The effect of femtosecond laser repetition rate on the self-organization of micro- and nano-scale surface features

There are many applications within the field of interfacial science where controlled micro- and nano-scale surface roughness has been shown to enhance properties. Applications including single- and two-phase heat transfer, extreme wetting, emissivity, catalysis, anti-icing, corrosion resistance and more all stand to benefit from the inclusion of finely-tuned surface roughness. One intriguing method of inducing this multi-scale, hierarchal roughness is with use of Femtosecond Laser Surface Processing (FLSP). Surfaces functionalized with FLSP have the distinct advantage of permanent finely-controlled roughness formed from the original material without disturbing the bulk properties. While FLSP techniques were being developed at the research scale, a new generation of high repetition rate, high pulse energy (⪆ 1 mJ) industrial femtosecond lasers were developed, increasing the potential for future scalability of femtosecond laser-based surface functionalization techniques like FLSP. This study focuses on how the final topography of surfaces processed by FLSP changes with increased repetition rates. In this work, substrates including silicon wafers and aluminum, stainless steel, titanium, and copper alloys were processed with a femtosecond laser using repetition rates ranging from 1 to 100 kHz, but otherwise identical laser processing parameters. Physical characteristics of the resulting microstructures like peak-to-valley structure height were quantified using laser scanning confocal microscopy. The microstructures and nanoparticle layers on the laser processed surfaces were also qualitatively studied using scanning electron microscopy.