Terahertz (THz) time-domain spectroscopy was used to study the optical properties of two series of oxyfluorosilicate (OFS) glasses. The experimentally measured refractive indices are analyzed by using the Clausius–Mossotti equation to retrieve information about polarizability of the glass. Compared with previously studied oxide-based glasses and chalcogenides, OFS glasses exhibit a balance of relatively low absorption coefficients (6–9 cm−1) and high refractive indices (2.9–3.7) at 0.5 THz. The value of 3.7 is the highest among silicate glasses reported to date and are comparable to those of La3+:chalcogenide glasses. The relatively high values of refractive indices have been attributed to the great increase of the glass polarizability in OFS glasses, which offsets the effect of an increase in molecular volume caused by multicomponent modification of the glass structure. Comparatively low THz absorption of OFS glasses is explained by the structural relaxation effect of fluorine, which effectively suppress the charge fluctuation in the glass structure. The high refractive index and low absorption loss properties of the present OFS glasses should be useful for quasi-optic components such as lens and waveguide devices application in the sub-THz and millimeter wave region.

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