超短パルスレーザアブレーション現象の分子動力学解析

Abstract

Laser ablation phenomena of aluminum with the fourth harmonics of Nd:YAG laser is analyzed using the modified molecular dynamics (MD), which has been developed previously by Ohmura and Fukumoto. In the modified MD method, the MD calculation for metal is carried out compensating the heat conduction by free electrons at each time step. Main conclusions are summarized as follows: (1) When the fluence is constant, there is an optimal irradiance of the laser pulse to make the number of evaporated atoms be the maximum. The ratio of evaporation energy to the laser energy absorbed in the material becomes the maximum at this optimal irradiance. (2) There are two types in laser ablation process. One is explosive evaporation which occurs when pulse width is extremely short, and the other is relatively calm evaporation which occurs when pulse width is comparatively long. In the former process, a comparatively large particles scatter, and in the latter process, the size of scattering particles is relatively small. (3) For the ultrafast laser irradiation, the evaporation threshold fluence of aluminum is about 20 mJ/cm2.