Determination of thermal and elastic coefficients of optical thin-film materials

In the aim of describing the thermo-mechanical behaviour of a thin-film filter, simplified models have been studied. An easy way is to determine the evolution of the optical properties of each layer composing the stack, and thus to derive the features change at the component level. Therefore, the knowledge of physical parameters describing each material, such as the coefficient of thermal expansion, the thermo-optic coefficient, the Poisson's ratio and the elasto-optic coefficients, is required. The main challenge is to evaluate these parameters at a single layer level. We propose here a new optical method, based on the analysis of the thermal behaviour of two dedicated Fabry-Perot (FP) structures including a thin disk of the material under study. In parallel, we show through modelling, that we can determine the physical properties of this material with a high accuracy, only by measuring the shift of FP resonance wavelengths. However, we have to take into account the mechanical deformation of the Fabry-Perot structures induced by the thin-film deposition process as well as its evolution with the temperature change (thermal stresses). In this goal, we carried out an accurate study of the thermo-mechanical behaviour of our interferometric structures by using a Finite Element Method.