Films with Tunable Graded Refractive Index Consisting of Spontaneously Formed Mesoporous Silica Nanopinnacles

Coatings with surfaces packed with pointed projections with subwavelength sizes are known as "moth-eye", whose graded index structure gives superior antireflection (AR) properties with a small dependence on wavelength and incident angle. However, the optical interface between the AR coating and a substrate still causes reflection, unless the refractive index of the coating is matched to that of the substrate. Here, we show a new AR coating with both graded and tunable refractive index, which completely eliminates distinct optical interfaces. The coating is made of a mesoporous silica film, whose surface is spontaneously converted to an assembly of pointed nanostructure by reactive ion etching, and its refractive index is matched universally to that of a substrate by controlled incorporation of titanium dioxide into the mesopores. This AR coating enables universal ultralow reflection on a substrate over the index range from 1.2 to 1.8 and will be applied widely in practical optics. (Figure Presented).