# New Candidate Multicomponent Chalcogenide Glasses for Supercontinuum Generation
## 3 Composition, Morphology, and Optical Properties: Glasses Towards Glass Ceramics
As an example of composition, morphology, and eventual microstructure engineering possibilities for candidate SC material development, we discuss a model system based on a multi-component ChG system initially developed by the University of Rennes [23; 86; 87; 88; 89], where controlled nucleation and growth of secondary phase(s) were used to modify physical and optical properties. In the GAP-Se system where PbSe concentrations have been varied from 0 to 50 mol%, we have investigated base (homogeneous) glass as well as glasses following controlled nucleation and growth protocols to microstructurally vary physical and optical properties [54; 59; 63; 65; 66]. The optical performance of these GAP-Se glasses depends on the parent glass' morphology and post-heat treatment microstructure; specifically, each composition and fraction of co-existing phases. Based on examination of optical properties (bulk and film) evaluated to date, we explode how such composite materials and the understanding of these factors could be further exploited to find desirable candidate materials for SC. As a key first step to establish a process-structure-property relationship, Figure 2a shows the morphology phase diagram of GAP-Se glass with PbSe concentrations from 0 to 50 mol% [54; 56; 58; 66], illustrating an immiscibility dome across a wide range of Pb concentrations from \(\sim\)5-10 to \(\sim\)45% at room temperature. Within these composition ranges, glasses melt-quenched from a temperature above a melting point to room temperature exhibit unstable liquid-liquid phase separation, yielding a homogeneous glass containing two amorphous phases. For lower PbSe content from 5-10 to 25-30 mol%, Pb-rich secondary phases have been shown to emerge within a Pb-deficient matrix while the morphology is inverted, becoming Pb-deficient secondary phases within a Pb-rich matrix as PbSe content increases beyond 25-30 up to 40-45 mol%. These regions are illustrated by two schematics in the diagram. Figure 2b,c show dark field (DF) transmission electron microscope (TEM) images collected from the glasses with 20 and 40 mol% of PbSe and linear profiles of atomic percentages for four constituents along the lines drawn in the DF TEM images. Specifically, the DF TEM image collected from the glass with 20 mol% PbSe includes bright, circular phases in a dark matrix, as shown in Figure 2b. The contrast in brightness between the secondary phases and the matrix in the DF TEM image suggests that the atomic percentage of heavy constituents in the secondary phases was greater than that in the matrix. The difference in weights of the secondary phase and the matrix is consistent with linear profiles where the atomic percentage of Pb, which is the heaviest constituent, was relatively high and low in the secondary phase and the matrix, respectively. In contrast, the DF TEM image and linear composition profiles collected from the glass with 20 mol% PbSe indicated an inverse microstructure with Pb-deficient secondary phases and a Pb-rich matrix, as shown in Figure 2c. Here, we discuss the potential opportunity to extend the range of available SC materials that could be optimized for use through consideration of how, for example, two glass compositions within this composition space could be modified to form optical nanocomposites suitable for use in SC applications. Specifically, we consider these two glasses with 20 and 40 mol% PbSe, for two reasons: firstly, it is desirable that candidate materials have high linear and nonlinear refractive indices, which in general increase with the concentration of PbSe; here, such an increase must occur while maintaining the glass' transparency in the MWIR [58]. The refractive index-transparency criteria suggest that the concentration of PbSe needs to be high to increase linear and nonlinear refractive indices of the glasses, though below a certain point (\(\sim\)50-55 mol% PbSe), at which the entire volume of the material starts to become crystalline upon quenching, leading to optical losses [61; 62].