Antireflection Coating for MWIR on Calcium Fluoride Using Ion-Assisted E-Beam Deposition

This research outlines the design, analysis, and fabrication of a multilayer anti-reflective coating on a calcium fluoride (CaF2) substrate, specifically for mid-wavelength infrared uses, employing the ion-assisted electron-beam evaporation method. A 2-layered multilayer structure in the form of SiO2/Ge was created on CaF2, consisting of low refractive index silicon dioxide (SiO2) and high refractive index germanium (Ge) thin films with a total thickness below 1 μm. The design was optimized for the 3.6-4.9 μm MWIR range, and an average transmission of 98.39% and an average reflectance of 0.93% were simulated at a broadband spectral width of 1300 nm. After the fabrication process using the ion-assisted physical vapour deposition (IAPVD) technique, the experimental results showed an average transmission of 98.13% and a reflectance value of 1.19% within the 3.6-4.9 μm range. The simulation design and experimental results were found to be very close to each other, with a difference of only 0.26%. This work provides a high-efficiency solution for AR coatings in the MWIR region on CaF2 surfaces. To our knowledge, the Ge/SiO2 multilayer structure on CaF2 has not been reported before in the literature and the results obtained will be an alternative for CaF2-based optical systems.