ABSTRACT Megahertz is the highest femtosecond laser repetition rate that the state-of-the art technology can achieve. In this article, a single femtosecond laser pulse is burst into multiple femtosecond laser pulses to process aluminum. The temporal gap between two consecutive burst pulses is 2 picoseconds, which is much shorter than the temporal gap between two consecutive pulses at the repetition rate of megahertz. By taking the thermophysical scenarios of femtosecond laser induced of electron thermalization, electron heat conduction, electron–phonon-coupled heat transfer and atomic motion into account, a multiscale framework integrating ab initio quantum mechanical calculation, molecular dynamics and two-temperature model are constructed. The effect of femtosecond laser pulse number on the incubation phenomenon is studied. Comparing with the single pulse-processing aluminum film, the femtosecond laser in burst mode leads to smaller thermal stress, which is favorable to reduce the thermal mechanical damage of the material beneath the laser-irradiated surface. Appreciable differences among the simulation results by using electron thermophysical parameters from ab initio quantum mechanical calculation and those from experimental measurement, empirical estimation and calculation are found, indicating the essentials to precisely model the electron thermal response subject to femtosecond laser excitation.

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飞秒激光脉冲在突发模式下加工铝的多尺度研究

摘要 兆赫兹是最先进技术所能达到的最高飞秒激光重复频率。在本文中，单个飞秒激光脉冲突发为多个飞秒激光脉冲来加工铝。两个连续突发脉冲之间的时间间隔为 2 皮秒，这比兆赫重复频率下两个连续脉冲之间的时间间隔要短得多。通过考虑飞秒激光诱导电子热化、电子热传导、电子声子耦合传热和原子运动的热物理场景，构建了一个集量子力学计算、分子动力学和双温度模型于一体的多尺度框架。研究了飞秒激光脉冲数对孵化现象的影响。与单脉冲处理铝膜相比，突发模式的飞秒激光产生更小的热应力，有利于减少激光照射表面下方材料的热机械损伤。发现使用从头算量子力学计算的电子热物理参数与实验测量、经验估计和计算的模拟结果之间存在明显差异，表明精确模拟飞秒激光激发下的电子热响应的要点。