基于Pokker-Planck方程, 建立一个描述短脉冲激光作用下电介质材料中导带电子能量分布随时间变化的模型, 用数值方法计算电子能量分布与电子数密度随时间的演化过程, 根据临界等离子体密度准则得到了不同激光脉冲宽度和波长下电介质材料(以SiO2为例)的破坏阈值。结果发现, 尽管激光波长通过3种途径对材料破坏阈值的确定产生影响, 但三者的共同作用导致在激光波长分别为1 060, 800, 532 nm时材料破坏阈值随脉宽变化曲线十分接近。讨论了碰撞电离、光致电离两种机制对材料激光破坏分别所起的作用: 当激光脉宽大于1 ps时, 碰撞电离对材料的破坏起主要作用; 当激光脉宽小于1 ps时, 光致电离对破坏阈值的影响越来越明显, 但是碰撞电离仍然不可忽略, 碰撞电离与光致电离同时起重要作用。

Based on the Fokker-Planck equation, a theoretical computational model is established to describe the change with time of the energy distribution of conduction electrons in a dielectric material under the irradiation of short laser pulses. Using this model, the evolution of electron density in a dielectric material (taking SiO2 glass as an example) is calculated; with the critical plasma density damage criterion, the material damage thresholds are determined for various laser wavelengths and pulse durations. For laser wavelengths of 1 060, 800 and 532 nm, the obtained damage threshold curves of SiO2 are rather close to each other. The role of impact ionization and photoionization in material damage is also investigated. It is found that when the laser pulse duration is greater than 1 ps, impact ionization dominates the material damage behaviour; when the laser pulse duration is shorter than 1 ps, the effect of photoionization becomes more and more notable, while with impact ionization still of significance, both types of ionization mechanisms play an important role.