IR reflectance spectra and optical constants of Al-Cu-Fe quasicrystalline thin films

The optical properties of Al-Cu-Fe quasicrystalline films and, for comparison, crystalline films of similar composition are studied using middle-and far-IR reflectance spectroscopy. Measurements are performed with 0.1-to 0.3-μm-thick films on sapphire substrates. The complex dielectric function of the films is calculated from experimental data. It is found that the real part of the dielectric function is negative for the crystalline films but positive and weakly frequency dependent, except in the range near 245 cm−1, for the quasicrystalline films. The optical conductivity of the quasicrystalline films does not feature the Drude peak observed for crystalline films and exhibits a peak at 245 cm−1, which can be assigned to optical phonon excitations and is absent for crystalline films.