Infrared reflectance studies on a Fe3O4 film deposited on a MgO substrate: Observation of the substrate longitudinal optic phonon resonance peak in the film geometry

Infrared reflectance spectra were measured on a Fe3O4 him deposited on a bulk MgO substrate. For an obliquely incoming light with TM polarization, the reflectance spectra showed a peak structure near a longitudinal optic (LO) phonon frequency of MgO. For TE polarization, such a resonant structure was not observed. This phenomenon is quite different from the ''Berreman effect'' [D. W. Berreman, Phys. Rev. 130, 2193 (1963)], where an absorption process related to a LO phonon of the film occurs instead of that of the substrate. For our film geometry, the electric field component normal to the film induces charge oscillations and a coupling between the charge carriers in the film and the LO phonon of MgO results in the resonance phenomenon. The resonance peak occurs at a frequency where the substrate dielectric constant becomes the same as sin(2) theta(0), where theta(0) is the incident angle. This newly observed phenomenon might be useful to investigate a LO phonon of a solid, where other methods are difficult to be applied.