Far-infrared reflection studies in ordered GaInP2

We report on room-temperature polarized far-infrared reflectance in partially ordered GaInP performed with the incidence plane parallel to the growth and ordering directions. Measurements in the s-polarization geometry show only one feature related to the vibrations in the plane of ordering. On the contrary, those performed using p polarization display a structure appearing in the range of InP-like vibrations as the order parameter increases, which we relate to a resonance with longitudinal-optic mode character due to the anisotropy of the longitudinal phonons.