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Free-electron generation in laser-rradiated dielectrics

机译：激光辐照电介质中的自由电子生成

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When transparent solids are irradiated with laser intensities above a certain threshold, strong absorption of laser energy occurs. The increasing absorptivity is caused by the formation of a free electron gas in the conduction band of the dielectric. The transient free electron density is a fundamental parameter for numerous theoretical and experimental investigations and applications. We study the mechanisms of free-electron generation in the frame of different approaches. A full kinetic treatment reveals a non-stationary behavior, which is neglected when applying the standard rate equation. A new model, the multiple rate equation, keeps track of the non-stationarity of the electron energy distribution while maintaining the conceptual and analytic simplicity of standard rate equation. We present the analytical asymptotic solution of the multiple rate equation which yields an expression for the avalanche coefficient and provides information about the validity of the standard rate equation. The numerical calculation shows the transient distribution of free electrons and the effect of the non-stationarity of its shape on the impact ionization probability. We study the role of different ionization processes and its dependence on laser pulse duration. The fraction of impact-ionized electrons is found to depend only on the product of pulse duration and intensity, i.e. on the fluence. A remarkable effect of the shape of the laserpulse on the total free electron density and the conditions for dielectric breakdown is found.
机译：当透明固体被超过一定阈值的激光强度照射时，会强烈吸收激光能量。吸收率的提高是由于在电介质的导带中形成了自由电子气。瞬态自由电子密度是众多理论和实验研究与应用的基本参数。我们在不同方法的框架内研究自由电子产生的机理。完全的动力学处理显示出不稳定的行为，在应用标准速率方程时可以忽略不计。一个新的模型，即多重速率方程，在保持标准速率方程的概念和分析简单性的同时，可以跟踪电子能量分布的非平稳性。我们提出了多重速率方程的解析渐近解，它给出了雪崩系数的表达式，并提供了有关标准速率方程有效性的信息。数值计算表明了自由电子的瞬态分布及其形状的非平稳性对碰撞电离几率的影响。我们研究了不同电离过程的作用及其对激光脉冲持续时间的依赖性。发现碰撞离子化电子的分数仅取决于脉冲持续时间和强度的乘积，即取决于注量。发现了激光脉冲的形状对总自由电子密度和介电击穿条件的显着影响。

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