Antireflective nanostructures for low-index substrates

Antireflective structures (ARS) with features of sub-wavelength size are appropriate as an alternative to interference coatings for obtaining antireflective properties on optical surfaces. A nanostructured surface can act either as an effective layer with a more or less constant low effective index or as a graded-refractive-index (GRIN) layer with decreasing index from substrate side to the ambient medium. In any case, for broadband antireflection or a wide range of incidence angles, a distinct thickness of the layer together with a very low structure size is required, which is especially difficult to realize on low-index substrates. A combination of interference layers with a nanostructured top-layer may help to overcome this problem.