Influence of the number of double layers on the damage threshold of Al2O3/SiO2 and LaF3/MgF2 mirrors at 248 nm

KrF-laser induced damage thresholds (LIDT) of HR stacks were investigated as a function of the number of quarterwave layers. The findings were interpreted in terms of calorimetric absorption measurements and scatter defect density counts as well as an analysis of the standing wave electric field. Higher numbers of high-purity Al203/SiO2 layers resulted in enhanced LIDT by shielding the substrate surface increasingly well. Contrary to this, LIDT decreased waith increasing numbers of e-beam evaporated LaF3/MgF2 layers. This was accompanied by elevated absorptance, defect density and conditionability at higher stack numbers.