Infrared Absorption in Highly Transparent Glasses Based on Hafnium Fluoride

There has been considerable research activity recently in the preparation and characterization of multicomponent glasses based on heavy metal fluorides. These glasses offer excellent prospects for multispectral transmission (from 5–7μm to 0.2–0.3μm), are amenable to the incorporation of a wide variety of atomic species, possess a moderately low refractive index (∼1.5), and are readily forged into optical components. In previous work we reported investigations of the infrared edge of fluorozirconate glass. In this paper, we report investigations of the infrared absorption in the transparent regime of several new hafnium fluoride glasses developed recently at our laboratories. We find, as expected, that the infrared edge is shifted to longer wavelengths compared to Zr-based glass, but with most other spectral features remaining similar for both families of glasses. One interesting feature, for example, is a shoulder in the vicinity of 1300–1400 cm⁻¹ in the infrared edge spectrum of these glasses. We discuss the influence of processing conditions on the mid-IR absorption; it appears that at this time that RAP processing utilizing CCl₄ yields the best glasses, with somewhat steeper IR edges and reduced absorption in the 3μm regime. An analysis of the infrared edge data in terms of multiphonon theory is utilized to estimate the intrinsic limiting absorption in the 3–5μm regime. Finally, the infrared edge measurements are correlated with the fundamental vibrational spectra of the glasses deduced from Raman and reflectivity studies.