Absolute infrared absorption measurements in optical coatings using mirage detection

Absolute measurements of the optical absorption at lambda = 10.6 mu m of BaF2 thin film grown on a ZnSe substrate and bare ZnSe substrate surface were performed by collinear photothermal deflection technique. Absorption values were obtained in an absolute manner by fitting experimental data to the theoretical expressions of beam deflection for two particular cases: when thermal wave extends far from the laser spot and the opposite, when the heated region does not stretch beyond the laser spot. The validity of the theoretical models were tested in the range of modulation frequencies from 20 Hz to 2700 Hz. This method was also applied to the analysis of fused silica and sapphire bulk samples in order to be used as reference absorbing media in the infrared spectral range. Optical absorption of BaF2 thin film and ZnSe substrate surface were also deduced from the same theoretical approach using sapphire as a bulk reference medium. From both calculation methods an absorptance of 1200 ppm for RaF2 thin film and of 425 ppm for ZnSe plate surface were measured. (C) 1998 Elsevier Science S.A, All rights reserved.