Enhancing the expansion of a plasma shockwave by crater-induced laser refocusing in femtosecond laser ablation of fused silica
Enhancing the expansion of a plasma shockwave by crater-induced laser refocusing in femtosecond laser ablation of fused silica
复制标题
通过飞秒激光烧蚀熔融石英中的火山口诱导激光重聚焦增强等离子体冲击波的扩展
DOI：
10.1364/prj.5.000488
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发表时间：
2017-10-01
影响因子：
7.6
通讯作者：
Lu, Yongfeng
中科院分区：
文献类型：
--
作者：
Wang, Qingsong;Jiang, Lan;Lu, Yongfeng
使用时间分辨阴影成像技术研究了熔融石英的两个飞秒激光脉冲烧蚀过程中等离子体和冲击波膨胀的动力学。实验结果表明，在对第一脉冲引起的弹坑进行第二次脉冲照射时，等离子体和冲击波在纵向上的膨胀增强。结合等离子体模型和菲涅尔衍射理论，考虑表面形貌和瞬态材料特性的变化来计算激光强度分布。理论结果表明，脉冲上升沿诱发的自由电子密度达到临界密度后，原本透明的表面转变为瞬态高反射率表面（金属态）。因此，具有凹透镜状形态的凹坑可以极大地反射和重新聚焦激光脉冲的后半部分，从而产生强度甚至高于入射强度的强激光场。这种强烈的重新聚焦激光脉冲会导致更强的激光诱导空气击穿，并增强等离子体和冲击波的后续膨胀。此外，在凹微透镜的单脉冲烧蚀中也记录了类似的阴影图，为增强机制提供了实验证据。
The dynamics of plasma and shockwave expansion during two femtosecond laser pulse ablation of fused silica are studied using a time-resolved shadowgraph imaging technique. The experimental results reveal that during the second pulse irradiation on the crater induced by the first pulse, the expansion of the plasma and shockwave is enhanced in the longitudinal direction. The plasma model and Fresnel diffraction theory are combined to calculate the laser intensity distribution by considering the change in surface morphology and transient material properties. The theoretical results show that after the free electron density induced by the rising edge of the pulse reaches the critical density, the originally transparent surface is transformed into a transient high-reflectivity surface (metallic state). Thus, the crater with a concave-lens-like morphology can tremendously reflect and refocus the latter part of the laser pulse, leading to a strong laser field with an intensity even higher than the incident intensity. This strong refocused laser pulse results in a stronger laser-induced air breakdown and enhances the subsequent expansion of the plasma and shockwave. In addition, similar shadowgraphs are also recorded in the single-pulse ablation of a concave microlens, providing experimental evidence for the enhancement mechanism.