König J, Nolte S, Tünnermann A
Opt Express. 2005 Dec 26;13(26):10597-607. doi: 10.1364/opex.13.010597.
We report on time-resolved measurements of the plasma evolution during metal ablation with ultrashort laser pulses in the range from 200 fs to 3.3 ps. The plasma transmission exhibits two distinctive minima. Almost total attenuation is observed a few nanoseconds after the ablation pulse, while a second decrease of the transmission to approximately 50% is observed after about 150 ns. Images taken with a gated ICCD-camera confirm these data and allow determining the expansion velocity of the plasma plume. The attenuation in the first nanoseconds can be attributed to electrons and sublimated mass emitted from the target surface, while attenuation after several 10 ns is due to particles and droplets after a thermal boiling process. The possibility of a normal or an explosive boiling process, also called phase explosion, is discussed. Despite of the physical insight into the ablation process, these data provide valuable information for scaling the speed of ultrashort pulse laser materials processing in a fluence regime of several J/cm2 since they allow estimating the maximum usable pulse repetition rate.
我们报告了在200飞秒至3.3皮秒范围内用超短激光脉冲进行金属烧蚀时等离子体演化的时间分辨测量结果。等离子体透射率呈现出两个明显的最小值。在烧蚀脉冲后几纳秒观察到几乎完全衰减，而在约150纳秒后观察到透射率第二次下降至约50%。用门控ICCD相机拍摄的图像证实了这些数据，并能够确定等离子体羽流的膨胀速度。最初几纳秒内的衰减可归因于从靶表面发射的电子和升华物质，而几十纳秒后的衰减则是由于热沸腾过程后的颗粒和液滴。文中讨论了正常沸腾或爆炸沸腾过程（也称为相爆炸）的可能性。尽管对烧蚀过程有了物理认识，但这些数据为在几焦耳每平方厘米的能量密度范围内缩放超短脉冲激光材料加工速度提供了有价值的信息，因为它们可以估计最大可用脉冲重复率。
