Computational Investigation of Optical Properties of Oxygen Non-Stoichiometry in Amorphous TiO₂:GeO₂

Amorphous metal oxides are ubiquitous in optical thin-films. Mixtures of amorphous TiO₂:GeO₂ (a-TiO₂:GeO₂) have high, tunable refractive indices and can exhibit mechanical loss 4x lower than state-of-the-art a-TiO₂:Ta₂O₅ films, making them excellent candidates for high-index layers in interference coatings for the Laser Interferometer Gravitational-wave Observatory (LIGO). However, inconsistent optical loss between a-TiO₂:GeO₂ samples presents a barrier to implementation in LIGO. While various sources have been proposed—including transition metal impurities and oxygen non-stoichiometry—the scope of understood loss mechanisms remains insufficient. The effects of oxygen deficiency in a-TiO₂:GeO₂ are particularly unclear, yet are suspected to be substantial. Using molecular dynamics and density functional theory, we generate models of Fe_y doped a-TiO_x:GeO_x and analyze their electronic structure. Our findings reveal that oxygen non-stoichiometry can induce intra-gap defect states in the electronic density of states, causing optical absorption. These defect states arise not only from obvious structural defects such as dangling bonds, but also from subtler topologies in the metal-oxygen network, particularly when combined with transition metal impurities.