Towards perfect MWIR transparency using oblique angle deposition

•A simple antireflective design over 3.7–4.8 µm MWIR range is reported for Si substrate.•Ge-MgF2 bilayer prepared by e-beam evaporation gives high performances (Tav = 98.56%).•The refractive index of the Ge layer is tuned by Oblique Angle Deposition (OAD).•Key insights about nanostructure, pollution and optical properties are provided.•New routes are proposed to keep on improving the performances.