# New Candidate Multicomponent Chalcogenide Glasses for Supercontinuum Generation
## 3 Composition, Morphology, and Optical Properties: Glasses Towards Glass Ceramics  
Optical (nano)composites in the context of the present discussion are not homogeneous glasses; rather, they are multi-phase glass ceramics where a glass matrix contains embedded (nano)crystallites of desired refractive index, size, and size distribution throughout the composite's volume. The most commonly known optical composite is Zerodur [85], engineered for its transparency and superior (low) CTE. Zerodur is a lithia-alumino-silicate glass where nanocrystals with a small (-75 nm) size, narrow size distribution, and positive and negative CTE crystal phases are dispersed to realize the 'effective' physical property of interest, here, a near-zero CTE. The 'effective' aspect of the property of interest, CTE, can be approximated by the individual properties of the parent glass phase following the precipitation of the resulting crystal phases and the respective volume factions of each of the constituent phases (glass and crystallites). The refractive index mismatch between the crystal phase(s) and parent glass is small, leading to low loss in the spectral regime of interest. Similar efforts at making IR glass ceramics (composites) have been extensively studied by the researchers at the University of  
Figure 1: (**a**) Variation in \(\eta\)-T behavior as a function of composition within the binary As-Se system. The colored box is shown as a guide to the eye to illustrate the typical viscosity range for fiber drawing. Figure reproduced with permission from [83], © 2011 American Institute of Physics. (**b**) Viscosity data compilation (probe penetration method) for commercial reference materials over temperature range suitable for precision glass molding including relative position of glass transition temperature, \(T_{\text{g}}\), as reported from the product data sheets. All curves are based on Sellmeir-paramaterization. (Curves for IRG23 and IRG25 show non-physical curvature over a broad temperature range and are only shown to illustrate the variation in temperature position and slope as a function of composition). Rennes [23; 86; 87; 88; 89] and others, as part of efforts to create low expansion substrates for optics suitable for use in applications in space as mirror substrates [25; 90; 91]. However, these composites were largely optimized for thermal and mechanical properties and not for their transmissive characteristics. There are few optical nanocomposites for use as transmissive bulk, film, or fiber components, such as those required for SC applications in the IR. This includes suitable MWIR and LWIR materials based on oxides or non-oxides. In this discussion, we do not consider tellurite glass materials, as they are not truly broadband. However, their high intensity performance as bulk and fiber may be attractive to SC application where MWIR performance is desired [92; 93]. Exploitation of microstructural engineering in ChGs can enhance the thermal, mechanical, and optical properties for SC generation, and such tailoring presents vast opportunities for new optical function including as hosts for SC generation. In optical composites for SC generation, both the parent glass and the crystal phases impact the resulting linear/nonlinear optical, thermal, and mechanical properties; if composition, refractive index, crystallite type, size, and distribution are all carefully controlled and optimized, low loss transmissive components can be realized. Depending on the volume fraction and IR absorption characteristics of crystallites combined with their thermal (conductivity) characteristics, one could envision that laser damage behavior could also be enhanced by a secondary phase. Such thermal property engineering is being used with index matching strategies to create novel nanocrystal doped glasses for fiber laser applications [94; 95; 96; 97; 98; 99].