GRIN materials can help compensate chromatic aberrations and enhance athermalization in an optical system, leading to more lightweight and compact lens assemblies, often with a lower number of elements. For several years, Umicore and ISCR in Rennes have developed GRIN materials for systems, using several approaches to modify the index in a chalcogenide glass. We will mention two of the most promising paths in this review: partial and spatially controlled crystallization by fast heating of the perimeter of a glass rod and ionic exchange which modifies locally the glass composition, leading to local changes in refractive index.
The last twenty years have seen a dramatic improvement in the performance of infrared detectors, especially those with uncooled microbolometer arrays. These make new commercial applications possible, from the Internet of things to drones for crop monitoring. Such emerging markets are constantly pushing the requirements on the existing technology to improve cost, performance, weight and size. In this context, gradient index (GRIN) lenses can offer a new degree of freedom compared to their homogeneous counterparts. For example, recent papers have shown how GRIN lenses could help reduce the number of elements in a system, by helping correct lens chromatism and athermalization. While the interest in using GRIN is obvious, mastering their fabrication with infrared materials remains highly challenging. Encouraging progress has been made in that direction, for instance by stacking different materials or by laser writing. Very recently, it has been demonstrated that partial crystallization of chalcogenide glass allows for tuning the refractive index. In this paper, we introduce a new technique based on the controlled formation of nano-crystallites in chalcogenide glass to fabricate the first macroscopic radial GRIN in the infrared. We also propose a simple way to test the index gradient value by interferometry and demonstrate GRIN with index contrast of ▵n ~ 3⋅10-2 between the center and the edge of the disk. The process is easily repeated and opens the possibility for a rapid transfer to the industry.
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