Design and fabrication of PbTe/BaF2 hydrophobic high-efficiency broad-band antireflection coating on Ge substrate in long-wave infrared region

Hydrophobic high efficiency broad-band antireflection (BBAR) coating in long-wave infrared (LWIR) region (7.5-11 µm) was designed and fabricated on germanium substrate by PVD technique. The multi-layer coating materials used in the design were Lead Telluride (PbTe) as high index material and Barium Fluoride (BaF2) as low index material. Hydrophobic high-efficiency BBAR coating with an average water contact angle of 102° was obtained only by the intrinsic surface roughness of BaF2 layer and without any surface treatment. AFM and FTIR spectrophotometer were used to investigate the surface topography and average reflection of the multi-layer coating, respectively. The average roughness obtained about 13 nm. The average reflection of the hydrophobic high-efficiency BBAR coating was 0.72% in the wavelength range from 7.5 to 11 µm that leads to eliminate Narcissus effect. Infrared optical windows and domes, having hydrophobic BBAR coatings, can effectively be applicable to humid or marine environments.