Silicon single crystals have a wide transmission range from NIR to LWIR. Due to its low thermal conductivity, Silicon performs exceptionally well in laser systems. Additionally, with its low density (2.329 g/cm3, half that of Ge or ZnSe), silicon is ideal for weight-sensitive The Knoop Hardness of silicon is 1150, in fact, it is one of the hardest optical materials which are transparent to lights in the 1000-6000nm region. The silicon crystal grown using the Czochralski method is particularly excellent for applications in MWIR (3-5μm) spectrum.
After optimizing the AR coating for the wavelength range, the reflection of CZ silicon windows can be significantly reduced. However, CZ silicon exhibits a strong absorption band at 9μm. In contrast, Float Zone silicon is intentionally doped with Nitrogen and purified during production to minimize its carbon and oxygen content, thereby attenuating absorption at 9μm. As a result, FZ silicon windows are suitable for use in the LWIR (8-12μm) range.