Enhanced Durability, Wide-Bandwidth Transmission, ZnSe Windows Incorporating AR Microstructures

The infrared transmitting material Zinc Selenide (ZnSe) is used for windows and lenses in many military systems because of its high transmission and low absorption of infrared light.  ZnSe has a particularly low absorption level throughout the infrared spectral region where it is used extensively for imaging lenses in multi-spectral and hyper-spectral sensor systems.  One long-standing issue with ZnSe is its low environmental durability that necessitates the use of some form of hardening to protect the window or lens from damage due to rain and sand impacts.  Typical hardening methods involve the application of thin-film material coatings that must also serve as an anti-reflection (AR) treatment. Such coatings however limit the transmission bandwidth forcing a tradeoff between the level of transmission required and the expected lifetime. As mission profiles become more demanding the further utility of ZnSe will depend upon the continued evolution and development of hardening strategies to improve durability under increasingly adverse environmental conditions.  In a recent experimental study, an innovative AR treatment based on surface relief microstructures was shown to have great potential for increasing the survivability of zinc sulfide (ZnS) windows operating in abrasive environments. This Phase I project proposes to investigate the durability of AR microstructures built in ZnSe windows that meet the wide bandwidth, high transmission performance requirements of multiple airborne sensor programs. Several design variants that combine AR microstructure technology with hard oxide coatings will be fabricated in ZnSe coupons and subjected to a set of rain and sand exposure conditions that are typical of aircraft ground and flight operations. Six coupons of the most promising designs will be delivered to the Government for further erosion testing.  A statistically meaningful number of coupons will be fabricated with the most durable AR microstructures found during the initial screening work, and further rain and sand erosion testing will be conducted.  A further demonstration of AR microstructure fabrication in a small-scale ZnSe lens surface will be made to demonstrate the ability to apply AR microstructures to the curved surfaces needed for imaging lenses.  A close collaboration with Raytheon Missile Systems and the prominent ZnSe manufacturer Rohm & Haas, will be maintained throughout the multi-phase program to ensure the rapid transition of the technology into Air Force sensor platforms. BENEFIT: The rain erosion resistance of microstructure-based AR treatments combined with the proven sand erosion resistance of hard oxide coatings will provide a dramatic increase in the transparency and lifetime of ZnSe windows and lenses used in a growing number of Air Force sensor systems.  The Joint Strike Fighter and many other Air Force programs that currently use sapphire windows to protect sensor packages such as Enhanced Vision Systems and ladar, would benefit from lower cost ZnSe windows that exhibit wider bandwidth transmission with the enhanced durability provided by AR microstructures.  ZnSe windows with enhanced durability could also allow for dual-band functionality transmitting both mid and long wave infrared light in a common aperture system.  In addition, the design principles developed for ZnSe will apply to a wide range of important window materials such as germanium, silicon, and glass. The commercial market for more durable, higher transmission windows is large, ranging from automobiles and firefighter and external security cameras, to solar panels, displays, cell phones and biomedical devices.