Temperature dependence of optical modes of vibration in MgO thin film

The authors observe the thermal emission spectra of the metal-backed thin film of MgO at the emission angle of 25 degrees . The film deposition on a platinum plate is performed by means of an electron beam technique in a high-vacuum system. The observed spectra measured at elevated temperatures are analysed by using the response function based on the virtual-mode theory of Kliewer and Fuchs for the ionic slab to estimate the temperature dependence of the lattice dynamical quantities such as the phonon frequency and the damping. The dielectric function derived from the theory based on the thermodynamic Green function technique is considered in the spectrum analysis. No significant difference is found in the values of the parameters between a film and a bulk single crystal. The damping around the TO frequency shows the linear temperature dependence, while the damping around the LO frequency varies nonlinearly with temperature. The quartic anharmonic contribution to the phonon self-energy is significant as well as the cubic contribution around the TO frequency.