## 1 Introduction  
Silicon photonics offers cost-effective and scalable solutions to a wide range of mature and emerging applications, such as telecommunication and datacom [1; 2; 3], high-performance computing [4], quantum communications and computation [5; 6; 7], LIDAR (Light Detection And Ranging) [8; 9; 10; 11], and nonlinear optics [12; 13; 14]. In the last few years, silicon platforms have been enriched with complementary functionalities by integrating other optically active materials into the platform, while preserving the complementary metal-oxide-semiconductor (CMOS) compatibility. For example, we have reported the heterogeneous integration of III-V materials in Si platforms to realize III-V/Si hybrid laser sources [15; 16; 17]. In addition, fast and efficient photodiodes can be obtained by growing Ge on Si [18]. It is then possible to combine the optoelectronic properties of these materials with high-speed and efficient Si modulators based on p-n junction [19]. A particularly interesting example of a CMOS-compatible waveguiding material is silicon nitride (SiN). Indeed, several demonstrations of monolithic integration into silicon photonics platforms have been reported [20; 21; 22; 23; 24], as SiN presents particularly attractive properties for passive photonic devices. Thanks to its low optical index (n\({}_{\text{SiN}}\)\(\sim\) 2 when n\({}_{\text{Si}}\)\(\sim\) 3.5), the index contrast with the SiO\({}_{2}\) cladding is reduced compared to Si. As a result, SiN monomode waveguides are less sensitive to edge roughness and therefore have lower propagation losses; for the same reason, SiN-based devices are less sensitive to fabrication variations. In addition, SiN has a much broader wavelength transparency range (between 500 nm and 3.7 \(\upmu\)m [25]), and photon absorption is also absent over a wide wavelength range. Moreover, SiN presents a rather large Kerr nonlinearity (10 times that of silica [13]). Finally, the thermo-optical coefficient of SiN is one order of magnitude lower than that of Si [26], thus making possible the realization of nearly athermal devices [27; 28; 29]. These properties can be used advantageously in a photonics platform for optical transceivers; strategies for increasing the data rate of optical transceivers often rely on wavelength division multiplexing (WDM) [30]. However, a major challenge for integrated silicon (de-)multiplexers is the shift in spectral response when fabrication variations and environment temperature fluctuations occur (for example between the transmitter and receiver circuits). The use of a SiN-based multiplexer can address favorably both issues [28; 29; 31]. Another, more disruptive way to increase the data rate of the optical link is to use the nonlinear properties of SiN to generate a frequency comb for massively parallel telecommunication wavelength channels. The nonlinear properties of SiN can also be useful for spectroscopy and metrology [14; 32]. For frequency comb generation in the C-band, a fundamental concern is the absorption caused by residual N-H bonds remaining in a non-stoichiometric SiN film. This problem can be solved by long and high-temperature annealing [12], but this rules out compatibility with active optoelectronics devices such as photodiodes and p-n junction silicon modulators, as such a high temperature would cause undesirable dopant diffusion. To address this incompatibility, we recently developed a crack-free and annealing-free deposition method to grow relatively thick (over 700 nm) SiN film [32; 33]. Finally, its wide wavelength transparency range make SiN a useful material for photonic applications outside the usual telecom bands, for example for the realization of integrated optical phased arrays (OPAs) for near-infrared LIDAR applications [34]. This is very attractive, as integrated OPAs have the advantage of enabling non-mechanical, arbitrary beam shaping and steering, thus creating a path for low-cost systems with a small physical footprint [9; 10; 11].