Laser-induced damage of 355 nm high-reflective mirror caused by nanoscale defect

Al2O3/SiO2 multilayer high-reflective (HR) mirrors at 355 nm were prepared by electron beam evaporation, and post-irradiated with Ar/O mixture plasma. The surface defect density, reflective spectra, and laser-induced damage characteristics were measured using optical microscopy, spectrophotometry, a damage testing system, and scanning electron microscopy (SEM), respectively. The results indicated that moderate-time of irradiation enhanced the laser-induced damage threshold (LIDT) of the mirror, but prolonged irradiation produced surface defects, resulting in LIDT degradation. LIDT of the mirrors initially increased and subsequently decreased with the plasma processing time. SEM damage morphologies of the mirrors revealed that nanoscale absorbing defects in sub-layers was one of the key factors limiting the improvement of LIDT in 355 nm HR mirror.