Quasi‐Bound States in the Continuum Supported in Titanium Oxide Metasurfaces with Nanoslits Fabricated by Thermal Oxidation

Recent advances in titanium oxide (TiO 2 ) nanophotonics have highlighted this material's exceptional potential for low‐loss visible and near‐infrared photonic applications, yet challenges in microstructuring have hindered its broader adoption. Herein, a transformative approach for creating subwavelength TiO 2 structures through controlled thermal oxidation of titanium (Ti) is demonstrated. Spectroscopic characterization confirms that the oxide product exhibits excellent behavior, closely resembling that of natural TiO 2 , and the geometrical extension of the subwavelength structures during the oxidation can be precisely predicable following some found rules. As a demonstration, a metasurface consisting of a periodic array of slitted TiO 2 disks supporting a quasi‐bound state in the continuum to enhance fluorescence is designed and fabricated, and the experimental result is well consistent with the theoretical prediction. It is striking that the width of the slits may be reduced below 10 nm, difficult for conventional top‐down fabrication methods, and the quality factor of the resonant metasurface can reach the level of 1000. This oxidation‐mediated nanofabrication strategy is valuable for forming sub‐10 nm gaps and establishes a potential pathway for large‐scale, cost‐effective production of subwavelength TiO 2 devices.