Wideband antireflection coatings combining interference multilayers and subwavelength structures prepared by reactive ion etching

Abstract

To further reduce the intensity of the Fresnel reflections of optical components, subwavelength structures prepared by reactive ion etching of SiO 2 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. Keyword list Antireflection coatingsSubwavelength structures Hybrid coating system Nanostructured Thin Films VIII, edited by Akhlesh Lakhtakia, Tom G. Mackay, Motofumi Suzuki, Proc. of SPIE Vol. 9558, 955804 · © 2015 SPIE · CCC code: 0277-786X/15/$18 ·

Modeling of the synergistic anti-reflection effect in gradient refractive index films integrated with subwavelength structures for photothermal conversion

“…Due to the facile growth on the glass substrate and intrinsically low extinction coefficient, 12,22 SiO 2 material (n = 1.42) 23 is selected as the model for the subwavelength structures. As an added benefit, SiO 2 exhibits excellent thermal stability, making it highly suitable for their applications in high-temperature environments.…”