Linz Norbert, Freidank Sebastian, Liang Xiao-Xuan, Vogel Alfred
Institute of Biomedical Optics, University of Luebeck, Peter-Monnik Weg 4, 23562 Luebeck, Germany.
Rep Prog Phys. 2025 Jul 31;88(8). doi: 10.1088/1361-6633/adedb3.
We present an in-depth analysis of the energy dependence of optical breakdown in water by tightly focused laser pulses, from plasma formation to shock waves and cavitation. Laser pulses of fs to ns durations and UV to IR wavelengths are aberration-free focused through microscope objectives. Photography captures luminescent plasmas with submicrometer resolution, and bubble threshold and size are determined via probe beam scattering. The energy dependence of mechanical effects is quantified through the maximum bubble radius. We find three key scenarios depicting the interaction between multiphoton and avalanche ionization, recombination, and thermal ionization from nanoeffects near threshold to extreme energy densities. They include a previously unknown scenario that emerges with single-longitudinal-mode UV ns pulses from compact lasers. It enables cost-effective creation of nanoeffects, as demonstrated on corneal tissue and glass. High-resolution color photography revealed new insights in the spatiotemporal dynamics of plasma formation, with an interplay of breakdown waves, string formation by local instabilities of avalanche ionization, and radiative energy transport. Plasma volume data from photographs together with absorption measurements show that the average energy density of luminescent fs and ns plasmas is similar, ranging between 10 and 40 kJ cm. However, small hot regions with up to 400 kJ cmare formed in ns breakdown. From the hot regions, energy is spread out via x-ray bremsstrahlung, forming a luminescent halo. Well above threshold,scales withacross all scenarios, with 15%-20% conversion of laser energy into bubble energy. With increasing plasma energy density, an ever-larger energy fraction is converted into shock wave energy (75% at 40 kJ cm). We discuss guidelines for parameter selection in laser surgery and material processing in bulk media as well as for laser ablation and breakdown spectroscopy in liquids. Finally, we suggest roadmaps for future experimental and modeling work, and for broadening applications.
我们深入分析了紧聚焦激光脉冲在水中产生光学击穿时的能量依赖性，涵盖从等离子体形成到冲击波和空化的过程。持续时间从飞秒到纳秒、波长从紫外到红外的激光脉冲通过显微镜物镜进行无像差聚焦。摄影以亚微米分辨率捕捉发光等离子体，并通过探测光束散射确定气泡阈值和尺寸。通过最大气泡半径对机械效应的能量依赖性进行量化。我们发现了三种关键情况，描述了多光子与雪崩电离、复合以及从阈值附近的纳米效应到极端能量密度的热电离之间的相互作用。其中包括一种此前未知的情况，它出现在紧凑型激光器发出的单纵模紫外纳秒脉冲中。如在角膜组织和玻璃上所展示的那样，它能够经济高效地产生纳米效应。高分辨率彩色摄影揭示了等离子体形成时空动力学的新见解，包括击穿波的相互作用、雪崩电离局部不稳定性导致的弦形成以及辐射能量传输。照片中的等离子体体积数据与吸收测量结果表明，发光飞秒和纳秒等离子体的平均能量密度相似，范围在10至40千焦/立方厘米之间。然而，在纳秒击穿中会形成能量密度高达400千焦/立方厘米的小热点区域。能量从这些热点区域通过X射线韧致辐射扩散开来，形成一个发光晕。远高于阈值时，在所有情况下 与 成比例，有15% - 20%的激光能量转化为气泡能量。随着等离子体能量密度的增加，转化为冲击波能量的能量份额越来越大（在40千焦/立方厘米时为75%）。我们讨论了在激光手术、块状介质中的材料加工以及液体中的激光烧蚀和击穿光谱学中参数选择的指导原则。最后，我们提出了未来实验和建模工作以及拓展应用的路线图。
