Off-normal infrared reflectivity in uniaxial crystals: α-LiIO3andα-quartz

We discuss the influence of experimental conditions in the analysis of the infrared reflectivity due to lattice modes in bulk single crystals of α-${\mathrm{LiIO}}_3$ and α-quartz. The analysis leads to the appearance of additional bands in the usual off-normal incidence of the incoming radiation in the infrared reflectivity spectrum that do not belong to the symmetry under study. These bands occur only in the p-polarized component of the radiation and they are associated with the longitudinal-optic (LO) modes of different symmetry. Depending upon the positions and the relative strengths of these modes, they appear as ‘‘dips’’ or ‘‘peaks’’ in the reflectivity spectra. The computed infrared reflectivity spectra using appropriate Fresnel equations are in very good agreement with our experimental observations.