission response of the realized samples was measured in a collaborating laboratory and showed a good agreement with simulation addressing the possibility to design and realize such kind of THz optical components. Finally, preliminary experiments were carried out on CdZnTe, a direct and wide band gap ternary semiconductor alloy of possible interest for room-temperature detection of nuclear-radiation and infrared light and for which fs laser surface structuring has been scarcely investigated to date. To conclude, the observations based on the experiments carried out in the course of this thesis provide valuable and interesting results for supporting this challenging field of research supporting a deeper understanding and a larger control on the laser induced surface structures