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Femtosecond Laser Surface Processing
Femtosecond (fs) lasers have been recognized as a powerful tool for precision machining and micro/nanofabrication due to their unique advantages, including minimal thermal introduction, high precision, high flexibility, high repeatability, etc. Nowadays, surface processing, including the laser polishing of additively manufactured parts and the nanofabrication of surface-enhanced Raman spectroscopy (SERS) substrates, is of great importance in various fields, including automobile, aerospace, biomedicine, biosensing, etc. However, the potential of fs lasers on surface processing is not fully explored, especially fs lasers with high average power and shorter wavelength. Therefore, the research efforts described in this dissertation mainly focused on the fs laser surface processing from the following aspects. 1) Fs laser polishing at grazing incidence was established for the post-processing of additively manufactured (AM) parts. The challenge of removing three levels of roughness was addressed, achieving mirror-like surfaces with a Sa = 200 nm on stainless steel parts with initial roughness = 25 µm, equivalent to > 99% improvement on the surface finish. 2) Combined fs-ns laser polishing was investigated for the post-processing of AM parts. A further reduction in surface roughness of Sa = 43 nm was achieved by the ns laser polishing of fs laser polished surfaces. The evolution of nanoripple morphology and roughness under ns irradiation was investigated. The optimized processing parameters of ns laser polishing were established. Besides, the microstructure evolution at the atomic level of combined fs-ns processing was characterized and analyzed. 3) Nanofabrication of SERS substrates by a UV fs laser was investigated. Nanoripples induced in open air by a UV fs laser on Silicon substrates with gold coatings were used as SERS-active substrates. The influences of laser fluence and hatching distance on nanoripple morphology and enhancement factor were analyzed. SERS EFs up to 2.4 × 107 were achieved for Rhodamine 6G (R6G) molecules on the laser-textured Si substrates with Au coatings of 34 nm thick.
Li, Nan, "Femtosecond Laser Surface Processing" (2022). ETD collection for University of Nebraska - Lincoln. AAI30000053.