In Situ Surface Assembly Derived Ultralow Refractive Index MgF₂–SiO₂ Hybrid Film for Tri‐Layer Broadband Antireflective Coating

For the broadband antireflective (AR) coating working in specific wavelengths, the precise control over refractive index and film thickness of each layer is critical to guarantee the peak position locating at the targeted wavelengths. In this paper, a simple sol–gel procedure without post‐treatment is used to prepare MgF₂–SiO₂ hybrid coating with tunable refractive index for the fabrication of tri‐layer tri‐wavelength broadband AR coating. The MgF₂–SiO₂ coating with ultralow refractive index of 1.12 is realized through in situ surface assembly of negatively charged silanol groups on vesicle‐like MgF₂ particles to obtain porous crosslinking structure. The electrostatic attraction between the negatively charged silanol groups and positively charged MgF₂ colloidal particles plays a key role. Furthermore, the MgF₂–SiO₂ coating with a wide range of refractive index between 1.20 and 1.44 is achieved by adjusting the addition amount of acid‐catalyzed silica sol into the vesicle‐like MgF₂ sol. According to the theoretical design, a tri‐layer broadband AR coating with refractive index model of 1.12/1.26/1.36 is prepared, which can enhance the transmittance of quartz substrate to near 100% simultaneously at 351, 527, and 1053 nm.