对1064 nm纳秒激光辐照下熔石英元件后表面损伤过程进行了时间分辨诊断。利用基于偏振原理的单发双帧阴影成像系统捕捉了脉冲上升沿开始到数百纳秒尺度内的瞬态材料响应,并结合剪切干涉成像系统分析了空气端等离子体微喷现象。结果表明,损伤在脉冲上升沿就已经发生,此时空气端等离子体的膨胀速度高达20 km/s,同时材料内部也存在高速扩张的不透明损伤区域,但其扩张过程在脉冲结束后迅速停止；损伤发生后数十纳秒后,空气端出现大量中性物质喷发。基于激光支持的固态吸收波前模型与相爆炸理论对这些现象进行了讨论。对空气端等离子体扩张过程进行了数值模拟,结果表明空气端等离子体压强、温度与密度等参数值随延迟增加迅速下降,其瞬态压强最高达600 MPa；模拟结果还预测了向内扩张的内激波的形成。研究结果对理解熔石英元件的损伤机理有重要意义。%Material response and the launch of laser plasma during the 1064 nm nanosecond laser pulse induced damage to the exit surface of fused silica are investigated. Employing a polarization-based two-frame shadowgraphy setup with∼60 fs probing resolution, the transient material responses from the rising part of nanosecond pumping pulse to several hundred nanosecond timescale are captured. Using a shearing interferometry setup, the evolution of transient phase shift of laser plasma in the expansion process to the ambient air is also investigated. Inhomogeneous distribution of phase shift caused by the electrons and neutrals in the plasma is quantitatively resolved by employing the fast Fourier transform based filtering algorism. To demonstrate the evolutions of important plasma parameters such as pressure, temperature and density, a continuum hydrodynamic model is numerically solved. The initial pressure of plasma is estimated according to the point-explosion model, and the initial plasma temperature is achieved by calculating the difference between simulating shockwave front radius and experimental value at the same delay. The optimal temperature is chosen when the radius difference is minimal. Main conclusions are as follows. 1) Abundant suprathermal electrons are excited in the early energy deposition process. Part of these electrons contribute to the thermal transport process and produce the laser supported solid-state absorption front (LSSAF) which propagates into the bulk silica. Other electrons escape to the air side and contribute to the formation of air plasma through the impact ionization process. Plasma expansion speed is about 20 km/s during this phase. 2) When the pump pulse is terminated, the LSSAF and air plasma lose their energy supplied and experience a rapid decline of the temperature and expansion velocity. As a result, the final damage crater depth exhibits seldomly no increase compared with the transient crater depth during this phase. Hot bulk plasma formed in this phase becomes the damage precursor and induces the ejection of abundant neutrals probably due to the phase explosion mechanism. Inhomogeneous distribution of stress is formed by Rayleigh-Taylor instability at the interface between hot bulk plasma and surrounding bulk material during the expansion of LSSAF. Radial and circumferential cracks are formed due to the release of stress. 3) Evolution of air plasma follows the conventional evolution process of laser-induced plasma, i. e. , internal pressure, temperature and density decrease quickly with time delay. The simulated transient highest pressure is about 600 MPa. Simulation also predicts the formation of the internal shockwave. Our work will be helpful in understanding the laser damage mechanism of the fused silica optical window.
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