Broadband Antireflection Coatings for Optical Lenses

Reducing the reflected light in optical systems is a basic aim of photonics. Reflected light causes losses in the intensity of transmitted light and can generate ghost images and stray light. To reduce these aberrations, antireflection (AR) interference multilayers are typically used. The application of sub-wavelength structures represents an alternative approach. Suitable nanostructures can be produced via plasma etching on surfaces consisting of organic materials. The combination of interference coatings with nanostructured layers has been further developed within the scope of the BMBF joint project FIONA.

The application of AR coatings on the surface of strongly curved lenses is a challenge. Vapor deposited layers aregenerally thinner on inclined areas. As a consequence, the reflectance spectrum in the inclined regions is shifted to shorter wavelengths which can increase the reflectance in the visible spectral range. To ensure sufficient performance in this range on inclined surfaces, the spectral range of an AR coating can be extended to include the near infrared region, thus enabling coverage of the visible range over the entire lens.

To achieve broadband AR performance, several coatings comprising inorganic layers and organic nanostructures have been developed. One of the newly developed broadband AR designs consists of alternating high-index and low-index layers accomplished by a plasma-etched nanostructured organic layer (see scanning electron micrograph Fig. 2). A coated lens is shown in Figure 1. A residual average reflectance below 0.3 % was achieved in the spectral range from 400 nm to 1500 nm, which is significantly lower than that achievable with classical interference systems.

 

Authors: Ulrike Schulz, Friedrich Rickelt, Peter Munzert, Christiane Weber