Effect of irradiation wavelength on femtosecond laser-induced homogenous surface structures

Advances in femtosecond laser micromachining have shown that various hierarchical surface structures can be fabricated with this single step method. Yet, the effect of wavelength as a machining parameter on the formation of hierarchical surface structures still remains unclear. In this work, we micromachined copper and titanium at four different wavelengths: 275 nm, 400 nm, 800 nm and 1200 nm and compared the structure threshold fluence at the four wavelengths. On both Cu and Ti, the structure thresholds strongly depend on the material's absorbance spectra. However, in contrast to results from single spot ablation which suggest a positive correlation between ablation threshold and wavelength, our results rather demonstrate negative correlation with a material's absorbance across the UV–Vis-IR range. The structure formation threshold fluences on Cu were lowest at 400 nm and increased below and above this value, which coincides with the absorbance peak of Cu located at 369 nm. Similarly, our results for Ti across the Vis-IR range showed that as absorbance decreases, the formation thresholds increase for the typical laser-induced structures on Ti. Yet below Ti's absorbance peak at 294 nm, we observed completely different structure types that could not be compared to the structures obtained at higher wavelength.