Zinc Selenide (ZnSe)
Zinc selenide is a clear yellow polycrystalline material with a grain size of approximately 70µm, transmitting in the range 0.5-15µm. It is essentially free of extrinsic impurity absorptions, providing extremely low bulk losses from scatter. The main identifiable extrinsic bulk absorption is zinc hydride, whose free diatomic molecule has a vibrational mode at 1608cm-1. Having a very low absorption of energy makes it useful for optical components in high power laser window and multispectral applications, providing good imaging characteristics. ZnSe is also useful in high resolution thermal imaging systems, where it is used to correct for colour distortion which is often inherent in other lenses used in the system. An observed electronic absorption edge at approximately 0.476µm (~ 2.6eV) at 300K and far infrared multi-phonon absorption edge commencing at approximately 22.2µm.
The multi-phonon lattice absorption of ZnSe has been extensively investigated since the first transmission and reflection measurements were made by Aven et al on cubic ZnSe in 1961. Since neutron-scattering and Raman-scattering have become available in the early 1970's, as has the increased availability of CVD ZnSe, investigations of ZnSe has demonstrated that in the three and four-phonon regions, ZnSe exhibits a characteristic structure consistent with predicted calculations by Bendow et al.
At wavelengths between the visible and infrared regions, ZnSe behaves as a dielectric material, with a refractive index decreasing with increasing wavelength. The mean value of n being approximately 2.4 at 300K. Over this same region, the extinction coefficient k is very small (<10-5), providing uniformly high transmission. As a result of these characteristics, ZnSe has been used extensively for optical components and windows. Marple et al derived the following Sellmeier type dispersion equation at 300K with a claimed experimental error of ± 0.002 ;
The effects of temperature on this refractive index profile was investigated across a narrow temperature range (243-343K) by Barron using a Buchdahl refractive index polynomial.