| Abstract |
Utilizing laser light for materials processing came along with the first laser invention and revolutionized a plethora of technological applications and scientific fields. It did not take long for lasers to be applied on a vast range of diverse fields such as manufacturing, imaging, chemical analysis-synthesis, medicine and biology. However, the fundamentals of lasing and coherent light manipulation, along with the scientific understanding of the physics involved are under continuous investigation given that any advance in laser science and instrumentation follows significant breakthroughs in most research fields. In particular, since the development of laser sources with ultrashort wave-packets so rapid that can yield timescales of a few femtoseconds (10-15 sec), the field of laser ablation and micro-structuring was revolutionized as today there are many ways to induced surface morphological changes with the use of laser light exclusively.
Among the various and fascinating phenomena that can take place during the interaction of intense ultrashort laser pulses with solid matter such as ablation, welding, sintering and thermal annealing, it is found that in some conditions, tiny self-arranged and reproducible surface patterns can occur. These surface structures beat the typical laser beam diffraction limit and since their first observation they paved the way for a new laser processing field during the past decades due to their implementation in numerous technological and industrial applications.
This thesis explores the formation of a particular type of self-assembled structures, the laser induced periodic surface structures (LIPSS), their morphological features, and possible ways to control them by introducing vectorial polarization states as a new parameter among the various laser parameters such as the laser wavelength, energy, etc. of the laser pulses is investigated. Moreover, this work explores the effect of complex polarization states on the LIPSS formation mechanism, along with the reversibility of the observed surface morphologies utilizing ideal material case, which can permit under specific irradiation conditions the formation and deletion of high ordered LIPSS. Finally, the yet unexplored utilization of these type of structures on sub-micrometer sized metal films is investigated. A few examples of the use of laser-based patterned surfaces with femtosecond (fs) pulses are discussed as they can be further utilized in applications such as laser marking, the direct realization of planar meta-optics and polarizers and potentially in fs-data storage The first Chapter is focused on a review of LIPSS and their classification based on the distinct features of the induced structures. A review of the most prominent physical mechanisms that lead to the formation of LIPSS is presented along with the most important applications that have risen from this field.
In Chapter 2, the experimental protocols and procedures are presented. The intensity distribution of the laser beam is described along with the polarization conversion process. A brief overview of the fundamentals of Gaussian and Cylindrical vector beams, their use for dynamic surface processing as well as the development of the experimental set up are presented. Moreover, the details of all material targets and the characterization methods (along with digital image analysis) that are involved in this thesis are listed.
In chapter 3 the experimental and theoretical results on irradiations of metals well known for their use in industry such as Nickel (Ni) and steel surfaces are presented and discussed. A systematic study was conducted with LIPSS formation with linear, radial and azimuthally polarized laser pulses. The dependence of such exotic polarization states in the formation mechanism is explored along with the experimental conditions that lead to complex structures of multiple orientations exhibiting omnidirectional angle independent diffractive properties.
For chapter 4, the use of controlled irradiation conditions leads to high-regularity, erasable and re-writable periodic surface patterns on silver metaphosphate glass (AgPO3) by means of ultrashort pulsed laser processing. The so-formed periodic patterns can be readily erased upon further exposure to femtosecond laser irradiation under irradiating conditions that can selectively permit or deny the physical process that leads to LIPSS formation. This all-laser inscription and deletion protocol allows the reversible patterning of the phosphate glass surface by employing a single laser beam and could potentially be utilized for data storage.
Finally, in the last chapter, we explore the direct realization of LIPSS, on sub-micrometer thin Ni films which are deposited on a dielectric substrate. The experimental investigation showed that it is possible, under specific conditions to realize high regular LIPSS on the thin film while the substrate remains unaffected. The fabricated surfaces were optically and morphologically characterized and exhibit significant response on the Infrared (IR) and The Mid-IR electromagnetic spectrum exactly as a wire grid polarizer. The effective spectrum of the polarizer can be controlled with the thin film ripple period which can be controlled precisely with the laser wavelength. Furthermore, due to the simplicity of this technique and the versatility, it can be potentially applied in almost all thin metal surfaces regardless of the substrate characteristics. Whereas, this approach can be with further development revolutionize the current way we produce wire grid polarizing plates with no need for chemicals and time-consuming multi-step processes.
|
Search
