Refractive index patterning of infrared glass ceramics through laser-induced vitrification [Invited]

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, multi-component 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.