## 3 Future CONERSTONE Platforms
### Silicon Nitride  
In recent years, silicon nitride has gained momentum as one of the preferred mid-index CMOS compatible materials for PICs. It is typically used in the form of hydrogenated amorphous films with thicknesses up to 1 \(\upmu\)m. One of the most attractive characteristics of Si\({}_{3}\)N\({}_{4}\) is its wider bandgap, which sets the lower limit of its transparency window to 450 nm in the visible spectrum and extends it all the way to the MIR [83]. This much larger bandgap results in negligible two photon absorption (TPA) at 1550 nm, which makes Si\({}_{3}\)N\({}_{4}\) a viable and more efficient material for non-linear processes, as it can support higher optical powers than Si with lower non-linear losses and a reasonable non-linear Kerr coefficient [84]. In terms of its physical properties, Si\({}_{3}\)N\({}_{4}\) has a thermo-optic coefficient (10\({}^{-5}\) \({}^{\circ}\)C), which is one order of magnitude lower than that of Si in the near-infrared regime [85]. As a result, Si\({}_{3}\)N\({}_{4}\) devices exhibit spectral shifts as small as 11 pm/\({}^{\circ}\)C under variable temperature environments [86], which is a feature that makes them attractive for applications that require a high tolerance to temperature variations. When combined with SiO\({}_{2}\) cladding, the refractive index contrast that can be achieved with Si\({}_{3}\)N\({}_{4}\) is lower than that of the SOI platform. However, it is enough to achieve a good optical confinement (60-80%) with the added advantage of providing a lower sensitivity to dimensional variations and a higher tolerance to surface roughness [87; 88]. Hence, Si\({}_{3}\)N\({}_{4}\) devices represent a compromise between the level of integration and a flexible dimensional control, despite their increased footprint. The different properties discussed above have made Si\({}_{3}\)N\({}_{4}\) an ideal candidate for the realization of a various linear and non-linear PICs spanning from visible to mid-infrared wavelengths. Si\({}_{3}\)N\({}_{4}\) has been widely used to demonstrate passive linear devices for datacom and visible wavelengths [89; 90], including low loss waveguides [90; 91], multimode splitters [83; 92] and Mach-Zehnder interferometers [93]. Many research groups have worked on (de)multiplexers such as array waveguide gratings [94], angled-multimode interferometers [95; 96] and echelle gratings [94; 97]. These devices exploit Si\({}_{3}\)N\({}_{4}\) to achieve a high tolerance to temperature variations for wavelength (de)multiplexing in datacom circuits and optical spectrometers [94; 98]. Si nitride has also become one of the key materials for nonlinear applications and many efforts have geared towards the demonstration of wavelength conversion and optical parametric amplification in the telecom band. Additionally, optical delay lines have been realized to enable digital filtering, pulse shaping and data storage for all optical signal processing [99; 100; 101]. Si\({}_{3}\)N\({}_{4}\) has also been attractive for frequency comb generation [97; 99; 102] that can be used as an optical source for high-capacity data transmission [99; 103], spectroscopy and optical metrology [104]. Supercontinuum generation on Si\({}_{3}\)N\({}_{4}\) has also been explored to obtain ultrabroadband spectra spanning from the visible to the mid-infrared [105; 106]. As the transparency window of Si\({}_{3}\)N\({}_{4}\) covers the spectrum between 730 and 920 nm, several research efforts have focused on creating highly sensitive biosensors [107; 108; 109]. Other biomedical applications have also benefited from Si\({}_{3}\)N\({}_{4}\) circuits that can provide optical sources and (de)multiplexers in a compact size [110; 111], including super-resolution microscopy [112], point-of-care diagnostics based on flow cytometry [110; 113] and optical coherence tomography [114; 115].