Air Nanocolumn-SiO2 composite film with adjustable anisotropic refractive index

Low refractive index (RI) porous dielectrics at optical frequencies, serving as subwavelength effective media for versatile photonic utilization, still face great challenges in realizing flexible refractive index adjustment, and balancing low RI performance and material robustness. These mainly stem from difficulty in precisely controlling their porosity, as well as high porosity (excessive air exposure) induced refractive index shift and weakened rigidity. To address these issue, ultrafine vertically aligned and perforated air nanocolumn-SiO2 composite films are elaborately designed and fabricated, namely, ultrafine (the diameter <10 nm) nanoholes embedded in the SiO2 matrix. The simple preparation process refers to self-organization growth of Ag nanowire (NW) arrays in SiO2 matrix followed by chemical etching for silver removal. Owing to high aspect ratio-induced depolarization effect and porosity modulation, their RIs not only possess anisotropic feature, but also are continuously tunable (from 1.40 to 1.15 for ordinary component and 1.41 to 1.19 for extraordinary component). Due to their low-RI nature and nanoscale microstructural characteristic, broadband (380 nm–1700 nm) high transmittance properties are demonstrated with ultralow (<0.5%) haze. More importantly, our proposed films are mechanically robust with water-proof and antifouling in nature. This work provides a new scheme for constructing anisotropic low RI materials that should be intriguing for diverse photonic applications.