Suppression of nano-absorbing precursors and damage mechanism in optical coatings for 3ω mirrors.

Damage precursors in the 3ω (351 nm) mirror for a high-power laser system are investigated as well as the relevant damage mechanisms. The precursors are classified into two ensembles according to the different laser resistance and damage features. The former is nano-absorbing precursors, which are sensitive to the standing wave electric field and vulnerable to the laser irradiation. The latter is submicrometer nodular defects, which have higher laser resistance and are sensitive to the adhesion strength between the fluoride coatings and oxide coatings. The damage due to nano-absorbing precursors is efficiently suppressed with the double stack design that screens the electric field in the oxides. Currently, the nodular seed is major originating from the Al2O3/SiO2 stack. Even for the same defect type and mirror, the final damage features are dependent on the local mechanical properties at the irradiation location. The investigations of the damage mechanisms provide a direction to further improve the laser-induced damage threshold of the 3ω mirror.