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

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 decay process 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 Nb2O5/SiO2 chirped mirrors excited by 800 nm, 1 kHz 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 Nb2O5 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.