The dependence of the ablation rate of metals on nanosecond laser fluence and wavelength

The dependence of ablation rate of aluminium, titanium and copper on the nanosecond laser fluence at 532 nm and, respectively, 1064nm wavelengths is investigated in atmospheric air. The wavelength is varied by exchanging the fundamental and second harmonic modules of a Q-switched Nd-YAG laser system, while the fluence of the pulses is varied by changing the diameter of the irradiated area at the target surface. The results indicate an approximately logarithmic increase of the ablation rate with fluence for both wavelengths, and an approximately double ablation rate in the case of visible pulses as compare to infrared pulses. By extrapolating the ablation rate vs. fluence fitting curve toward zero, we estimate the ablation threshold fluence, F_th. The ablation threshold fluence is strongly dependent on the wavelength and target material. Thus, F_th in the case of infrared pulses is twice as large as F_th corresponding to the visible pulses, while F_th is lower for the aluminium and titanium as compared to the copper. A different behavior occurs in the case of copper, for which we obtained a two times smaller F_th when using infrared pulses comparative with the visible pulses.