关键词: 激光烧蚀/
平面冲击波/
半球型冲击波/
大气环境
English Abstract
Femtosecond laser ablation of an aluminum target in air
Kang Xiao-Wei,Chen Long,
Chen Jie,
Sheng Zheng-Ming
1.Laboratory for Laser Plasmas, Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11374210).Received Date:14 September 2015
Accepted Date:20 December 2015
Published Online:05 March 2016
Abstract: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.
Keywords: laser ablation/
planar shock wave/
hemispherical shock wave/
air environment
