Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection

In 1880, by studying light passing through Earth's atmosphere, Lord Rayleigh mathematically demonstrated that graded-refractive-index layers have broadband antireflection characteristics(1). Graded-index coatings with different index profiles have been investigated for broadband antireflection properties, particularly with air as the ambient medium(2-4). However, because of the unavailability of optical materials with very low refractive indices that closely match the refractive index of air, such broadband antireflection coatings have not been realizable. Here we report the fabrication of TiO2 and SiO2 graded-index films deposited by oblique-angle deposition, and, for the first time, we demonstrate their potential for antireflection coatings by virtually eliminating Fresnel reflection from an AlN-air interface over a broad range of wavelengths. This is achieved by controlling the refractive index of the TiO2 and SiO2 nanorod layers, down to a minimum value of n = 1.05 in the case of the latter, the lowest value so far reported.