Infrared reflectance spectra were measured on a ${\mathrm{Fe}}_3$${\mathrm{O}}_4$ film deposited on a bulk MgO substrate. For an obliquely incoming light with TM polarization, the reflectance spectra showed a peak structure near a longitudinal optic (LO) phonon frequency of MgO. For TE polarization, such a resonant structure was not observed. This phenomenon is quite different from the ‘‘Berreman effect’’ [D. W. Berreman, Phys. Rev. 130, 2193 (1963)], where an absorption process related to a LO phonon of the film occurs instead of that of the substrate. For our film geometry, the electric field component normal to the film induces charge oscillations and a coupling between the charge carriers in the film and the LO phonon of MgO results in the resonance phenomenon. The resonance peak occurs at a frequency where the substrate dielectric constant becomes the same as ${\sin }^2$${\mathrm{\theta }}_0$, where ${\mathrm{\theta }}_0$ 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. © 1996 The American Physical Society.

• Received 14 December 1995

DOI:https://doi.org/10.1103/PhysRevB.53.10310