Abstract

The heavy metal fluoride (HMF) glasses are a new class of optically useful but environmentally unstable materials. These materials possess desirable optical properties, including a broad transmittance range from the mid-IR (~7 μm) to the near-UV (~0.3 μm), low absorption and scatter losses, low dispersion, low index of refraction and low thermal distortion.¹ However, despite their promising optical characteristics, many HMF compositions are hygroscopic and relatively soft. Thus, hermetic coatings are required that will protect HMF glasses without degrading their optical properties. The problem is compounded by the fact that HMF materials have low softening temperatures, typically ~200 °C. This precludes standard thin film deposition techniques that require a substrate temperature of ~300°C. It is important to note that optical coatings conventionally deposited at low temperature usually exhibit reduced refractive index, increased optical absorption, poor environmental stability and less resistance to abrasion.

© 1987 Optical Society of America