Transparent porous films with real refractive index close to unity for photonic applications.

Herein, we demonstrate mechanically stable large-area thin films with a purely real refractive index () close to 1 in the optical range. At specific wavelengths, it can reach values as small as = 1.02, the lowest reported for thin solid slabs. These are made of a random network of interwoven spherical silica shells, created by chemical vapour deposition of a thin layer of silica on the surface of randomly packed monodisperse polymer nanoparticles that form a film. Thermal processing of the composites results in highly porous silica-based transparent thin films. We demonstrate the potential of this approach by making novel photonic materials such as strong optical diffusers, built by integrating scattering centers within the ultralow transparent films, or highly efficient light-emitting slabs, in which losses by total internal reflection are practically absent as a result of the almost null optical impedance at the film-air interface.