Silicon-Rich Nitride Refractive Index as a Degree of Freedom to Maximize Nonlinear Wave Mixing in Nanowaveguides

Silicon nitride is widely used in integrated photonics for optical nonlinear wave mixing due to its low optical losses combined with relatively high nonlinear optical properties and a wide-range transparency window. It is known that a higher concentration of Si in silicon-rich nitride (SRN) magnifies both the nonlinear response and optical losses, including nonlinear losses. To address the trade-off, four-wave mixing (FWM) is implemented in over a hundred SRN waveguides prepared by plasma-enhanced chemical vapor deposition in a wide range of SRN refractive indices varying between 2.5 and 3.2 (measured in the C-band). It is determined that SRN with a refractive index of about 3 maximizes the FWM efficiency for continuous-wave operation, indicating that the refractive index of SRN is indeed a crucial optimization parameter for nonlinear optics applications. The FWM efficiency is limited by large nonlinear optical losses observed in SRN waveguides with indices larger than 2.7, which are not related to two-photon absorption. Finally, the third-order susceptibility ${\chi }_3$ and the nonlinear refractive index $n_2$ are estimated for multiple SRN refractive indices, and, specifically, the nonlinearities as large as ${\chi }_3=(12.6\pm 1.4)\times {10}^{-19}\hspace{0.17em}{m}^2\hspace{0.17em}{V}^{-2}$ and $n_2=(7.6\pm 0.8)\times {10}^{-17}\hspace{0.17em}{m}^2\hspace{0.17em}{W}^{-1}$ are estimated in a waveguide with an SRN refractive index of 3.2.