初始电子密度对光学薄膜激光损伤机制的影响

光学元件的激光损伤问题是激光器件向高功率密度方向发展中必须认识和克服的问题.基于Forkker-Planck方程,研究了激光与材料相互作用时的雪崩电离机制、多光子电离机制以及联合两种机制的情况.雪崩电离的产生需要一定密度的初始自由电子存在,该自由电子可以是材料中原本就存在的,也可能是光电离产生的.着重分析了材料中的初始自由电子对材料电离机制的影响.结果表明,雪崩过程在激光作用一段时间后会达到一个稳定的电离阶段（以自由电子平均能量不随时间变化为特征,且此时雪崩电离为材料电离的主导机制）,该时间与光电离速率、材料中初始自由电子密度有关.材料中的初始自由电子可以在一定程度上掩盖光电离的作用效果.
关键词:初始电子;激光损伤;光电离;雪崩电离

The Influence of Inherent Seed Electron on Laser Induced Damage Mechanism of Optical Films*

The interaction of a short or ultrashort pulse and optical films is a hot research field.Prevalent opinion is that the breakdown mechanism of optical films induced by laser pulse is avalanche ionization or photon ionization or the combination of them.Based on Forkker-Planck equation, these damage mechanisms were studied.The theoretic results indicate that it needs some time for a stable avalanche ionization process to form when avalanche ionization dominates the damage process.This stable avalanche ionization is characterized by the unaltered average electronic energy.The damage mechanism of optical films affected by the electrons induced by photon ionization in the formation phase of the stable avalanche ionization were studied.Besides, as the focuses of this article, for the purpose of comparison, the influence of the number density of the inherent seed electrons in optical films on damage mechanism of optical films was studied also.Results reveal that the relative magnitude of the inherent electrons and the electrons induced by photon ionization before the formation of the stable avalanche ionization determines the damage mechanism of optical films.