Development of high efficient and ultra-broadband antireflection coating on calcium fluoride for electro-optical applications

Abstract

This study aims to demonstrate the development of multilayer antireflection (AR) coating on calcium fluoride (CaF₂) optics. AR coating functions over ultra-broadband spectrum from 3.4 to 4.8 μm at mid-wave infrared (MWIR). The design of AR coating has five layers and a physical thickness less than 4 μm. Multilayer AR coating consists of aluminium oxide and germanium named as low and high refractive index materials respectively. Multilayer stack was coated by plasma-assisted e-beam evaporation with optimized process parameters. The results obtained in this study are found to be better ones exhibiting lower base reflectance values 0.05% and 0.025% at 3550 and 4450 nm respectively. Additionally, 0.121% average reflectance over 1400 nm width in MWIR also offers better spectral performance than the previous works reported. Besides high efficiency, the number of layers and overall thickness of design offer an efficient AR solution for CaF₂ optical elements for electro-optical applications.