Nanoporous antireflection coating for high-temperature applications in the infrared

Antireflection (AR) coatings are essential to the performance of optical systems; without them, surface reflections increase significantly at steep angles and become detrimental to the functionality. AR coatings apply to a wide range of applications from solar cells and laser optics to optical windows. Many times, operational conditions include high temperatures and steep angles of incidence (AOIs). The implementation of AR coatings is extremely challenging in these conditions. Nanoporous coatings made from high-temperature-tolerant materials offer a solution to this problem. The careful selection of materials is needed to prevent delamination when exposed to high temperatures, and an optimal optical design is needed to lower surface reflections at both the normal incidence and steep AOIs. This paper presents nanoporous silicon dioxide and hafnium dioxide coatings deposited on a sapphire substrate using oblique angle deposition by electron beam evaporation, a highly accurate deposition technique for thin films. Developed coatings were tested in a controlled temperature environment and demonstrated thermal stability at temperatures up to 800°C. Additional testing at room temperature demonstrated the reduction of power reflections near optimal for AOIs up to 70° for a design wavelength of 1550 nm. These findings are promising to help extend the operation of technology at extreme temperatures and steep angles.