Strong red-light scattering from colloidal copper in ZBLAN fluoride glass

Abstract

Following extended thermal processing under reducing conditions, fluoride glasses with a composition close to the standard 53% ZrF 4-20% BaF 2-4% LaF 3-3% AlF 3-20% NaF (ZBLAN) formulation scatter red light in a band centred at around 600 nm, together with an associated extinction band centred at around 570 nm. Some samples show a dichroic effect of appearing red in back-scattered light but blue in transmitted light. The spectra are attributed to a dilute colloidal precipitate of copper, present as a residual impurity in the starting material with a concentration of around 1 ppm by volume. Calculations of the extinction and scattering spectra for copper colloidal particles show that the light scattering efficiency peaks at around 600 nm, but is insignificant compared with absorption for particle radii less than about 30 nm. For particle radii between about 30 and 50 nm, the scattering dominates the beam-particle interaction and is still peaked in the red region. For larger radii, the peak in the scattering efficiency becomes invariant and broadens. Fittings based on the calculations suggest that the copper particles responsible for the observed spectra in ZBLAN glass have radii in the range of 16 to 45 nm.