Optical characteristics of passivation layers for ultra-low-loss on-chip devices in the mid-infrared

On-chip photonics offers substantial potential for miniaturizing mid-infrared (mid-IR) optical systems, advancing molecular science and technology across diverse practical applications. However, strong interaction between mid-IR light and chemical species, while beneficial for molecular detection, can also leave these devices, such as ones made with chalcogenide glass, vulnerable to gradual increases in propagation loss caused by the adsorption and diffusion of airborne chemicals. Dense, nanometer-scale atomic layer deposited (ALD) films are commonly employed as protective barriers against surrounding contaminants, but their effectiveness in preserving initial optical loss performance has not been quantitatively assessed. In this study, we investigate both the protective capabilities of ALD coatings and their influence on mid-IR propagation losses. By depositing Al2O3 and TiO2 films on ultra-low-loss microresonators and monitoring changes in propagation loss over several days, we demonstrate that these ALD films effectively prevent long-term loss degradation. Nonetheless, the coating also introduces initial optical losses through two distinct mechanisms, absorption from precursor-derived impurities within the film and increased surface-adsorbed water due to higher surface hydrophilicity. By analyzing the dependence of additional optical loss on film thickness and two distinct precursor chemistries, we clearly distinguish these mechanisms. Our results provide a comprehensive evaluation of mid-IR passivation materials in terms of both passivation capability and optical loss impact, highlighting the critical importance of careful selection and process optimization of passivation films for further performance improvements in integrated mid-IR photonic devices.