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    • Electron‐Ion Relaxation, Phase Transitions, and Surface Nano‐Structuring Produced by Ultrashort Laser Pulses in Metals

    N. A. Inogamov,V. V. Zhakhovsky,Yu. V. Petrov,V. A. Khokhlov,S. I. Ashitkov,K. V. Khishchenko,K. P. Migdal,D. K. Ilnitsky,Yu. N. Emirov,P. S. Komarov,V. V. Shepelev,C. W. Miller,I. I. Oleynik,M. B. Agranat,A. V. Andriyash,S. I. Anisimov,V. E. Fortov

    Contributions to Plasma Physics（2013）

    Russian Acad Sci

    Cited 43|Views27
    Abstract
    Fundamental physical phenomena in metals irradiated by ultrashort laser pulses with absorbed fluences higher than few tens of mJ/cm(2) are investigated. For those fluences, laser-produced electron distribution function relaxes to equilibrium Fermi distribution with electron temperature T-e within a short time of 10-100 fs. Because the electron subsystem has T-e highly exceeding much the ion subsystem temperature T-i the well-known twotemperature hydrodynamic model (2T-HD) is used to evaluate heat propagation associated with hot conductive electron diffusion and electron-ion energy exchange. The model coefficients of electron heat conductivity (?, T-e, T-i) and electron-ion coupling parameter (?, T-e) together with 2T equation of state E (?, T-e, T-i) and P (?, T-e, T-i) are calculated. Modeling with 2T-HD code shows transition of electron heat wave from supersonic to subsonic regime of prop-agation. At the moment of transition the heat wave emits a compression wave moving into the bulk of met al. Nonlinear evolution of the compression wave after its separation from the subsonic heat wave till spallation of rear-side layer of a film is traced in both 2T-HD modeling and molecular dynamics (MD) simulation. For fluences above some threshold the nucleation of voids in frontal surface layer is initiated by strong tensile wave following the compression wave. If the absorbed fluence is approximate to 30 % above the ablation threshold than void nucleation develops quickly to heavily foam the molten met al. Long-term evolution of the metal foam including foam breaking and freezing is simulated. It is shown that surface nano-structures observed in experiments are produced by very fast cooling of surface molten layer followed by recrystallization of supercooled liquid in disintegrating foam having complex geometry. Characteristic lengths of such surface nanostructures, including frozen pikes and bubbles, are of the order of thickness of molten layer formed right after laser irradiation. ((c) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
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    Laser-matter interaction,phase transitions,surface nano-structuring produced by femtosecond pulses
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    【要点】：该论文研究了用超短激光脉冲辐照金属时，当吸收的激光剂量高于几十毫焦耳每平方厘米时，基本的物理现象。研究采用了双温度流体动力学模型（2T-HD）来评估与热传导电子扩散和电子-离子能量交换有关的热传播。发现了电子热波从超音速传播过渡到亚音速传播时会发射压缩波，并在一定条件下，金属表面层会出现空洞和纳米结构。
    

    【方法】：采用了双温度流体动力学模型（2T-HD）和分子动力学（MD）模拟来研究压缩波在金属中的传播及其对表面层的影响。
    

    【实验】：实验使用分子动力学（MD）模拟和双温度流体动力学模型（2T-HD）来研究超短激光脉冲辐照下金属中的热波传播，发现了一定的激光剂量下，电子热波会从超音速过渡到亚音速，并产生压缩波，进而在金属表面产生纳米结构。实验结果显示，表面纳米结构的产生与激光辐照后金属表面熔层快速冷却并重新结晶有关。