A model for calculating the effect of nanosized pores on refractive index, thermal shift and mechanical stress in optical coatings

A simple model is presented that relates optical and mechanical properties of optical coating materials deposited by typical physical vapour deposition techniques to the porosity of the layers. In terms of this model, a clear correlation between thermal shift, mechanical stress and refractive index could be simulated. As a practical conclusion, it is found that a certain fraction of small nanosized pores in the films is essential in order to get a valuable balance between optical and mechanical coating properties.