A group at the Fraunhofer Institute of Applied Optics and Precision Engineering (Jena, Germany) has developed a method of creating gradient-index antireflection (AR) coatings on glass that contain plasma-etched nanostructured organic layers. The layers have a low effective refractive index that can be tuned down to approximately 1.1 to produce effective AR properties. The coatings contain inorganic and organic materials, and can have a gradient-index profile or a stepwise reduction in refractive index. Extreme broadband (400–1200 nm) AR behavior is the result.
The nanostructured material is produced using optical-coating equipment (a box coater with a plasma source). The structure itself is created by plasma-etching organic materials; for example, a cycloolefin polymer. Melamine, another organic material, is then deposited and etched to create a double structure. In this particular example, the average residual optical reflection is 1.4% from 400 to 800 nm at incidence angles of 0°, 45°, and 60°. Other configurations include a two-layer inorganic index step-down coating topped by a double organic layer, and an organic nanostructure coated with inorganic material such as silica, which increases durability. Substrate areas of a square meter can be coated in one run; the process can also be used on curved surfaces. The coatings can be cleaned with methanol; the more-durable coating also survived a tape-peel test. Reference: U. Schulz et al., Opt. Mater. Express (2015); doi:10.1364/ome.5.001259.
