Comparison of the production of nanostructures on bulk metal samples by picosecond laser ablation at two wavelengths for the fabrication of low-reflective surfaces

摘要

Nanostructure formation on bulk metals (copper, gold, and silver) by picosecond (FWHM = 10 ps) Nd:YAG laser irradiation was studied aiming at the production of low-reflectivity surfaces. The experiments were performed at two distinct wavelengths (λ = 355 and 1064 nm) using 20 kHz repetition frequency. The fluence was varied in the 1-11 J/cm2 range, while the samples were shot by an average pulse number from 0 to 50 depending on the scanning speed of the applied galvanometric scanner. The reflectivity of the treated surfaces was recorded with a visible near-infrared microspectrometer in the 450-800 nm range. Morphological investigations of the irradiated metal surfaces were performed with scanning electron microscope, which showed that mainly two types of nanostructures can be responsible for the reflectivity decrease depending on the type of the illuminated metal. Finite element modeling was performed to simulate the laser absorption-induced heating of the illuminated samples, which helps in the understanding of the structure formation process. It was found that two main processes take place in the production of micro-and nanostructures on the surface; the ejection and falling back of molten metal droplets, and the back scattering and aggregation of nanoparticles.