For most IR applications in the wavelength range from 780 - 3600 nm synthetic fused silica is the material of choice.
The optical performance is mostly influenced by the purity and the OH content of the material. Metallic impurities as well as OH-molecular vibrational or rotational excitations lead to light absorption in the material. As consequence transmission decreases, the material heats up and in IR laser material processing for example an unintented focus shift of the final lens can be observed. Heraeus Quarzglas provides fused silica materials with lowest absorption for all IR laser applications.