Wavelength selection for femtosecond laser processing of materials: Comparison of ablation efficiency and surface quality

Abstract

We investigated the effects of the femtosecond laser wavelengths λ = 1035, 515, and 345 nm on the laser ablation of 64FeNi alloy (Invar). The ablation rate and ablation efficiency for the three wavelengths were obtained through material-removal experiments of stationary irradiation and laser scanning, respectively. Among laser–matter interaction parameters, the effective penetration depth (α−1) and ablation threshold fluence (Fth) could be estimated from the experiments and were found to be proportional to λ. The ablation rate and ablation efficiency for the peak fluence range of 0.2 J/cm2 < F0 < 1.0 J/cm2 and F0 > 1.0 J/cm2 were the highest at λ = 515 and 1035 nm, respectively, which could be confirmed by using analytical models with α−1 and Fth. The smoothest (roughest) processed surfaces could be obtained at λ = 345 nm (1035 nm) owing to the shortest (longest) periodicity of laser-induced periodic surface structures. Thus, surface smoothing at λ = 345 nm over a certain ablation depth after ablation at λ = 1035 nm, i.e., double-wavelength ablation, can be used for improving surface quality. These results can help in the selection of the proper λ for implementing precise laser micromachining with high productivity.