Interfacial optical absorption in TiO_2–SiO_2 multilayer coatings prepared by rf magnetron sputtering

A narrow bandpass filter consisting of nineteen alternate layers of TiO2 and SiO2 was fabricated by rf magnetron sputtering without thickness monitoring. Analysis of the measured transmittance spectrum suggests that energy loss exists at interlayer boundaries. A digital thin film consisting of 182 alternate layers showed additional absorption in the visible to near-infrared range. The additional absorption disturbs fabrication of flip-flop digital thin films by rf sputtering, and it might be a cause of the laser-induced damage in optical coatings which has been a problem for a long time.