Time- and Position-Dependent Breakdown Volume Calculations to Explain Experimentally Observed Femtosecond Laser-Induced Plasma Properties
Time- and Position-Dependent Breakdown Volume Calculations to Explain Experimentally Observed Femtosecond Laser-Induced Plasma Properties
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DOI：
10.1021/acsphotonics.2c00746
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发表时间：
2023-02
期刊：
影响因子：
7
通讯作者：
A. M. Hossain;M. Ehrhardt;Martin Rudolph;P. Lorenz;D. Kalanov;K. Zimmer;A. Anders
中科院分区：
文献类型：
--
作者：
A. M. Hossain;M. Ehrhardt;Martin Rudolph;P. Lorenz;D. Kalanov;K. Zimmer;A. Anders
飞秒激光脉冲在气体中的聚焦可以产生不同的气体击穿现象，这取决于聚焦条件：从简单的光学击穿（如激光“火花”）到非线性光学击穿（如折射）。脉冲曝光后此类等离子体的动态特性取决于激光在击穿体积中沉积的能量。使用的时间和位置相关的击穿模型，我们估计的击穿体积，并表明，由激光沉积在这个击穿体积的能量决定了在脉冲曝光制度后的激光诱导等离子体的特性。实验上，我们发现，对于不同的焦距，存在一个阈值的能量密度，超过该阈值可以观察到从椭球形到球形的过渡，然后由所产生的等离子体的环形膨胀。当电子密度和电子温度表示为能量密度的函数时，观察到偏离对辐照度的抛物线依赖性。它们意味着当激光脉冲的峰值功率高于空气中自聚焦的临界功率时，在等离子体体积中发生多光子电离的额外电离。这项实验和理论研究的相关性是为了防止在材料加工过程中出现不期望的自聚焦条件，这是朝着在没有激光烧蚀的情况下良好控制的激光等离子体蚀刻迈出的一步。
Focusing of femtosecond laser pulses in gases can produce different gas breakdown phenomena depending on the focusing conditions: from simple optical breakdown like laser “sparks” to a nonlinear optical breakdown like filamentation. The dynamics of such plasmas after the pulse exposure is dependent on the energy deposited by the laser in the breakdown volume. Using a time- and position-dependent breakdown model, we estimate the breakdown volume and show that the energy deposited by the laser in this breakdown volume determines the characteristics of laser-induced plasma in the post-pulse exposure regime. Experimentally we find that for different focal lengths there exists a threshold value of the energy density beyond which a transition from an ellipsoidal shape to a spherical shape can be observed, followed by a toroidal expansion of the produced plasma. When electron density and electron temperature are expressed as a function of the energy density, deviations from the parabolic dependence on irradiance are observed. They imply additional ionization by multiphoton ionization in the plasma volume that occurs when the peak power of the laser pulse is above the critical power for self-focusing in air. The relevance of this experimental and theoretical study is to prevent undesired self-focusing conditions during material processing, a step toward well-controlled laser-plasma etching without laser ablation.