Silicon nitride (SixNy) fabricated via plasma-enhanced chemical vapor deposition (PECVD) has emerged as a highly promising platform for CMOS-compatible integrated photonics. The stoichiometric form, Si3N4, offers key advantages such as ultra-low optical losses (< 1 dB/cm), a wide optical transparency range, and the absence of two-photon absorption (TPA) in the telecommunication spectral band. These characteristics make Si3N4 highly suitable for applications requiring high optical power, including broadband nonlinear wave mixers, modulators, and optical switches. Additionally, Si3N4 has a high optical damage threshold, enabling efficient nonlinear optical devices. Despite being an amorphous material, Si3N4 also exhibits second-order nonlinear effects. However, Si3N4 has its own limitations, including a low third-order nonlinear susceptibility, a small refractive index contrast with the SiO2 cladding, and a larger effective mode area, which collectively reduce the efficiency of nonlinear wave mixing. To address these limitations, silicon-rich nitride (SRN), achieved by increasing silicon concentration, has been explored as a material with enhanced nonlinearities. SRN has enabled efficient demonstrations of nonlinear processes such as FWM, supercontinuum generation, intermodal frequency generation, and the strong DC Kerr effect. This thesis explores the impact of the SRN refractive index on key waveguide parameters, including optical nonlinearities, linear and nonlinear losses, and mode confinement. Our findings reveal a trade-off: increasing the refractive index enhances optical nonlinearities but also raises optical losses. Notably, we demonstrate that nonlinear losses become the primary limitation for four-wave mixing (FWM) efficiency at high optical power when the SRN refractive index exceeds 3. We identify free carrier absorption (FCA) as the dominant source of nonlinear losses and show that its dynamics are strongly influenced by the refractive index of SRN. Furthermore, we demonstrate significant bidirectional optical trimming of the SRN refractive index using visible light. This ability to precisely tune the refractive index is crucial for phase-sensitive devices, such as demultiplexers, as highlighted in this thesis. These findings establish SRN as a highly reconfigurable and versatile platform for photonic applications.