KAROLIS RATAUTAS, PAULIUS GEČYS, MINDAUGAS GEDVILAS, and GEDIMINAS RAČIUKAITIS
On its 60th anniversary, the fields of laser application, particularly in material processing, are expanding steadily, and most of the advanced solutions come with the use of ultrashort-pulse (USP) lasers, a type of laser for which Lithuanian companies are well recognized globally.
The Department of Laser Technologies at FTMC – the Center for Physical Sciences and Technology (Vilnius, Lithuania) is one of the active places in Europe where new technologies and applications for lasers are born. The following is a review of the department’s recent high-maturity level developments ready to be directly installed by customers.
Glass processing technologies
Novel laser glass processing techniques, offering high speed and precise fabrication, have become an attractive alternative to conventional methods, such as mechanical dicing, diamond saw, and waterjet cutting. With the evolution of modern industrial lasers, laser-based technologies are ensuring high throughput as well as higher quality.
One of the most flexible glass processing techniques is laser rear-side processing. The process is initiated by focusing the laser beam through the glass sample to the back surface of a transparent substrate. It is a very efficient process that could reach >120 mm3/min material removal rate with a laser emitting just 20 W. Utilizing this technique, the typical kerf width can be remarkably reduced compared to mechanical diamond tool processing. Therefore, feature sizes as small as 150 µm can be laser-machined. The laser cuts are taper-less, and it is feasible to fabricate extremely high-aspect-ratio features. Excellent cutting quality can be obtained with surface chipping <50 µm, and the chamfering process also can be realized. The method was validated on float glass, borosilicate glass, and fused silica. The process is accomplished via galvanometer scanner beam positioning—therefore, there are almost no process shape restrictions. Furthermore, process stitching is implemented to extend the fabrication field to industrial requirements.
FIGURE 1 presents the laser system for glass processing working in the production of displacement and rotation encoders.
The focused laser beam can be applied not only for cutting and drilling, but 2.5D milling processing is possible as well.
An exciting emerging field of application for the technique is the fabrication of gas injection nozzles. Gas nozzles are used in various laser-plasma experiments for laser wakefield acceleration (LWFA), secondary x-ray, and γ-light generation toward industrial tomography and medical treatment applications. Tailored plasma targets require the implementation of the complexed-shaped nozzles with converging-diverging and cylindrical throats to get modulation of gas concentration along to the laser beam propagation. Such complex shapes are complicated or, in many cases, impossible to produce with conventional techniques. Further, the channel wall roughness has a crucial impact on the nozzle performance, and fused silica is much more resistant compared with metals. The Department of Laser Technologies has developed a hybrid laser processing technique, where nanosecond laser processing is combined with the femtosecond laser-induced selective etching (FLISE) technique (FIGURE 3). Such a unique combination allowed us to reach high submicron-scale resolution together with high-speed material processing.1
Precise and high-efficiency laser ablation
Nature-inspired surfaces and their replication are getting a huge amount of interest in science, technology, and medicine due to their unique functional properties. Particular attention is paid to surfaces that replicate the shark-skin texture, which reduces friction with liquids or gases and has antibacterial properties. Our team at FTMC has developed an efficient laser-ablation technology, keeping a maximal material removal rate together with ultimate processing quality. The technology was validated by replication of shark-skin-like structures over large areas at high speeds.
Findings of the theoretical way to predict an optimal set of processing parameters for the most efficient laser ablation advanced in the readiness level of laser milling technology to real use in biomimetical applications. Achievement of the highest-known laser milling ablation efficiency for copper, using a state-of-the-art bi-burst femtosecond laser,2 together with the minimal roughness of the laser-milled surface and best machining quality,3 the new laser technology is advanced over its competitors. The friction with air, of laser structured shark-skin-like surfaces studies, showed a drag-force decrease by up to 6%. The ability to mimic the bio-inspired surfaces of shark skin-like structures, drag-reducing trapezoidal-riblet structures (FIGURE 4), and fish scale-like structures or other complex tree-dimensional shapes at high fabrication speed while maintaining lowest-possible surface roughness has enabled the efficient laser ablation technology to successfully compete with conventional industrial techniques.