Thermally induced porosity in CSD MgF2-Based optical coatings:: An easy method to tune the refractive index

A very simple and robust method is described to prepare MgF2-based optical thin films exhibiting refractive indices ranging from 1.08 to 1.2. These coatings were deposited on solid substrates through chemical solution deposition from a single and extremely stable (up to few years old) sol-gel solution composed of magnesium acetate and trifluoroacetic acid dissolved in a mixture of ethanol and water. The present work shows that the generation of the porosity responsible for the ultralow refractive indices of these materials is triggered by the thermal decomposition of the metallic precursor ligands. The resulting nanobubbles of gas are then frozen-in by the thermally induced condensation and partial crystallization of the mineral network around them. We demonstrate that such a process is governed, and can be perfectly controlled, by the heating rate and the atmospheric water content applied during calcination, through which nanobubble size, quantity, and the material's chemical makeup are tuned. The resulting vesicle-like porous materials demonstrate a range of tunable compositions with varying refractive indices and combine good mechanical properties with high chemical and thermal resistance. This work is supported by a novel thermal ellipsometry analysis (TEA) method that allows for in situ monitoring of the films' optical constants, structure, and thickness during thermal treatment. This is the first time, to our knowledge, that this technique has been used in the dynamic characterization of evolving unstructured porosity, allowing valuable insight into the thermal decomposition, condensation, crystallization, and sintering processes occurring during calcination.