Refractive index modification in glass or crystalline materials typically involves conversion of state (amorphous to crystalline or crystalline to amorphous) through a homogeneous, external stimulus such as laser-or current-induced heating, melting, or localized (resonant) bond modification. With the exception of traditional phase change materials that exploit reversibility, usually at high speeds and over multiple cycles, localized patterning of the refractive index is most frequently employed to induce a complete change of phase to enable the creation of embedded or surface optical structures. The present effort employs a novel, laser-induced vitrification (LIV) process developed to spatially modify the refractive index in a fully homogeneous glass ceramic material. Such processing leads to a local re-vitrification of the pre-existing nanocrystalline microstructure within the material to realize spatially-defined, refractive index profiles. Post-processing refractive index modification on the order of ∆n ~-0.062 was realized in a partially crystallized, multicomponent chalcogenide glass ceramic nanocomposite, subjected to bandgap laser exposure. Spatially-varied phase modification in the lateral and axial directions within a bulk glass ceramic is quantified and the optical function of the resulting structure is demonstrated in the formation of an infrared grating. The underlying mechanism associated with the resulting local refractive index modification is explained through quantification of the multi-phase material attributes including parent glass properties, crystal phase identity and phase fraction as determined through micro-XRD and electron microscopic analysis. This correlation validates the proposed mechanism associated with the modification. A threshold power density for LIV in the starting glass ceramic has been determined based on exposure conditions and material attributes.
Optical materials capable of advanced functionality in the infrared will enable optical designs that can offer lightweight or small footprint solutions in both planar and bulk optical systems. The University of Central Florida's Glass Processing and Characterization Laboratory, together with our collaborators, have been evaluating compositional design and processing protocols for both bulk and film strategies employing multicomponent chalcogenide glasses (ChGs). These materials can be processed with broad compositional flexibility that allows tailoring of their transmission window, physical and optical properties, which allows them to be engineered for compatibility with other homogeneous amorphous or crystalline optical components. We review progress in forming ChG-based gradient refractive index (GRIN) materials from diverse processing methodologies, including solution-derived ChG layers, poled ChGs with gradient compositional and surface reactivity behavior, nanocomposite bulk ChGs and glass ceramics, and metalens structures realized through multiphoton lithography. We discussed current design and metrology tools that lend critical information to material design efforts to realize next-generation IR GRIN media for bulk or film applications.
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