Enhancing laser-induced damage threshold of picosecond laser mirrors using graded-refractive-index multilayer structures

Graded-refractive-index (n) multilayer architectures are examined as a means of improving the laser-induced damage threshold (LIDT) of picosecond laser mirrors. The Al2O3 and Al2O3/HfO2 nano-laminates are employed as high-n materials to form a mirror structure, where the n values of the high-n layers gradually increase along the direction of the electric-field attenuation. Compared with a conventional M-Constant mirror based on a fixed-n Al2O3–HfO2 mixture, the M-Graded mirror shows a 1.49-fold increase in LIDT under picosecond laser irradiation, while the reflection bandwidth above 99.5% reflectivity decreases by only ∼6.9%. Observation of laser-induced damage sites indicates that damage initiation is mainly associated with nanoscale absorbing defects on the coating surface, suggesting that redistribution of the material composition profile through graded-refractive-index multilayers helps suppress damage and improve the LIDT of high-power picosecond laser mirrors.