Titanium Dioxide Mono-Material Multilayer Optical Coatings

Advanced optical functionalities are typically achieved using interference multilayer coatings composed of alternating materials with contrasting refractive indices. Here, we propose a mono-material concept in which the required refractive-index contrast is generated within a single material through a multilayer stack of porous and dense microstructured titanium dioxide (TiO₂). A key challenge in such architectures is the deposition of dense layers on porous ones without intermixing or excessive roughness. To address this, titanate 2D nanosheets are introduced as an interfacial layer. To demonstrate this concept, a visible near-infrared optical filter was designed using optical simulations and experimentally realized. The desired refractive-index contrast was achieved by tuning the porosity of consecutive amorphous TiO₂ layers, with ∼1 nm crystalline Ti_0.87O₂ titanate nanosheet interlayers enabling sequential stacking of dense and porous TiO₂. This approach enabled the fabrication of a six-layer mono-material optical filter and establishes a sustainable strategy for designing versatile mono-material optical coatings.