Wideband antireflection (AR) coatings—those for which the ratio of the upper wavelength limit to the lower wavelength limit is greater than 2—are difficult to fabricate, especially on low-index substrates, because of the limited availability of low-index thin-film materials. Knowing that the refractive index of the last low-index layer deposited on a multilayer interference stack has a dominant influence on the reflectance obtained for a given bandwidth, researchers at the Fraunhofer Institute of Applied Optics and Precision Engineering (Jena, Germany) fabricated a 250-nm-thick polymethylmethacrylate subwavelength (and self-organized by an etching step) nanostructured coating with a refractive index of 1.49 on top of a common multilayer substrate.
Reflectance values for substrates with the nanostructured coating were typically 0.35%, while substrates coated with conventional multilayers saw higher reflectance values, typically greater than 1.0%. While such nanostructured coatings are easily fabricated, future coatings must be developed with improved ruggedness and resistance to mechanical abrasion and cleaning fluids. Contact Ulrike Schulz at [email protected].