高反Al_2O_3/SiO_2膜内超快激光预损伤动力学过程

Ultrafast laser pre-damage dynamics process in Al_2O_3/SiO_2 high reflectors

利用紫外飞秒激光光谱技术研究了Al_2O_3/SiO_2高反射膜内的超快载流子动力学。通过实验,发现该反射膜Al_2O_3层的载流子动力学在紫外反射膜的激光诱导损伤中起着至关重要的作用。通过泵浦-探测实验,发现紫外飞秒激光与光学薄膜作用后,光学薄膜反射率有所下降,且探测光反射率变化的峰值在约2.3 ps的时间内从417 nm左右转移到402 nm左右。为了更好的解释激光诱导载流子动力学,一个具体的理论模型被提出来,该模型指出导带自由电子弛豫过程中与晶格相互作用,产生距导带一个光子能量的中间缺陷态,其初始电子密度影响了材料损伤阈值高低。通过该理论模型得出的激光损伤阈值数据和实验数据吻合得很好。

Ultrafast carder dynamics in Al2O3/SiO2 high reflectors was investigated by UV femtosecond laser. It was identified by laser spectroscopy that, the carder dynamics contributed from the front few layers of Al2O3 played a dominating role in the initial laser-induced damage of the UV reflector. Time- resolved reflection decrease after the UV excitation was observed by pump-probe experiment, and the peak value of the variation of the reflectivity of the probe light changed from 417 nm to 402 nm in 2.3 ps. To interpret the laser induced carrier dynamics further, a specific theoretical model including multiphoton ionization（MPI）, avalanche ionization（AI）, and the mid-gap defect state was built to simulate the evolution process of the electron density in the conduction band, it pointed out that during the conduction band free electron relaxation process, the mid-gap defect state locating about one photon below the conduction band was formed because of the interaction between electrons and lattice. The initial electron density effect of the mid-gap defect state has important influence to the damage threshold of the Al2O3/SiO2 high reflectors. This model agrees very well with the experimental results.