Preparation and properties of five-layer graded-refractive-index antireflection coating nanostructured by solid and hollow silica particles

This study examined large-area omnidirectional broadband antireflective nanostructured double-sided silica coatings fabricated by dip-coating on glass substrates. The coating consisted of five layers of graded-refractive-index thin films, and the total thickness of the coating was 250 nm. The refractive indices of the five layers of the coating changed from 1.33 to 1.11 with the thickness of each layer 50 nm. We prepared a series of 50 nm silica (SiO2) particles with hollow diameters ranging from 0 nm to 38 nm using the sol-gel method, and the sols for coating were composed of a mixture of 50 nm SiO2 nanoparticles sol and SiO2 sol derived from acid-catalyzed tetraethyl orthosilicate. The separations of Fresnel reflection coefficients and polarized transmittances within a small range demonstrate a significant advantage of achieving omnidirectional broadband antireflective performance for the coated substrate. At a wavelength range of 380-1600 nm, the average transmittance is 99.04% at normal incidence and the maximum unpolarized transmittances exceed 99% at wide angles of incident from 0 degrees to 45 degrees, and the increment in average unpolarized transmittances reach around 17.51% at an incident angle of 75 degrees.