飞秒激光蚀除金属的分子动力学模拟

Molecular Dynamics Simulation on Femtosecond Laser Ablation of Metal

采用耦合一维双温模型的分子动力学方法,从连续及原子级的角度详尽描述了飞秒激光与金属的相互作用过程。建模时不仅考虑了激光能量的吸收方式、电子热传输及电子 晶格间的能量交换,而且对底部边界进行了特殊处理,采用简单而有效的速度减幅技术,既能防止靶材底部断裂,又能保证有效蚀除。分析了靶材内部温度、压力的分布特征及应力约束条件下的蚀除机制。模拟结果表明,靶材是在卸载波及被反射的压力波的共同作用下发生断裂;较高脉冲能量密度下蚀除产物由多个大团簇组成,而较低能量密度下只包含单个团簇,这是应力约束条件下金属蚀除的主要特征。同时,深入探讨了激光诱导压力波的成因及其传播规律,预测了压力波的波速。确定了蚀除阈值,同实验数据完全吻合,并证明了有过热现象的存在。

Using molecular dynamics method combining with two-temperature model, the progress of femtosecond laser-metal interaction is described completely at both continuum and atomistic length scales. The laser energy absorption mode the thermal transport sustained by free electrons and the energy exchange between electrons and lattice are taken into account in simulation model. Furthermore, special treatment is used at the bottom boundary with velocity-dampening technique in order to avoid spallation at the back side of the target and to ensure efficient ablation. The temperature and stress distribution as well as ablation mechanism under stress confinement are analyzed. Simulation results show that the spallation is due to the collective actions of an unloaded wave and the reflected stress wave. The ablation yield is composed of a few big clusters at higher fluences of the pulse energy, while a single cluster at lower fluences, which are all the characters of ablation under stress confinement.The reason for laser-induced stress wave and its propagation rule are also further investigated, and the velocity of stress wave is predicted simultaneously. The ablation threshold is predicted in good agreement with experiment. Superheating phenomenon is also discovered.