Optical characterisation of femtosecond laser-induced periodic surface structures fabricated on stainless steel substrate material utilising static linearly polarised laser pulses
The optical characterization of Laser-induced periodic surface structures LIPSS was studied on polished stainless steel surfaces following irradiation with static linearly polarized fs-laser pulses at a pulse duration = 300fs, a laser wavelength = 1025nm, laser fluences F = 0.8, 1.7, 2.4, 3.4 and 6.6J/cm2 and at repetition rates frep = 250Hz, 20.8kHz and 41.7kHz. Line scans with scanning velocities v = 0.5mm/s, 0.5m/s and 1m/s corresponding to the repetition rates were used in the fabrication process and in ambient air environment. Optical and scanning electron microscopies were used in the surface characterization of the LIPSS. 2D fast Fourier Transforms were also performed to evaluate the periodicity of LIPSS using the Gwyddion Software. Using the integrating sphere with a spectrophotometer, the wavelength dependent reflectance (diffuse and total) was obtained. By means of a diffraction grating setup, the color effect of LIPSS was also investigated. The results confirm that a change in the laser fabricating parameters induces a change in the surface morphology of LIPSS. Ideal gratings were generated at lower F, larger line spacing and higher v and these subsequently led to high light absorption enhancement factors LAEFs. The high LAEF arise from the surface plasmon polaritons SPP-generated localized regions of the high-intensity electric field and the effective refractive index and Fabry-Perot resonances could be used to explain the mechanism. This study thus has potential applications in generating efficient solar cells, antireflective coatings and in the biomedical industry.