Ion energy induced absorption reduction for the upper limit of LWIR optics in the IAD process

Long wavelengths for 7.5-12μm and particularly above the upper limit of 12μm applies a huge challenge for LWIR (long wavelength infrared) optics. Of the known materials, sulfides and particularly selenides can work up to these wave lengths, but they are high index materials. For HL(High-Low) optical designs low index materials with low absorption remain a challenge. Fluorides have low index with bandgaps and outperforms other known LWIR materials, but absorption exists with traditional conditions at the upper limits of LWIR. Some fluorides can be better but because of the toxicity and durability their applications are restricted. In this research we successfully improved LWIR optics transmissions in wavelengths of 7.5-13.6μm and above by optimizing fluorides IAD (Ion assisted deposition) processing conditions. This study clearly shows that fluorides, for example YbF₃, there is ion energy threshold for the absorption edge above which the material absorption can be greatly reduced regardless the approach of the evaporation methods, such as e-beam or thermal evaporation. This threshold is also closely related to stress and microstructure of the coating layer, above which the layer tends to be a dense and low defect microstructure by TEM and low tensile by stress analysis. With optimized ion energy in the IAD process, LWIR optics can increase transmission by 3.5%, 2.0% and 7.5%, at wavelength of 7.5, 10.5, and 13.5μm respectively. Compared to the traditional approach, single layer absorptions at the same wavelength from the optimized IAD process drop about 3%, 6% and 14%, respectively.