While widely used in the visible, radial gradient refractive index (GRIN) lenses are still elusive in the IR waveband. In this paper we introduce a new method based on spatially resolved crystallization of chalcogenide glass to produce such lenses. Optical and structural properties of 80 GeSe 2-20 Ga 2 Se 3 glass ceramic samples are measured. A shift of refractive index is induced by increasing the density of nanocrystals. By placing the sample into a tailored thermal profile, spatially controlled crystallization is achieved. To our knowledge this constitutes the first fabrication of an optically functional radial GRIN in the IR. We also introduce a method to characterize the index profile non-destructively, which is a necessary step for embedding GRIN into commercial systems. midwave and longwave infrared. This fabrication is achieved by using spatially controlled crystallization of chalcogenide glass in an annular furnace. A non-destructive way to characterize such lenses is also introduced.
In order to decrease the number of lenses and the weight of thermal imaging devices, specific optical design are required by using gradient refractive index (GRIN) elements transparent in the infrared waveband. While widely used for making visible GRIN lenses with silicate glasses, the ion exchange process is very limited when applied to chalcogenide glasses due to their low Tg and relatively weak mechanical properties. In this paper, we develop chalco-halide glasses based on alkali halide (NaI) addition in a highly covalent GeSe2–Ga2Se3 matrix, efficient for tailoring a significant and permanent change of refractive by ion exchange process between K+ and Na+. Optical and structural properties of the glass samples were measured showing a diffusion length reaching more than 2 mm and a Gaussian gradient of refractive index Δn of 4.5.10–2. The obtained GRIN lenses maintain an excellent transmission in the second (3–5 µm) and third (8–12 µm) atmospheric windows.
TOC graphicCrystallisation of a 62,5GeS2-12,5Sb2S3-25CsCl glass-ceramic was investigated using different techniques from macro to nano-scale. A two-step crystallization process was evidenced, allowing a direct correlation between the microstructural features and the evolution of the mechanical and optical properties. This two-step crystallization process starts with the nucleation of spherical CsCl crystals that progressively evolve toward a disc shaped morphology when they grow. This transformation goes along with a local enrichment of Sb leading also to a significant increase of the lattice parameter of the CsCl phase. These crystals significantly improve the mechanical behaviour of the glass with only a small reduction of the infrared transmission properties.
The Te 20 As 30 Se 50 (TAS) glass has been studied under the extreme conditions of high energy milling. Starting from the raw materials, an amorphization process occurs progressively reaching an unstable intermediate state and the final stable state then. Despite its high resistance against crystallization when made using silica tubes, an intermediate state is reached when mechanically alloyed into powder. The evolution of the glass structure has been investigated using Raman spectroscopy and MAS NMR of 77 Se according to the milling time. Optimized parameters and conditions to compact milled powders by Spark Plasma Sintering show the possibility to design infrared windows transparent from 2 to 20µm. A correlation to the thermo-mechanical properties of densified powder using hot pressing has been made.
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