Al2O3 coatings for 193nm:: A nonlinearly absorbing material

New absorption measurements for aluminum oxide optical coatings at 193nm are presented. Apart from the strong linear absorption at this wavelength the data indicate a nonlinear absorption within the thin dielectric layer which increases nonlinearly with the layer thickness. By varying the layer thickness, the intrinsic contribution of the layer material to the overall absorption was separated from the contribution of the substrate and the interface. In addition, the conditioning behaviour of the coatings was examined. A strong long term conditioning in the linear absorption was found for Al2O3 containing systems. Comparing the absorption and conditioning behaviour of the single layers and a high-reflective system, we can show that the absorption properties of the HR-system are determined by its Al2O3 layers.