Influence of nodular defects on the laser damage resistance of optical coatings in the femtosecond regime.

The influence of nodular coating defects on the sub-picosecond laser damage resistance of multilayer coatings is investigated. The study is conducted on engineered nodules from monodisperse silica microspheres in HfO2/SiO2 high-reflective mirrors, at 400  fs/1030  nm. We demonstrate through an experimental study coupled with 3D finite-difference time-domain simulations that nodules in dielectric multilayer coatings are a main concern for the damage resistance of femtosecond optics. The nodules, and hence possibly other defects that produce E-field enhancement in coating materials, induce damage initiation at very low fluences (0.1  J/cm2 in the case under study) compared to the intrinsic damage threshold of the component (1.4  J/cm2 for the present mirror). After initiation, the damage sites grow catastrophically at a determined threshold, reducing significantly the damage resistance (0.6  J/cm2) but allowing a "safe" operating fluence to be defined.