The Photothermal Deflection Technique is used for mapping absorption in optical coatings. We have observed a strong influence of the nature of the substrate material on measured absorptance. Different single layer films deposited in the same conditions, at the same time, on different substrates located at the same distance from the rotating axis, have very different absorptances: the films deposited on fused silica or calcium fluoride substrates absorb weakly while those deposited on glass substrates (BK7, C2036*, D2050*) can absorb five to fifty times more. This effect is observed with different coating materials: TiO2, Ta2O5, SiO2 prepared by different techniques: I.A.D., I.P. and E.B.. We interpret these phenomena in terms of film contamination by the substrate: metallic ions present in glass substrates can be responsible for enhanced absorptance. A simple model taking into account thin absorbing interface layers is developed. We determine both interface and bulk absorptance of the studied thin films.
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