Glass formation in the Te-enriched part of the quaternary Ge–As–Se–Te system and its implication for mid-infrared optical fibres

Abstract

We have identified a region of excellent glass formation in the Te-enriched part of the Ge–As–Se–Te (GAST) quaternary system. The best glass compositions did not show crystallisation features on differential analysis curves, have a glass transition temperature, T g, at about 200 °C and did not devitrify when quenched from the liquidus, under vacuum, and held at 50–100 °C above T g for up to 20 h. Thus these glasses are potentially good candidates for drawing into mid-infrared (IR) transparent fibres. The IR transmission window of bulk glasses extends from 1.5 up to 23 μm, which is the broadest, and penetrates the furthest into the IR, amongst glasses considered for mid-IR fibre drawing to date, such as chalcogenide, heavy metal halide, or oxide, glasses. The refractive index of these glasses can be varied from 3.2 to 3.4 by changing the Te/Se or Ge/As ratios allowing tailoring of the refractive index for core-clad structured fibres. We have identified a relatively small, ellipsoid-shaped region of excellent glass formation in the Te-enriched GAST part of the quaternary phase diagram, and multiple crystallisation temperatures have been detected in the glass compositions of the surrounding regions, indicating their poor glass stability. Crystalline phases found in these devitrified glasses are hexagonal Te and As 2Ge 2Te 5.