Subwavelength Etched Colloidal Monolayers: A Model System for Tunable Antireflective Coatings

Colloidal monolayers represent a versatile material class to fabricate nanostructures with high quality. The length scale of the nanostructured film is given by the size of the colloidal nanoparticles. Importantly, colloidal monolayers, though being of hexagonally close packed symmetry, still embody a high amount of free volume. This reduces the effective refractive index of thin colloidal monolayer films significantly. For particles and periodicities <200 nm, the heterogeneous layer can be approximated by an effective medium theory. The amount of free volume can be further fine‐tuned by a controlled size reduction of the constituting spheres, for instance by plasma etching. This can be utilized to realize an optimum refractive index for the application of colloidal monolayers as antireflective (AR) coatings. In contrast, previously reported >200 nm monolayers demonstrate distinct extinction peaks due to grating diffraction. Rational design by the use of differently sized particles further allows shifting the best performance across the visible spectrum. Colloidal monolayers, though representing single‐layer AR coatings, exhibit broadband AR properties and are ideally suited to demonstrate the influence of refractive index and layer thickness, independently.