Dynamics of deep short pulse laser drilling: Ablative stages and light propagation

Dynamics of ablation rate and laser light propagation under deep laser dril ling of chemical vapor deposited (CVD) diamond by picosecond (300 ps) and n anosecond (9 ns) pulses at 1078 nm wavelength has been studied. Based upon the ablation rate behavior, the drilling process consists of four stages: ( 1) ablation in a shallow crater (depth-to-diameter ratio L/D < 1-3) at the constant and relatively high ablation rate; (2) formation of a deep conical -like crater (1-3 < L/D < 5-7), when the ablation rate reduces by 1-2 order s of magnitude, (3) the main rise of the channel depth (L/D > 7) under low and quite stable ablation rate, and (4) quick reduction of the ablation rat e down to zero and the ablation process stopping. In the frame of this gene ral scheme the absolute value of the ablation rate substantially depends on pulse energy and pulsewidth. The rise of laser fluence at the channel bott om at the channel deepening (L/D < 20) was revealed in the experiments. It has been shown that strong reduction of the ablation rate at L/D = 1-7 can be explained neither by the energy losses due to beam divergence nor by str engthening of plasma shielding effect. Strong bending of the experimental c hannels, which occurs under a certain orientation of the radiation polariza tion relating to the film surface, was also observed and discussed.