Ultra-broad band antireflection coating at mid wave infrared for high efficient germanium optics

This work mainly demonstrates the development process of an ultra-broad band antireflection (AR) coating that maximizes the optical performance of germanium optical elements. A multilayer stack exhibiting high efficient AR performance on Ge consists of thin multi-layers of aluminum oxide (Al2O3) and germanium (Ge) named as low and high refractive index layer materials. A three-layer design with a very tight thickness tolerance (3%) was optimized by Optilayer software. Al2O3 and Ge layers were deposited by a plasma assisted e-beam evaporation system. Ultra-high efficient multilayer AR coating on Ge has a base reflectance 0.005% at 3550 nm and an average reflectance of 0.256% over the entire mid wave infrared spectrum. Multilayer AR coating on Ge offers lower base and average reflectance than the works reported before. Moreover, AR coating on Ge offers a cost effective process cycle due to its number of layers and its thickness for providing ultra-broadband and high efficient optical performance for mid wave infrared electro-optical applications.