Recently, Novus Light Today published an article by Dr. Jonathan Levine, Director of Technical Business Development at Rubicon Technology, about sapphire quality. His article shares a thorough review of the measures of sapphire quality for optical-grade applications. Last week, we looked at the first two metrics, chemical analysis and X-ray rocking curves. This week, we’ll look at transmission quality.
Levine writes that the quality of a sapphire is determined by how closely the grown crystal matches the ideal structure with respect to the arrangement of atoms within the lattice, dislocations, defects, and stress. Root causes for these problems often originate from insufficient purity of the starting material and the growth process itself.
Sapphire exhibits excellent transmission in the ultraviolet (UV) to the mid-infrared (IR) range (~200 – 5000 nm). According to Levine, conditions within the sapphire growth furnace can induce subtle interactions between the molten sapphire and the growth environment. These interactions can produce bubbles, dislocations and other stresses that could impact optical performance. Levine says that carefully controlling the growth environment produces sapphire that maintains excellent transmission at 200 nm through the mid-IR wavelengths. He illustrates the impact of furnace interactions by comparing Rubicon’s ES-2 sapphire with another commercial sapphire maker’s crystal produced using a different growth method in the figure below. From the image in the post, you can see a sharp absorption peak at 200 nm for sapphire produced by the commercial maker that is absent in sapphire grown by Rubicon.
For Further Reading
Novus Light Today, Optical-Grade Sapphire, Where Quality Matters, http://www.novuslight.com/optical-grade-sapphire-where-quality-matters_N1596.html#sthash.giGipxT1.dpuf