Measurement of thin-film optical absorption at the air-film interface within the film and at the film-substrate interface

A measurement technique is described where the absorption introduced by the addition of a single-layer thin film is separated into three parts: absorption (a) within the bulk of the film; (b) at the air-film interface; (c) at the film-substrate interface. The technique employs a scanning adiabatic calorimeter to measure substrates which are partially coated on the entrance surface with films of continuously varying thickness. By measuring the absorption on the bare substrate and at various film thicknesses, and by making use of changes in relative power density within the film for λ/4 versus λ/2 optical thickness films, we have determined (a) αf, the bulk absorption coefficient of the film, (b) aaf, the specific absorptance of the air-film interface, and (c) afs, the specific absorptance of the film-substrate interface. The analysis assumes that the films are highly transparent and of known index of refraction. In addition, the film thickness must range from λ/4 to 3λ/4 optical thickness, or from λ/2 to λ optical thickness. Representative data are shown for absorptance measurements at 2.7 and 2.9 μm for a film of As2Se3 on CaF2. This example exhibits predominantly film-substrate interface absorption with very little of the absorption taking place in the bulk of the film.