Sapphire surface polariton splitting due to resonance with magnesium oxide film phonon

Surface polariton spectra of magnesium oxide films (of thickness ranging from 10 to 300 nm) prepared by chemical vapor deposition technique on sapphire substrates have been studied. The splitting of the dispersion curve of the sapphire surface polariton appears near 700 cm⁻¹ due to the resonance interaction of the sapphire substrate surface polariton with the film optical phonon. This splitting is proportional to the square root of the film thickness. Optical constants of ultrathin magnesium oxide films have been obtained from surface polariton spectra measurements. For the thinnest film (10 nm) they appear to be close to the bulk crystal values.