Optical constants and thermo-optic coefficients of nanocrystalline diamond films at 30–500°C

The refractive index and absorption index of nanocrystalline diamond (NCD) films were investigated using spectroscopic ellipsometry between 30 and $500°\mathrm{C}$ . Due to their high transparency the experimental spectra could be well fitted in the subgap region using a single-oscillator model with a four-phase layered structure. The single-oscillator model yields a small optical absorption in the band gap region. The temperature dependence of dispersion of the refractive index over the photon energy range of $1.15–4.75\mathrm{eV}$ was determined. Based on the Bose-Einstein model, a thermo-optic coefficient of $(1∕n)(\partial n∕\partial T)=6.5\times {10}^{-6}{\mathrm{K}}^{-1}$ at $300\mathrm{K}$ was obtained for the NCD film in the near-infrared region.