IR GRIN lenses prepared by ionic exchange in chalcohalide glasses

Claire Fourmentin,Yann Guimond,Xiang-Hua Zhang,Mathieu Rozé,Thierry Pain,Francis Gouttefangeas,Laurent Calvez,Enora Lavanant

doi:10.1038/s41598-021-90626-4

Abstract

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.

Highlights

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
To perform lightweight and compact IR systems for thermal imaging applications working in the third atmospheric window (8–12μm), this paper focuses on the development of radial IR GRIN lenses based on chalcogenide glasses
The glass Tg decreases with addition of sodium iodine while the crystallization temperature remains unchanged leading to a high increase of the glass stability against crystallization

Summary

Introduction

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. 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+. To perform lightweight and compact IR systems for thermal imaging applications working in the third atmospheric window (8–12μm), this paper focuses on the development of radial IR GRIN lenses based on chalcogenide glasses. The glass is reinforced by the surfaces in compression and the heart in a state of compensatory tension In this case, critical diffusion length of 25μm is reached before inducing strong deterioration of the base glass leading to irreversible damages due to inner mechanical constrains.

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Conclusion