Black and white fused silica: modified sol-gel process combined with moth-eye structuring for highly absorbing and diffuse reflecting SiO2 glass.

Diffuse reflecting (white) and highly absorbing (black) fused silica based materials are presented, which combine volume modified substrates and surfaces equipped with anti-reflective moth-eye-structures. For diffuse reflection, micrometer sized cavities are created in bulk fused silica during a sol-gel process. In contrast, carbon black particles are added to get the highly absorbing material. The moth-eye-structures are prepared by block copolymer micelle nanolithography (BCML), followed by a reactive-ion-etching (RIE) step. The moth-eye-structures drastically reduce the specular reflectance on both diffuse reflecting and highly absorbing samples across a wide spectral range from 250 nm to 2500 nm and for varying incidence angles. The adjustment of the height of the moth-eye-structures allows us to select the spectral position of the specular reflectance minimum, which measures less than 0.1%. Diffuse Lambertian-like scattering and absorbance appear nearly uniform across the selected spectral range, showing a slight decrease with increasing wavelength.

Combined 'moth-eye' structured and graded index-layer anti-reflecting coating for high index glasses.

We present a hybrid antireflective coating (ARC) providing a complete continuous graded refractive index (GRIN) transition from a high-index substrate down to ambient air. The ARC comprises a first GRIN layer of dense silicon-oxy-nitride with a varying, height adjusted material composition. Secondly, a layer of quasi-periodic nanopillars imitating AR-"moth-eye structure" is added to the dense GRIN layer. Demonstrated on a high index glass with a refractive index of ne=1.73 the hybrid GRIN-ARC is applicable to a broad material selection and allows to eliminate any step-like transition up to a refractive index of the substrate of ∼2.0. The ARC offers antireflective properties for large incidence angles and over an extremely broad spectrum ranging from 400 nm up to 2.5 µm. Compared to the sole substrate, the hybrid GRIN-ARC results in an increase of transmittance of more than 10% in the maximum, and more than 6% in the peripheral regions of the spectrum.