# New Candidate Multicomponent Chalcogenide Glasses for Supercontinuum Generation
## 2 Optical and Physical Properties and Composition: Glasses  
SC generation requires candidate materials to possess high linear and nonlinear refractive indices without appreciable optical degradation such as scattering and absorption losses [3]. Traditional homogeneous glasses have fixed values of properties that usually vary systematically with composition, with heavier, more polarizable species leading to higher linear and nonlinear properties. Losses are often caused by impurities in the parent host; however, these can also be related to nanoscale crystallization and interfacial defects, such as those seen in core/clad fibers of dissimilar refractive index, trapped inclusions or bubbles, or sidewall roughness such as that seen in planar waveguides or resonators [67], which can lead to scattering losses. While SC spectral width is typically defined by the host's transmission window, refractive index, and dispersion, broadband materials can suffer from laser damage induced by high intensity pump radiation in lossy structures [19; 51; 68; 69; 70; 71; 72; 73]. This damage can surface in nature in fibers (at end-faces or cleaved interfaces) or intrinsic within the bulk material due to impurities, inhomogeneities, or other defects. Homogeneous glass refractive index data and surface/bulk laser damage thresholds and mechanisms in ChGs are not widely reported in the MWIR and LWIR regions; where reported, typically measurements have been made in the band-edge region of most ChGs, near 1 \(\upmu\)m with CW or long-pulse (low intensity) regimes. Representative literature data useful for assessing suitability of glass composition for SC applications is summarized for a diverse range of IR transparent glasses in Table 1[7; 74; 75; 76; 77; 78; 79; 20; 21; 26; 28; 51; 54; 56; 58; 59; 60; 74; 75; 76; 77]. Included for comparison in Table 1 is data on glass(es), where reported, in bulk and film form measured at various IR wavelengths. As can be seen, the thermal history associated with the processing method used to create the film (or fiber) into its final form can impart a unique and variable structure to the resulting material. This structural variation for a constant composition has been shown to impact linear and nonlinear properties, and was first highlighted in [28]. Such structural variations are attributable to subtle variation in composition, bonding, and thus density, and polarizability, imparting refractive indices that can be processing method-specific. Additionally, since full dispersion data across the IR spectral window of SC generation is often difficult to measure, the properties available to model component behavior is often limited to data at a single wavelength. Hence, complete dispersion behavior is a desirable knob for optimization of material choice, and should be quantified [20]. For comparison to previously reported data in Table 1, the linear refractive index collected from GAP-Se bulk and films are noticeably greater than those reported for more simple binary or ternary compositions, impacted by the addition of Pb. Furthermore, Ge, which has been often used for nonlinear optical applications, has a nonlinear refractive index of 4.57 \(\times\) 10\({}^{-5}\) cm\({}^{2}\)/GW and a three-photon absorption coefficient of 1.027 cm\({}^{3}\)/GW\({}^{2}\) at 4.5 \(\upmu\)m [78], which are greater than those of GAP-Se glasses. However, the high nonlinear absorption for Ge could be a limitation on the purpose of all optical switching. Therefore, a trade-off between high \(n_{2}\) and low nonlinear absorption is strongly desired and requires the development of new candidate materials, such as GAP-Se glasses reported in this study. It should be noted that full broadband nonlinear property characterization of GAP-Se films in both their as-deposited amorphous and converted glass ceramic form, is ongoing. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Form**} & \multirow{2}{*}{ \begin{tabular}{c} **Composition** \\ **(mol\%)** \\ \end{tabular}