Optical and structural properties of Nb2O5–SiO2 mixtures in thin films

Mixed material thin films are used in gradient index optical systems as their refractive index can be tailored to meet requirements. For the system to meet the optical performance requirements, it is necessary to relate the refractive index profile of the design to the volume fraction of the components of the mixture. Here we report that for electron-beam co-deposited Nb2O5–SiO2 mixtures, the most appropriate effective medium theory is Lorentz–Lorenz theory. The optical properties of a mixture are related to the structure of the composite. The structural properties of the mixtures studied justify the appropriateness of Lorentz–Lorenz theory for this kind of effective medium.

► For studied Nb2O5–SiO2 mixtures the most appropriate effective medium theory is Lorentz–Lorenz.
► Structural analysis shows that the materials are well mixed, with no separation in phases.
► After annealing at 750 °C the materials in the mixture show phase separation and crystallisation.
► Separation remains unfinished and size of phase grains too small for another theory to be effective.
► Lorentz–Lorenz theory should be built in algorithms for design of gradient index coatings.