Subject classification: 78.30.Am; 78.40.Fy; S5
The combination of low absorption and extreme mechanical and thermal properties make diamond a compelling choice for some more extreme far infrared (8-14 mm) window applications. The optical properties of CVD diamond at elevated temperatures are critical to many of these extreme applications. The absorption coefficient of diamond has been assessed on polished diamond samples with a variety of optical characteristics by heating samples up to 550 C and measuring transmission in a grating IR spectrophotometer. High temperature transmission spectra in the range 5000-200 cm --1 (2-50 mm) for CVD diamond are presented. The transmittance at near IR wavelengths %4200 cm --1 (2.5 mm) appears to be only slightly affected by elevated temperatures and the three-and two-phonon regions exhibit the expected temperature dependence. In the far infrared, the transmission drops with a broad peak approximately centred at %1250 cm --1 (8 mm), a behaviour characteristic of all samples tested. The temperature dependency of the absorption coefficient has been deduced by assuming there are no additional scatter losses from the surface and calculating the refractive index at temperature, thus allowing the reflectance at that temperature to be estimated. Direct emissivity and photo-thermal deflection measurements are in agreement with these measurements. An empirical formula for predicting the variation of absorption coefficient at %1000 cm --1 (10 mm) with temperature, is presented.