飞秒激光作用下啁啾镜膜层内损伤相关动力学

Pre-damage dynamics of dielectric chirped mirror film layer excited by femtosecond laser

光学元器件的激光损伤问题,一直以来都困扰着超强超短激光系统的进一步发展。飞秒激光领域,激光脉冲引起的光学器件损伤主要由材料的本征特性决定。光学材料内的多光子电离、雪崩电离、导带电子弛豫等一系列非线性过程,与材料的激光损伤过程密切相关。利用泵浦探测技术,采用中心波长为800 nm,重复频率1 k Hz的飞秒激光脉冲,对Nb2O5/Si O2啁啾镜介质膜的内部与飞秒激光损伤相关的超快动力学进行了研究。发现强的泵浦光脉冲辐照结束后飞秒乃至几十皮秒的范围内,啁啾镜对探测光的反射率有一定程度的下降。反射率降低的主要原因是泵浦光在介质膜的Nb2O5层内激发的大量的自由电子对探测光吸收所致,且该过程对激光诱导损伤过程起主导作用。通过反射率的变化,对其介质膜内导带电子弛豫过程进行探究,测定得到其衰减寿命,分别为1.31、6.88、22.34 ps。

Femtosecond laser-induced damage of optical components seriously restricts the development of ultrashort and ultra-intense laser systems with ever higher average powers and peak intensities. In the ultrashort pulse regime, the damage of optical materials is closely related to the nonlinear ionization processes inside the materials, such as the multiphoton ionization, the avalanche ionization, and the decayprocess of electrons in the conduction band. In other word, the femtosecond laser induced damage in optical components display highly deterministic damage performance. To investigate the laser induced damage inside the chirped mirrors, ultrafast carrier dynamics in Nb2 O5/Si O2 chirped mirrors excited by 800 nm, 1 k Hz femtosecond laser had been investigated by pump-probe technique. Reflectance of dielectric chirped mirrors was found reduced after pump pulse excitation femtosecond and dozens of picosecond range. The reduction is mainly ascribed to the absorption of conduction electrons inside the Nb2 O5 layers in dielectric chirped mirror film, which plays an important role in laser induced damage. To investigate the ultrafast dynamics in dielectric film in detail, the relaxation lifetimes of electrons in the conduction band has been measured as 1.31 ps, 6.88 ps, and 22.34 ps, respectively.