Ultrashort pulsed lasers create a fundamentally different ablation mechanism than conventional pulsed lasers because of the ultrashort laser pulse's extreme intensity (> 1012W/cm2) and time duration ( 10–12s), which is shorter than the electron-lattice transfer time (>10–12S). Consequently, a thermally excited plasma is generated in a cool lattice. Assumptions used in conventional pulsed laser ablation are invalid for ultrashort pulses. In this work, computer modeling of ultrashort pulsed ablation was performed for diamond. A two-step model was developed in which a heat transfer, finite-difference model was formulated and tight-binding molecular dynamics simulations were performed to evaluate the dynamics of ablation events. The heat transfer model incorporated absorption of the laser light by the electrons and predicted the thermal profiles within the electrons from the start of a laser pulse to 1 ps. The tight-binding molecular dynamics predicted the threshold electron temperatures (room temperature lattice) and overall equilibrium temperature (electron and lattice at the same temperature) required for changes in structure and ablation to occur in the material. The results of both simulations were then used to predict ablation threshold, ablation volume, and the size of the heat-affected zone within the material.; Ultrashort pulsed laser ablation experiments were performed on chemical vapor deposited and on single crystal diamonds, as well as on highly-oriented pyrolytic graphite, in order to verify the model predictions. Scanning electron microscopy, atomic force microscopy, profilometry, and micro-Raman spectroscopy were employed to characterize the ablated surfaces. Results showed that ultrashort pulses, compared with nanosecond laser pulses, yield lower threshold fluences, higher material removal rates, and much more precise ablation, all of which are attributed to the increased absorption coefficient and improved energy coupling. The most significant observation is that the surfaces of diamond and graphite did not undergo phase transformation, demonstrating that chemical cleanliness is increased with use of ultrashort pulses rather than nanosecond or longer pulses. In addition, thermal damage and the associated debris and recast layer formation were non-existant with ultrashort pulses. The investigation further showed that ultrashort pulsed lasers significantly reduced the feature size and improved the feature resolution, leading to sub-micron machining, which is not achievable in nanosecond or longer pulsed lasers. These experimental observations are consistent with predictions base on the finite difference and molecular dynamics models.
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机译：由于超短脉冲的极高强度（> 10 12 W / cm 2 ）和持续时间（<10），超短脉冲激光器与传统脉冲激光器产生了根本不同的烧蚀机制。 –12 s），比电子晶格的传输时间（> 10 –12 S）短。因此，在冷晶格中产生热激发等离子体。传统脉冲激光烧蚀中使用的假设对于超短脉冲无效。在这项工作中，对金刚石进行了超短脉冲消融的计算机建模。建立了一个两步模型，在该模型中，建立了一个传热，有限差分模型，并进行了紧密结合的分子动力学模拟，以评估消融事件的动力学。传热模型结合了电子对激光的吸收，并预测了从激光脉冲开始到1 ps的电子内部的热分布。紧密结合的分子动力学预测了在材料中发生结构变化和烧蚀所需的阈值电子温度（室温晶格）和整体平衡温度（在同一温度下的电子和晶格）。然后，将两个模拟的结果用于预测材料中的烧蚀阈值，烧蚀体积和热影响区的大小。为了验证模型预测，对化学气相沉积和单晶金刚石以及高度取向的热解石墨进行了超短脉冲激光烧蚀实验。使用扫描电子显微镜，原子力显微镜，轮廓仪和显微拉曼光谱来表征烧蚀表面。结果表明，与纳秒激光脉冲相比，超短脉冲产生的阈值通量更低，材料去除率更高，烧蚀更加精确，所有这些都归因于吸收系数的增加和能量耦合的改善。最重要的观察结果是金刚石和石墨的表面未发生相变，表明使用超短脉冲而非纳秒或更长的脉冲可提高化学清洁度。此外，超短脉冲不存在热损伤以及相关的碎屑和重铸层的形成。研究进一步表明，超短脉冲激光器显着减小了特征尺寸并提高了特征分辨率，从而导致了亚微米加工，这在纳秒或更长时间的脉冲激光器中是无法实现的。这些实验观察结果与基于有限差分和分子动力学模型的预测一致。
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