To reduce the intensity of the Fresnel reflections of optical components, subwavelength structures prepared by reactive ion etching of SiO2 thin films were combined as the outermost layer with a multilayer system made of conventional thin-film materials. A hybrid coating was thus realized, with the nanoscaled structured outermost layer expected to further improve the antireflection properties of common interference stacks. The microscopic and optical spectroscopic analysis of the subwavelength structures revealed that pillar-shaped nanostructures formed during etching exhibit low-refractive-index properties and have a depth-dependent refractive index. To take into account the refractive-index gradient in the coating design, the optical properties of the nanostructures were modeled using the effective-medium approximation. The calculated average effective refractive index turned out to be 1.11 at 500-nm wavelength. A hybrid coating was designed to minimize the residual reflectance in the 400-nm to 900-nm spectral range for BK7 glass substrate. Experimental results demonstrated that the hybrid-coating approach yields a low residual reflectance with very good omnidirectional properties, owing to the properties of the nanostructured surface.
To further reduce the intensity of the Fresnel reflections of optical components, subwavelength structures prepared by reactive ion etching of SiO2 thin films are combined as outermost layer with a multilayer system made of conventional thin film materials and prepared by magnetron sputtering. In this approach, a hybrid coating is realized in which the nanoscaled structured outermost layer is expected to further improve the antireflection properties of common interference stacks. The subwavelength structures are examined by spectroscopic ellipsometry, spectral photometry and scanning electron microscopy. The microscopic and optical spectroscopic analysis revealed that pillar-shaped nanostructures are formed during etching which exhibit low-index properties and have a depth-dependent refractive index. To take into account the index gradient in the coating design, the optical properties of the nanostructures are modeled using the effective medium approximation. The calculated average effective refractive index is 1.11 at 500 nm wavelength. A hybrid coating was designed to minimize the residual reflectance in the 400 – 900 nm spectral range for BK7 glass substrate. Experimental results showed that the hybrid coating achieves a low residual reflectance with very good omni-directional properties, owing to the properties of its nanostructured surface. The residual reflection of the hybrid coating is found to be two times smaller than the reflection obtained by applying a common interference multilayer system which demonstrates the benefit of the use of hybrid systems for the realization of broadband antireflective coatings with wide-angle properties.
In this work we present a new technique in order to create antireflective surfaces with the help of statistical
nanostructures on fused silica. A specific plasma etching process was found to serve this purpose, as thereby
nanostructures are created through self-masking. Under specific etching conditions the micro-contaminations create
pillars on the surface with dimensions down to 20 nm, which act as an antireflective nanostructure. Those structures raise
the transmission of fused silica in the wavelength range from 370 nm to 500 nm to more than 99.5% (both sides etched).
Within the close UV range (from 200 nm up to 400 nm) the transmission can be raised by 4.8% to 97.1% on average.
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