# New Candidate Multicomponent Chalcogenide Glasses for Supercontinuum Generation
## 1 Introduction
Chalcogenide (ChG) glasses are superb candidates for SC applications, due to their high linear refractive indices (between 2.2 and 2.6 for sulfide, 2.4 and 3.0 for selenide, and 2.6 and 3.5 for telluride glasses [24]), low phonon energies, and high optical nonlinearity (typically \(\sim\)500 times more than traditional silica glasses [25]) attributable to the low vibrational energies of molecular bonds dictated by heavy, chalcogen atoms. The optical transparency of ChGs can extend from the visible up to 10 \(\upmu\)m for sulfides, 12-13 \(\upmu\)m for selenides, and 20 \(\upmu\)m for tellurides [23; 24; 25], making them suitable as fibers and waveguides for use in a wide range of infrared optical applications. The first studies on chalcogenide fibers in the mid-1980s reported high optical losses due to impurity absorption, which was a major limiting factor [36; 37]. With the development of low-loss chalcogenide fibers, SC sources that generate light in the MWIR (3-8 \(\upmu\)m) and LWIR (8-14 \(\upmu\)m) spectral regions gradually attracted further interest. In particular, experimental work on chalcogenide fibers for SC generation first focused on binary As\({}_{2}\)S\({}_{3}\) fibers, where a spectrum from 2-4.6 \(\upmu\)m was demonstrated in an As\({}_{2}\)S\({}_{3}\) step-index fiber [38; 39]. Since then, efforts to realize broad SC generation in MWIR using multi-component chalcogenide fibers have been also made. For example, different chalcogenide compositions such as Ge-As-Se, Ge-Sb-Se, and Ge-As exhibited ultra-broadband SC generation at 2-12 \(\upmu\)m [15; 40; 41; 42; 43; 44; 45]. In addition to fibers, chalcogenide on-chip waveguides have also been established as a major platform for SC generation. Waveguides provide greater flexibility in dispersion engineering compared to fibers; the ability to engineer the zero-dispersion wavelength to be very close to the pump wavelength greatly facilitates the generation of supercontinuum [46]. One of the earliest demonstrations of broadband SC generation at the MWIR was reported to show transparency up to 8 \(\upmu\)m [33], and a variety of compositions, such as Ge-Sb-S, Ge-Te-Sb, and As\({}_{2}\)S\({}_{3}\), have since been developed as waveguides to show broadband SC generation [33; 47; 48; 49]. Early demonstrations of chalcogenide waveguide-based SC generation were achieved using pumps located at the telecommunications wavelength [47; 48; 49]. Most recently, chalcogenide waveguides have been fully leveraged for their transparency far into the mid-long infrared wavelengths for SC generation [50]. Chalcogenide waveguides pumped at \(\sim\)4 \(\upmu\)m have been demonstrated to span from 2 \(\upmu\)m to \(>\)10 \(\upmu\)m [34], thus further leveraging the chalcogenide glass' optical transparency into the far-infrared. As shown in these examples, such properties can be engineered through composition tailoring, as in the case of one well-studied bulk ChG glass system, where the nonlinear refractive index of Ge-Sb-Se has been correlated with composition, and shown to increase with the concentration of Sb [7] as well as Se [18]. Chalcogen substitution of Se by S in simple binary As\({}_{40}\)Se\({}_{60}\) has been shown to lead to an increase in nonlinear refractive index of the glass at the expense of increased multiphoton absorption [22]. Furthermore, as one might expect, with the addition of highly polarizable glass constituents, an increase in the concentration of Te in Ge-As-Se-Te glasses has also been shown to increase glass' nonlinearity [51]. Since properties of a glass are strongly dependent upon its thermal history during fabrication, glass in fiber and film form often has optical properties different from those of their bulk counterpart. This variation is caused by processing conditions (imparting variations in material thermal history) to the final component form, and requires consideration in the design and fabrication of SC structures in planar (thin bulk or films) or fiber form, as key properties may vary from those reported for bulk glasses.