Emerging compositionally engineered complementary metal-oxide-semiconductor (CMOS)-compatible platforms have been employed for high efficiencies in various on-chip applications, including optical parametric amplification and wavelength conversion. Combining the novel nonlinear optics platforms such as ultra-silicon-rich nitride (USRN: Si7N3) with periodic waveguide structures can lead to further enhancement of material nonlinearities via the slow light effect and enable ultra-compact devices. Four-wave mixing in a USRN-based, CMOS-compatible, photonic crystal waveguide (PhCWg) leading to on/off optical parametric signal gain reaching 3 dB, and a large instantaneous idler conversion efficiency of -1 dB is explored experimentally. Enhancement of Kerr nonlinearity in the presence of a sizable and near-constant group index allows the findings on an ultra-compact, 97 mu m-long PhCWg, equivalent to a large on/off gain per unit length of 333 dB/cm. (C) 2020 Author(s).