Ripple formation on Nickel irradiated with radially polarized femtosecond beams

We report on the morphological effects induced by the inhomogeneous absorption of radially polarized femtosecond laser irradiation of nickel (Ni) in sub-ablation conditions. A theoretical prediction of the morphology profile is performed and the role of surface plasmon excitation in the production of self-formed periodic ripples structures is evaluated. Results indicate a smaller periodicity of the ripples profile compared to that attained under linearly polarized irradiation conditions. A combined hydrodynamical and thermoelastic model is presented in laser beam conditions that lead to material melting. The simulation results are presented to be in good agreement with the experimental findings. The ability to control the size of the morphological changes via modulating the beam polarization may provide an additional route for controlling and optimizing the outcome of laser micro-processingComment: 4 pages, 4 figures, 4 appendix page

Επιφανειακή δόμηση υλικών και λεπτών υμενίων με πολωμένους υπερβραχείς παλμούς λέιζερ

Controlling the interactions of light with matter is of paramount importance for the innovation and scalability of laser materials processing applications. The duration of the laser pulse is the most critical parameter and the treatment with ultrashort (<1 ps = 10^-12 s) laser pulses (YLP) paves the way for new, exciting possibilities for the creation of modern materials. Indeed, there are important applications for the processing of materials with YLP, such as the development of reduced friction surfaces, optical elements, micro-fluidic devices and biomedical systems. An important feature of the processing of a material with YLP is that the morphological characteristics of the induced structures are strongly correlated with the polarization of the laser beam. In this context, the polarization state during processing provides countless possibilities. The subject of this dissertation is the understanding of the effect of spatially alternating forms of polarization on laser induced periodic surface structures, the control of their direction and the application of this technique in the processing of materials. In addition, the possibility of reversibility of their creation is studied, as well as their application in thin metal films for the development of polarized optical elements in the infrared. It is expected that this dissertation will contribute to a deeper understanding of matter-light interactions and provide a new tool for constructing nanostructured materials leading to the development of functional surfaces without the need for chemicals and other precursors, and exclusively with the use of coherent laser light.Ο έλεγχος των αλληλεπιδράσεων του φωτός με την ύλη είναι υψίστης σημασίας για την καινοτομία και επεκτασιμότητα των εφαρμογών επεξεργασίας υλικών με λέιζερ. Η διάρκεια του παλμού λέιζερ είναι η πιο κρίσιμη παράμετρος και η επεξεργασία με Υπερβραχείς (<1 ps = 10^-12 s) Παλμούς Λέιζερ (ΥΠΛ) ανοίγει το δρόμο για νέες, συναρπαστικές δυνατότητες για την δημιουργία σύγχρονων υλικών. Πράγματι, υπάρχουν σημαντικές εφαρμογές της επεξεργασίας υλικών με ΥΠΛ, όπως η κατασκευή επιφανειών μειωμένης τριβής, οπτικών στοιχείων, συσκευές μικρο-ροών και βιοϊατρικών συστημάτων. Ένα σημαντικό χαρακτηριστικό της κατεργασίας ενός υλικού με ΥΠΛ είναι ότι τα μορφολογικά χαρακτηριστικά των επαγόμενων δομών συσχετίζονται ισχυρά με την πόλωση της δέσμης λέιζερ. Στο πλαίσιο αυτό, η κατάσταση της πόλωσης κατά τη διάρκεια της κατεργασίας παρέχει αναρίθμητες δυνατότητες. Αντικείμενο της παρούσας διατριβής είναι η κατανόηση της επίδρασης των χωρικά εναλλασσόμενων μορφών πολώσεων στις επαγόμενες δομές από λέιζερ, ο έλεγχος της κατεύθυνσης αυτών και η εφαρμογή της τεχνικής αυτής στην κατεργασία υλικών. Επιπλέον, μελετάται η δυνατότητα αναστρεψιμότητας της δημιουργίας τους αλλά και η εφαρμογή τους σε λεπτά μεταλλικά υμένια για την ανάπτυξη πολωτικών οπτικών στοιχείων στο υπέρυθρο. Αναμένεται ότι η παρούσα διατριβή θα συμβάλλει στην βαθύτερη κατανόηση των αλληλεπιδράσεων ύλης – φωτός και θα παρέχει ένα νέο εργαλείο για την κατασκευή νανο-δομημένων υλικών οδηγώντας στην ανάπτυξη λειτουργικών επιφανειών χωρίς την απαίτηση χημικών και άλλων πρόδρομων ουσιών, και αποκλειστικά με τη χρήση σύμφωνου φωτός λέιζερ

The Use of Lasers in Dental Materials: A Review

Lasers have been well integrated in clinical dentistry for the last two decades, providing clinical alternatives in the management of both soft and hard tissues with an expanding use in the field of dental materials. One of their main advantages is that they can deliver very low to very high concentrated power at an exact point on any substrate by all possible means. The aim of this review is to thoroughly analyze the use of lasers in the processing of dental materials and to enlighten the new trends in laser technology focused on dental material management. New approaches for the elaboration of dental materials that require high energy levels and delicate processing, such as metals, ceramics, and resins are provided, while time consuming laboratory procedures, such as cutting restorative materials, welding, and sintering are facilitated. In addition, surface characteristics of titanium alloys and high strength ceramics can be altered. Finally, the potential of lasers to increase the adhesion of zirconia ceramics to different substrates has been tested for all laser devices, including a new ultrafast generation of lasers

Biomimetic surface structures in steel fabricated with femtosecond laser pulses: influence of laser rescanning on morphology and wettability

The replication of complex structures found in nature represents an enormous challenge even for advanced fabrication techniques, such as laser processing. For certain applications, not only the surface topography needs to be mimicked, but often also a specific function of the structure. An alternative approach to laser direct writing of complex structures is the generation of laser-induced periodic surface structures (LIPSS), which is based on directed self-organization of the material and allows fabrication of specific micro- and nanostructures over extended areas. In this work, we exploit this approach to fabricate complex biomimetic structures on the surface of steel 1.7131 formed upon irradiation with high repetition rate femtosecond laser pulses. In particular, the fabricated structures show similarities to the skin of certain reptiles and integument of insects. Different irradiation parameters are investigated to produce the desired structures, including laser repetition rate and laser fluence, paying special attention to the influence of the number of times the same area is rescanned with the laser. The latter parameter is identified to be crucial for controlling the morphology and size of specific structures. As an example for the functionality of the structures, we have chosen the surface wettability and studied its dependence on the laser processing parameters. Contact angle measurements of water drops placed on the surface reveal that a wide range of angles can be accessed by selecting the appropriate irradiation parameters, highlighting also here the prominent role of the number of scans

Mimicking lizard-like surface structures upon ultrashort laser pulse irradiation of inorganic materials

Inorganic materials, such as steel, were functionalized by ultrashort laser pulse irradiation (fs- to ps- range) to modify the surfaces wetting behavior. The laser processing was performed by scanning the laser beam across the surface of initially polished flat sample material. A systematic experimental study of the laser processing parameters (peak fluence, scan velocity, line overlap) allowed the identification of different regimes associated with characteristic surface morphologies (laser-induced periodic surface structures, grooves, spikes, etc.). Analyses of the surface using optical as well as scanning electron microscopy revealed morphologies providing the optimum similarity to the natural skin of lizards. For mimicking skin structures of moisture-harvesting lizards towards an optimization of the surface wetting behavior, additionally a two-step laser processing strategy was established for realizing hierarchical microstructures. In this approach, micrometer-scaled capillaries (step 1) were superimposed by a laser-generated regular array of small dimples (step 2). Optical focus variation imaging measurements finally disclosed the three dimensional topography of the laser processed surfaces derived from lizard skin structures. The functionality of these surfaces was analyzed in view of wetting properties

Mimicking bug-like surface structures and their fluid transport produced by ultrashort laser pulse irradiation of steel

Ultrashort laser pulses with durations in the fsto-ps range were used for large area surface processing of steel aimed at mimicking the morphology and extraordinary wetting behaviour of bark bugs (Aradidae) found in nature. The processing was performed by scanning the laser beam over the surface of polished flat sample surfaces. A systematic variation of the laser processing parameters (peak fluence and effective number of pulses per spot diameter) allowed the identification of different regimes associated with characteristic surface morphologies (laser-induced periodic surface structures, i.e., LIPSS, grooves, spikes, etc.). Moreover, different laser processing strategies, varying laser wavelength, pulse duration, angle of incidence, irradiation atmosphere, and repetition rates, allowed to achieve a range of morphologies that resemble specific structures found on bark bugs. For identifying the ideal combination of parameters for mimicking bug-like structures, the surfaces were inspected by scanning electron microscopy. In particular, tilted micrometre-sized spikes are the best match for the structure found on bark bugs. Complementary to the morphology study, the wetting behaviour of the surface structures for water and oil was examined in terms of philic/ phobic nature and fluid transport. These results point out a route towards reproducing complex surface structures inspired by nature and their functional response in technologically relevant materials