Fourier transform spectroscopic transmissivity data in the far-infrared (10 - 350 cm-1) were used to derive a model for the complex refractive index (n - ik) of polycrystalline diamond films grown by microwave plasma enhanced chemical vapor deposition. Due to their polycrystalline nature, the diamond films exhibited a surface roughness with crystallite sizes ranging from 0.1 - 8 μm. Scattering of the far-infrared radiation introduced by the diamond's rough surface is shown to have a significant effect on the FTIR spectrum and is accounted for in the determination of the material's optical properties. Free carrier type absorption (k proportional to λ2) is observed at longer wavelengths (l/λ < 100 cm-1).
A. J. Gatesman, A. J. Gatesman,
"FTIR spectroscopy of CVD and natural diamond", Proc. SPIE 1514, 15th International Conference on Infrared and Millimeter Waves, 151423 (1 December 1990); doi: 10.1117/12.2301485; https://doi.org/10.1117/12.2301485