大气环境下飞秒激光对铝靶烧蚀过程的研究

利用时间分辨的光阴影成像技术研究了在大气环境下飞秒激光烧蚀铝靶的动态过程. 在入射激光能量为4 mJ, 激光光斑超过1 mm时, 激光烧蚀区表面物质以近似平面冲击波形式向外喷射; 在同样激光能量下、激光光斑较小时(约0.6 mm), 激光烧蚀区以近似半球型冲击波形式向外喷射. 当激光能量比较大时(7 mJ), 发现空气的电离对于激光烧蚀靶材有着重要影响. 在光轴附近烧蚀产生的喷射物具有额外的柱状和半圆型的结构, 叠加在平面冲击波结构上.

Femtosecond laser ablation of an aluminum target in air

The dynamics of laser ablation of solid target with ultrashort intense laser pulses is not only fundamentally interesting, but also relevant to a few important applications such as microfabrication, laser propulsion, laser induced breakdown spectroscopy, etc. By use of time-resolved pump-probe shadowgraphic imaging technology, we study the dynamic process of laser ablation of a planar aluminum target in air. The incident laser pulses are from a Ti: sapphire femtosecond laser amplifier system with a duration of 50 fs, central wavelength of 800 nm, pulse energy varying between 4 mJ and 7 mJ. Time-resolved shadowgraphs of material ejection from the aluminum target are recorded at the time delay up to a few nanoseconds after laser irradiation. By changing the distance between the target and the focal point of the incident laser, we obtain the shadowgraphs of the target ejection under different laser spot sizes. When the laser spot size is relatively large say, over 1 mm, the irradiated target surface is ablated in the form of a planar shock. However, when the laser spot size is relatively small, the ejection appears in the form of a hemispherical blast wave. It is found that the hemispherical blast wave well conforms to the Sedov's blast wave theory. When the laser energy is relatively large, it is found that ionization of air has a great effect on laser ablation. Additional small ejections appear as columnar and hemispherical structures near the laser axis, which are superimposed on the large planar shock. These can be attributed to the following processes. Firstly, as the ionization of air occurs near the laser axis, effective heat transfer from air plasma to the aluminium target leads to enhanced target temperature. This leads to the formation of a columnar structure on a picosecond time scale. Secondly, the columnar ejection and air plasma expansion near the laser axis result in the decrease of air density and pressure, which leads to the formation of the hemispherical structure on a nanosecond time scale.