We developed a high-precision direct laser micro-structuring system for surface modification, based on bursts of picosecond laser pulses.
Fundamental requirement for microfluidic experiments is fabrication of microfluidic arrays. In close collaboration with Laboratory for Experimental Soft Matter at the Faculty for Mathematics and Physics and Laboratory for Photonics and Laser Systems at the Faculty for Engineering, both University of Ljubljana, we developed a high-precision direct laser micro-structuring system. The developed method, which is highly efficient and precise, uses fibre-laser generated bursts of picosecond light pulses. The set-up allows structuring of single micrometer-sized objects with a nanometre resolution. The efficiency of the method was demonstrated on a copper layer and a significant reduction of typical processing time was shown. Bursts of pulses also contribute to high quality definition of structure edges and sides. By using bursts instead of single pulses, we are able to use up to ten times lower laser energy for microfluidic structuring. The work thus opens a window to utilise compact fibre lasers, reducing the overall system complexity, size and cost (J. Phys. D: Appl. Phys., vol. 50, 325104 (2017).
The developed method for laser ablation and surface modification, which is based on using a sequence of weaker laser pulses rather than one stronger one, was used also on silicone. We have shown that by changing the energy of the pulse, pulse position, and the number of pulses in a burst, we can successfully control the crystallinity of the structure. Depending on the parameters, a transition from no observable changes of the silicon at low pulse energies, through amorphisation below the ablation threshold energy, to the ablation with either complete, partial or non-existent amorphisation was observed. Single micrometer-sized areas of desired shape and crystallinity were defined on the silicon surface with sub-micron precision (Opt. Express, vol. 25, 26356 (2017).
Described research was part of postgraduate programme, which was successfully completed with a dissertation entitled Interaction of short laser pulses with matter during direct micro-structuring.
Part of the research was done in collaboration with Condensed Matter Physics Department. We focused on observations of optothermal effects, which can transport microparticles in confined liquid crystals. Optical tweezers were used for rapid movement of laser beam in a thin layer of a nematic liquid crystal and demonstrated the appearance of fluid flows. The observed phenomenon is caused by two different mechanisms, thermoviscous expansion effects and thermally induced local melting of the liquid crystal. The flow was controlled by variation of the velocity of the focused laser beam and its intensity. This enabled creation of directed microflows in a channel-free environment (Soft Matter, vol. 13, 2449 (2017).