Nanostructured top layer enables wideband antireflection coatings

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 ulrike.schulz@iof.fraunhofer.de.