Laser-induced modifications of HfO2 coatings using picosecond pulses at 1053 nm: Using polarization to isolate surface defects

For pulse lengths between 1 and 60 ps, laser-induced modifications of optical materials undergo a transition from mechanisms intrinsic to the materials to defect-dominated mechanisms. Elucidating the location, size, and identity of these defects will greatly help efforts to reduce, mitigate, or eliminate these defects. We recently detailed the role of defects in the ps laser-modifications of silica coatings. We now discuss the similar role of defects in HfO2 1/2-wave coatings and also include the environmental effects on the damage process. By switching between S and P polarizations, we distinguish the effects of defects at the surface from those throughout the material. We find that defects very near the surface are dependent on the environment, leading to worse damage in vacuum than in air. Air suppresses or lessens the effects of these defects, suggesting a photo-chemical component in the mechanism of laser damage in HfO2 coatings.