[论文解读] Interaction of the ultra-short Bessel beam with transparent dielectrics: Evidence of high-energy concentration and multi-TPa pressure

Eugene G. Gamaly, Andrei V. Rode+7 more|arXiv (Cornell University)|Aug 28, 2017

Laser Material Processing Techniques参考文献 6被引用 8

一句话总结

本研究证明，超短贝塞尔光束可在蓝宝石等透明电介质中诱导极端能量集中和多兆帕（TPa）量级的压力，其效果超过传统高斯光束。该效应源于动态介电常数梯度形成零实部介电常数表面，从而聚焦光线并产生激波，进而在材料中形成大尺寸圆柱形空腔和压缩壳层，实现桌面级高压物理研究并推动新型物相的发现。

ABSTRACT

It has been proven that the intense tightly focused Gauss beam (GB) generates pressures in excess of a few TPa creating the novel super-dense phases of Aluminium and silicon [1-5]. Recently it was demonstrated that the Bessel beam (BB) focused inside sapphire produced the cylindrical void being two orders of magnitude larger than that generated by the GB [6-8]. Analysis of the experimental data presented below allows making the remarkable conclusions based solely on the void size measurements without any ad hoc assumptions about the interaction process. First, the void size is direct evidence of strong (>40%) absorption of the pulse energy. Second, it is a direct experimental evidence of the high-energy concentration in the central spike of the focus. The unique features of the intense Bessel beam interaction then allow understanding the experimental observation. This interaction generates early in the pulse time the spatial distribution of excited permittivity changing from positive to negative values. Then the light interacts with zero-real-permittivity surface, separating plasma and dielectric areas, which leads to high energy concentration near the axis of cylindrical focus up to several MJ/cm3 (pressure range of 4-8 TPa). The effect depends on the angle between the permittivity gradient and the field polarisation. High pressure generates intense cylindrical shock/ rarefaction waves, which led to formation of void and compressed shell. We demonstrate that the Bessel beam proves to be an effective tool for producing extreme pressure/temperature conditions on the laboratory tabletop. It appears that adjusting polarisation and permittivity gradient might be a novel way for increasing the maximum pressure. This tool allows for search of novel high-pressure material phases, for the 3D laser machining and for creating Warm Dense Matter as those in star cores.

研究动机与目标

• 研究超短贝塞尔光束与透明电介质的相互作用，探索极端能量集中与压力生成机制。
• 确定贝塞尔光束是否能在蓝宝石等材料中产生高于传统高斯光束的压力量级与更大空腔。
• 在不依赖人为假设的前提下，阐明能量集中与激波特性的物理机制。
• 探索贝塞尔光束作为生成新型高压物相与温稠密物质条件的实验工具的潜力。

提出的方法

• 对超短贝塞尔光束在强度超过10^14 W/cm²条件下辐照蓝宝石后空腔形成的实验分析。
• 利用空腔尺寸测量作为能量吸收与压力生成的直接指标，无需依赖理论模型。
• 对脉冲期间动态介电常数变化的理论建模，显示在光束轴线附近实部介电常数由正变负的转变。
• 识别出分离离子化区与介电区的零实部介电常数表面，实现强光场约束与能量集中。
• 分析能量集中对介电常数梯度方向与电场偏振方向之间夹角的依赖性。
• 通过数值模拟与实验验证激波特性的动力学，揭示圆柱形空腔与压缩壳层的形成机制。

实验结果

研究问题

• RQ1 超短贝塞尔光束是否可在无需聚焦的条件下，在透明电介质中产生超过4 TPa的压力？
• RQ2 何种物理机制使得在贝塞尔光束辐照下蓝宝石中出现高能量集中与空腔形成？
• RQ3 光束偏振方向相对于介电常数梯度的取向如何影响能量局域化与压力生成？
• RQ4 空腔尺寸在多大程度上可作为吸收脉冲能量与峰值压力的直接实验测量指标？
• RQ5 贝塞尔光束能否作为可扩展平台，在实验室中实现温稠密物质条件的生成？

主要发现

• 贝塞尔光束在蓝宝石中产生了尺寸比高斯光束在相同条件下形成的空腔大两个数量级的圆柱形空腔。
• 空腔尺寸测量提供了直接实验证据，表明入射脉冲能量的吸收率超过40%。
• 能量集中程度可达每立方厘米数兆焦耳（MJ/cm³）量级，对应4–8 TPa范围的压力。
• 脉冲过程中零实部介电常数表面的形成实现了强光场约束，并增强了光束轴线附近的能量集中。
• 压力生成对电场偏振方向与介电常数梯度之间夹角高度敏感，表明该机制具有可调谐性。
• 该相互作用产生了强烈的圆柱形激波与稀疏波，导致压缩壳层与大尺寸中心空腔的形成。