Fused silica is an excellent material for laser optics: not only is it highly transmissive from the near-infrared to the ultraviolet, but it has a very high bulk dielectric breakdown threshold, meaning that pulse energies have to be very high to damage high-quality fused silica internally. But the laser-induced damage threshold (LIDT) for the surface of fused silica is quite a bit lower than for its interior. One reason for this is the surface damage inflicted by optical grinding and polishing of the fused silica. Surface treatments that include magnetorheological finishing (MRF), etching with hydrofluoric (HF) acid, ion-beam sputtering, and others can improve the post-polished surface quality of fused silica, thus raising its surface LIDT.
Scientists at the National University of Defense Technology (Changsha, China) and the China Aerodynamics Research and Development Center (Mianyang, China) have homed in on some of these surface-damage-reducing processes, doing some experiments to characterize them. Using a frequency-tripled Nd:YAG laser with a 7 ns pulse duration at a 1 Hz repetition rate, the group experimentally found that the LIDT of a fused-silica (Heraeus 312) surface already treated with HF acid to raise its damage threshold can be raised a further 30% by a round of ion-beam sputtering (IBS), but only if the IBS is done to a depth <1 μm. If done any further, the IBS causes additional chemical defects that start lowering the LIDT. Also, if an IBS-treated fused-silica surface is then immersed in deionized water, the LIDT drops by about 20%. The results may help to optimize the treatment of polished fused-silica surfaces to maximize their LIDT. Reference: M. Xu et al., Opt. Express (2017); https://doi.org/10.1364/oe.25.029260.