High-index and low-index thin-film antireflection (AR) coatings for use at 355 nm have been tested to determine their resistance to laser damage. All coatings consisted of dielectric materials which were vacuum deposited onto bowl-feed polished Suprasil II by evaporation. Laser damage studies were made by irradiating several sites on each sample with a 0.6 ns, 355 nm laser pulse. Each site was irradiated by only one pulse.
High-index coatings were made with both a high-index material and a low-index material. The high-index materials chosen for this study were scandia (Sc2O3), hafnia (HfO2), and zirconia (ZrO2), while the low-index materials were silica (SiO2) and magnesium fluoride (MgF2). All coatings had four non-quarterwave thick layers plus a halfwave-thick undercoat (barrier layer) of the low-index material. Average damage threshold values for four samples of each type ranged from 2.4 J/sq. cm to 3.1 J/sq. cm. Coatings made from scandia/magnesium fluoride had the highest damage thresholds.
Low-index coatings contained only low-index materials. The low-index materials chosen for this study were samarium fluoride (SmF3), lanthanum fluoride (LaF3) thorium fluoride (ThF4), silica, magnesium fluoride, and lithium fluoride (LiF). AR coatings containing three to six quarterwave-thick layers plus a halfwave-thick barrier layer were made from nine combinations of these six materials. Average damage threshold values for four to six samples of each type ranged from < 1.0 J/sq. cm to 2.8 J/sq. cm. Coatings made from samarium fluoride/magnesium fluoride had the highest damage thresholds.
One set of four scandia/silica coatings was measured at both 355 nm and 248 nm. Damage thresholds at 248 nm for these parts were determined using 20 ns laser pulses. The average threshold was 4.2 J/sq. cm.