光学薄膜激光诱导损伤机理研究

基于电子密度演化模型,借助数值方法,研究了飞秒激光作用下光学薄膜内的电子密度演化过程,讨论了初始电子密度Ni和激光脉冲宽度τ对光学薄膜激光损伤阈值Fth的影响,分析了激光诱导薄膜损伤过程中MPI和AI的性质和作用.研究结果表明,对应于一定的脉宽,存在一个临界初始电子密度,当Ni低于这一临界密度时,Fth不受Ni影响；当Ni高于临界密度时,Fth随Ni增加而降低.临界初始电子密度随着脉宽的减小而增加。对于FS和BBS介质薄膜,Fth随脉宽的增加而升高。初始电子密度Ni对BBS中的MPI和AI基本没有影响；同样Ni对FS中的AI基本不产生影响,但当Ni>1011 cm－3时,FS中MPI电子密度随Ni增加而降低.在所研究的脉宽范围τ∈［0.01,5］ps,AI是FS介质激光诱导损伤的主要机制.而对于BBS,当脉宽τ∈［0.03,5］ps,AI是激光诱导损伤的主要机制；当脉宽τ∈［0.01,0.03］ps,MPI在激光诱导损伤中占主导地位.

Mechanism of Laser-Induced Damage in Optical Dielectric Films

Based on electron density evolution model, the electron density evolution process induced by femtosecond laser pulse in optical dielectric films was studied by means of numerical method. The effects of initial electron density Ni and laser pulse width τ on damage threshold fluence Fth were analyzed respectively. Both the nature of multi-photon ionization (MPI) and the role of avalanche ionization (AI) were also discussed. The numerical analysis results show that, for a certain pulse width τ, there is a critical initial electron density below which Fth is not affected by Ni and above which Fth decreases with the increase of Ni The critical density rises with the decrease of pulse width τ. For the two dielectric films, the damage threshold fluence Fth rises with the increase of τ. Initial electron density Ni has little effect on MPI and AI for BBS. For FS, Ni has likewise little effect on AI, and MPI electron density decreases with the further increase of Ni for Ni>1011cm-3. For FS, the laser-induced-damage is dominated by AI for τ>0.01 ps, while for BBS, and MPI takes over for pulse durationτ below 0.03ps.