Influence of SiO2 undercoat on the laser-induced damage threshold of 355Â nm LaF3/AlF3 multilayer reflectors

► LaF3/AlF3 HR films with and without SiO2 undercoat were prepared and characterized. ► Increased laser-induced damage threshold was obtained on samples with undercoat. ► Total stress and density of surface defects was reduced with undercoat. ► Undercoat benefit 10-layer-pair fluoride coatings by shielding interface defects. ► The improvement in 15-layer-pair coatings results from reduced density of defects.