Off-normal-incidence effects in the infrared-reflectivity spectra of aluminum phosphate

The effects of finite off-normal-incidence angle on the polarized-reflectivity spectra of aluminum phosphate have been investigated in great detail. An unambiguous identification of the extra structure, which manifests itself at 525 ${\mathrm{cm}}^{\mathrm{-}1}$ in the ${\Gamma }_2$(p) or transverse-magnetic (TM) configuration, has been made. It comes from a strong ${\Gamma }_3$ mode, of which the LO component couples to the light for nonzero values of the angle of incidence. For typical experimental conditions, this results in the appearance of a pronounced extra structure which does not belong to the symmetry of the components under investigation. The corresponding intensity is well accounted for by the appropriate Fresnel equations. Nothing else similar to it appears in the higher-frequency range (where a second ${\Gamma }_3$ mode exists near 1200 ${\mathrm{cm}}^{\mathrm{-}1}$) or in the lower-frequency part of the spectrum (around 380 ${\mathrm{cm}}^{\mathrm{-}1}$, where a second additional band was previously identified). In the latter case, no obvious connection with a ${\Gamma }_3$ mode could be discerned. Finally, from the data collected in the pure s (transverse-electric) polarization for both ${\Gamma }_2$ and ${\Gamma }_3$ components, an improved series of transverse and longitudinal phonon frequencies, oscillator strengths, and broadening parameters could be deduced. This gives a better account of the dispersion of the refractive index of aluminum phosphate in the range from 100 to 1500 ${\mathrm{cm}}^{\mathrm{-}1}$.