Design, fabrication, and measured performance of anti-reflecting surface textures in infrared transmitting materials

Rugged infrared transmitting materials have a high refractive index, which leads to large reflection losses. Multi-layer thin-film coatings designed for anti-reflection (AR), exhibit good performance, but have limited bandwidths, narrow acceptance angles, polarization effects, high costs, and short lifetimes in harsh environments. Many aerospace and military applications requiring high optical transmission, durability, survivability, and radiation resistance, are inadequately addressed by thin-film coating technology. Surface relief microstructures have been shown to be an effective alternative to thin-film AR coatings in many infrared and visible-band applications. These microstructures, etched directly into the window surface and commonly referred to as "Motheye" textures, impart an optical function that minimizes surface reflections without compromising the inherent durability of the window material. Reflection losses are reduced to a minimum for broad-band light incident over a wide angular range. For narrow-band applications such as laser communications, a simpler type of AR surface structure called a sub-wavelength, or "SWS" surface, is used. In general, both the Motheye and SWS surface textures will exhibit the same characteristics as the bulk material with respect to durability, thermal issues, and radiation resistance. The problems associated with thin-film coating adhesion and stress, are thus eliminated by design. Optical performance data for AR structures fabricated in fused silica, sapphire, Clear ZnS, ZnSe, cadmium zinc telluride (CZT), silicon, and germanium, will be presented.