# New Candidate Multicomponent Chalcogenide Glasses for Supercontinuum Generation
## 4 Linear Optical Property Variation  
The desirable linear optical properties of the glasses with specific compositions and corresponding microstructures are pre-requisites for a variety of optical applications. Figure 3a shows Fresnel loss corrected transmittance spectra for bulk GAP-Se glasses with 20 mol% and 40 mol% PbSe at wavelengths up to 10 \(\upmu\)m. As can be seen, there is a clear difference in position of the glass' band edge, high energy tail states, and scatter can be observed, followed by near-100%, saturated transmittance at wavelengths from 2 to 10 \(\upmu\)m. The spectral shift in band-edge energy is likely to be associated with the addition of highly-polarizing Pb elements where their interaction with other constituents in the glass creates additional energy bands within a bandgap which leads to a decrease in band-edge energy [65]. The tail near the band-edge originates from both band tail electronic states and light scattering which is highly dependent upon a combined effect from the size, volume fraction, and composition of secondary phases present in the phase separated glass. Here, the inverse secondary phase/matrix morphology and compositions of the glasses with 20 and 40 mol% PbSe give rise to such a difference, where the Pb-deficient phase dictates the edge position and the scattering is dictated by the refractive index contrast between the two phases [54; 59]. The broad, high transparency window of these base glasses (no crystalline phase) across wavelengths from 2 to 10 \(\upmu\)m strongly suggests that the material systems shown are effective media with low optical losses in both the MWIR and LWIR for SC applications. Figure 3b shows linear refractive indices and extinction coefficients of the planar disks of bulk glass with 20 and 40 mol% PbSe. The magnitude of refractive indices for both glasses was distinctively greater than those of other ChG glasses due to the existence of PbSe in the material matrix [65]. As the concentration of PbSe increases from 20 to 40 mol%, the refractive indices of the glasses correspondingly increased. A transition from a planar bulk disks to a film geometry (both with 40 mol% PbSe) resulted in a minor shift in band-edge energy and tail states, though it still maintains their maximum, near-100% intensity at wavelengths from 2 to 10 \(\upmu\)m, as shown in Figure 3c. This is due to the fact that thin films of GAP-Se materials, regardless of composition, show no evidence of phase separation, and hence, reduced scatter loss. Meanwhile, Figure 3d shows that the refractive index of the film glass was greater than that of the planar bulk glass. This illustrates the impact of variation of thermal history and the more rapid cooling rate associated with film deposition and the resulting variation in bonding and density. As discussed earlier, the nature of materials with reduced geometries such as films was typically different from their bulk counterpart. This is due to a fact that source materials were condensed into an amorphous film with a relatively higher concentration of strained bonds on a room temperature or cold substrate, and therefore, vapor-deposited thin films were further from equilibrium compared to bulk counterparts [60]. In studies to date, some properties of GAP-Se in bulk and film have been quantified across the broad spectral region where these materials operate. For optical designers, these attributes are critical to assessing and predicting optical performance. Using data presented in previous work and new to this work we summarized many of these optical property performance metrics for these glasses as a function of composition, form, and spectral region. Comparing the linear and nonlinear property variation (discussed in the next section) along with the dispersion variation of these glasses (see Table 3), one can see that both composition and form (bulk versus film) provides a 'knob' which allows a designer to modify their choice in material and form for desired application.