s - and p -Polarized Infrared Specular Reflectance of Vitreous Silica at Oblique Incidences: Detection of LO Modes

Abstract

Infrared (IR) specular reflectance spectra of a semi-infinite sample of vitreous silica (v-SiO₂) were obtained with the use of both s- and p-polarized light and oblique incidence angles. The optical constants of the material and hence its longitudinal optic/transverse optic (LO-TO) functions were determined through the Kramers–Krönig analysis (KKA) of its s-polarized 20° off-normal reflectance spectrum. p-Polarized spectra had their reflection maxima blue-shifting as the incidence angle increased, while they remained unchanged for the s-polarized spectra. Since an LO mode generally lies at wavenumbers higher than its respective TO mode, such a blue shift may be due to the detection of the LO mode in addition to the TO mode as incidence angle increased. The only exception to this observation was the high-frequency shoulder, which underwent a sharp intensification as the incidence increased. The present work shows that it is indeed brought about by the weakly IR active asymmetrical mode (AS2) but only because it takes place immediately after the intense AS1 mode, which causes the refraction index spectrum to have a broad dip below unity. Such a dip is proven to be responsible for the sharp increase in the high-frequency shoulder of the reflectance spectra.